Organic Chemistry Principles and Techniques PDF

326 CHEMISTRY UNIT 12 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES In t...

326 CHEMISTRY UNIT 12 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES In the previous unit you have learnt that the element carbon has the unique property called catenation due to which it forms covalent bonds with other carbon atoms. After studying this unit, you will be It also forms covalent bonds with atoms of other elements able to like hydrogen, oxygen, nitrogen, sulphur, phosphorus and understand reasons for halogens. The resulting compounds are studied under a tetravalence of carbon and separate branch of chemistry called organic chemistry. shapes of organic molecules; This unit incorporates some basic principles and write structures of organic techniques of analysis required for understanding the molecules in various ways; formation and properties of organic compounds. classify the organic compounds; name the compounds according 12.1 GENERAL INTRODUCTION to IUPAC system of nomenclature and also derive Organic compounds are vital for sustaining life on earth their structures from the given and include complex molecules like genetic information names; bearing deoxyribonucleic acid (DNA) and proteins that understand the concept of constitute essential compounds of our blood, muscles and organic reaction mechanism; skin. Organic chemicals appear in materials like clothing, explain the influence of fuels, polymers, dyes and medicines. These are some of electronic displacements on the important areas of application of these compounds. structure and reactivity of Science of organic chemistry is about two hundred organic compounds; years old. Around the year 1780, chemists began to recognise the types of organic distinguish between organic compounds obtained from reactions; plants and animals and inorganic compounds prepared lear n the techniques of from mineral sources. Berzilius, a Swedish chemist purification of organic proposed that a ‘vital force’ was responsible for the compounds; formation of organic compounds. However, this notion write the chemical reactions was rejected in 1828 when F. Wohler synthesised an involved in the qualitative organic compound, urea from an inorganic compound, analysis of organic compounds; ammonium cyanate. understand the principles involved in quantitative analysis NH4 CNO  Heat NH2 CONH2 of organic compounds. Ammonium cyanate Urea The pioneering synthesis of acetic acid by Kolbe (1845) and that of methane by Berthelot (1856) showed conclusively that organic compounds could be synthesised from inorganic sources in a laboratory. ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 327 The development of electronic theory of necessary for a proper sideways overlap. covalent bonding ushered organic chemistry Thus, in H2C=CH2 molecule all the atoms into its modern shape. must be in the same plane. The p orbitals are mutually parallel and both the p orbitals 12.2 TETRAVALENCE OF CARBON: are perpendicular to the plane of the SHAPES OF ORGANIC COMPOUNDS molecule. Rotation of one CH2 fragment with 12.2.1 The Shapes of Carbon Compounds respect to other interferes with maximum The knowledge of fundamental concepts of overlap of p orbitals and, therefore, such molecular structure helps in understanding rotation about carbon-carbon double bond and predicting the properties of organic (C=C) is restricted. The electron charge cloud compounds. You have already learnt theories of the π bond is located above and below the of valency and molecular structure in Unit 4. plane of bonding atoms. This results in the Also, you already know that tetravalence of electrons being easily available to the carbon and the formation of covalent bonds attacking reagents. In general, π bonds provide by it are explained in terms of its electronic the most reactive centres in the molecules configuration and the hybridisation of s and containing multiple bonds. p orbitals. It may be recalled that formation and the shapes of molecules like methane Problem 12.1 (CH 4 ), ethene (C 2 H 4 ), ethyne (C 2 H 2 ) are How many σ and π bonds are present in explained in terms of the use of sp3, sp2 and each of the following molecules? sp hybrid orbitals by carbon atoms in the (a) HC≡CCH=CHCH3 (b) CH2=C=CHCH3 respective molecules. Hybridisation influences the bond length Solution and bond enthalpy (strength) in organic (a) σC – C: 4; σC–H : 6; πC=C :1; π C≡C:2 compounds. The sp hybrid orbital contains (b) σC – C: 3; σC–H: 6; πC=C: 2. more s character and hence it is closer to its nucleus and forms shorter and stronger Problem 12.2 bonds than the sp3 hybrid orbital. The sp2 What is the type of hybridisation of each hybrid orbital is intermediate in s character carbon in the following compounds? between sp and sp3 and, hence, the length and enthalpy of the bonds it forms, are also (a) CH3Cl, (b) (CH3)2CO, (c) CH3CN, intermediate between them. The change in (d) HCONH2, (e) CH3CH=CHCN hybridisation affects the electronegativity of carbon. The greater the s character of the Solution hybrid orbitals, the greater is the (a) sp3, (b) sp3, sp2, (c) sp3, sp, (d) sp2, (e) electronegativity. Thus, a carbon atom having sp3, sp2, sp2, sp an sp hybrid orbital with 50% s character is Problem 12.3 more electronegative than that possessing sp2 or sp 3 hybridised orbitals. This relative Write the state of hybridisation of carbon electronegativity is reflected in several in the following compounds and shapes physical and chemical properties of the of each of the molecules. molecules concerned, about which you will (a) H2C=O, (b) CH3F, (c) HC≡N. learn in later units. Solution 12.2.2 Some Characteristic Features of π Bonds (a) sp2 hybridised carbon, trigonal planar; (b) sp3 hybridised carbon, tetrahedral; (c) In a π (pi) bond formation, parallel orientation sp hybridised carbon, linear. of the two p orbitals on adjacent atoms is 328 CHEMISTRY 12.3 STRUCTURAL REPRESENTATIONS Similarly, CH3CH2CH2CH2CH2CH2CH2CH3 OF ORGANIC COMPOUNDS can be further condensed to CH3(CH2)6CH3. 12.3.1 Complete, Condensed and Bond-line For further simplification, organic chemists Structural Formulas use another way of representing the structures, in which only lines are used. In Structures of organic compounds are this bond-line structural representation of represented in several ways. The Lewis organic compounds, carbon and hydrogen structure or dot structure, dash structure, atoms are not shown and the lines condensed structure and bond line structural representing carbon-carbon bonds are drawn formulas are some of the specific types. The in a zig-zag fashion. The only atoms Lewis structures, however, can be simplified specifically written are oxygen, chlorine, by representing the two-electron covalent nitrogen etc. The terminals denote methyl bond by a dash (–). Such a structural formula (–CH3) groups (unless indicated otherwise by focuses on the electrons involved in bond a functional group), while the line junctions formation. A single dash represents a single denote carbon atoms bonded to appropriate bond, double dash is used for double bond number of hydrogens required to satisfy the and a triple dash represents triple bond. Lone- valency of the carbon atoms. Some of the pairs of electrons on heteroatoms (e.g., examples are represented as follows: oxygen, nitrogen, sulphur, halogens etc.) may or may not be shown. Thus, ethane (C2H6), (i) 3-Methyloctane can be represented in ethene (C2H4), ethyne (C2H2) and methanol various forms as: (CH3OH) can be represented by the following (a) CH3CH2CHCH2CH2CH2CH2CH3 structural for mulas. Such structural | representations are called complete structural CH3 formulas. (b) Ethane Ethene (c) Ethyne Methanol These structural formulas can be further abbreviated by omitting some or all of the dashes representing covalent bonds and by (ii) Various ways of representing 2-bromo indicating the number of identical groups butane are: attached to an atom by a subscript. The resulting expression of the compound is called a condensed structural formula. Thus, ethane, (a) CH3CHBrCH2CH3 (b) ethene, ethyne and methanol can be written as: CH3CH3 H2C=CH2 HC≡≡CH CH3OH (c) Ethane Ethene Ethyne Methanol ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 329 In cyclic compounds, the bond-line formulas may be given as follows: (b) Solution Condensed formula: Cyclopropane (a) HO(CH2)3CH(CH3)CH(CH3)2 (b) HOCH(CN)2 Bond-line formula: (a) Cyclopentane (b) chlorocyclohexane Problem 12.6 Problem 12.4 Expand each of the following bond-line Expand each of the following condensed formulas to show all the atoms including formulas into their complete structural carbon and hydrogen formulas. (a) (a) CH3CH2COCH2CH3 (b) CH3CH=CH(CH2)3CH3 Solution (b) (a) (c) (b) (d) Solution Problem 12.5 For each of the following compounds, write a condensed formula and also their bond-line formula. (a) HOCH2CH2CH2CH(CH3)CH(CH3)CH3 330 CHEMISTRY Molecular Models Molecular models are physical devices that are used for a better visualisation and perception of three-dimensional shapes of organic molecules. These are made of wood, plastic or metal and are commercially available. Commonly three types of molecular models are used: (1) Framework model, (2) Ball-and-stick model, and (3) Space filling model. In the framework model only the bonds connecting the atoms of a molecule and not the atoms themselves are shown. This model emphasizes the pattern of bonds of a molecule while ignoring the size of atoms. In the ball-and-stick model, both the atoms and the bonds are shown. Balls represent atoms and the stick denotes a bond. Compounds containing C=C (e.g., ethene) can best be represented by using springs in place 12.3.2 Three-Dimensional of sticks. These models are referred to as ball- Representation of Organic and-spring model. The space-filling model Molecules emphasises the relative size of each atom The three-dimensional (3-D) structure of based on its van der Waals radius. Bonds organic molecules can be represented on are not shown in this model. It conveys the paper by using certain conventions. For volume occupied by each atom in the example, by using solid ( ) and dashed molecule. In addition to these models, computer graphics can also be used for ( ) wedge formula, the 3-D image of a molecular modelling. molecule from a two-dimensional picture can be perceived. In these formulas the solid-wedge is used to indicate a bond projecting out of the plane of paper, towards the observer. The dashed-wedge is used to depict the bond projecting out of the plane of the paper and away from the observer. Wedges are shown in such a way that the broad end of the wedge is towards the observer. The Ball and stick model Framework model bonds lying in plane of the paper are depicted by using a normal line (—). 3-D representation of methane molecule on paper has been shown in Fig. 12.1. Space filling model Fig. 12.2 Fig. 12.1 Wedge-and-dash representation of CH4 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 331 12.4 CLASSIFICATION OF ORGANIC (homocyclic). Sometimes atoms other than COMPOUNDS carbon are also present in the ring (heterocylic). Some examples of this type of The existing large number of organic compounds are: compounds and their ever -increasing numbers has made it necessary to classify them on the basis of their structures. Organic compounds are broadly classified as follows: Cyclopropane Cyclohexane Cyclohexene Tetrahydrofuran These exhibit some of the properties similar to those of aliphatic compounds. Aromatic compounds Aromatic compounds are special types of compounds. You will learn about these compounds in detail in Unit 13. These include benzene and other related ring compounds (benzenoid). Like alicyclic compounds, aromatic comounds may also have hetero atom in the ring. Such compounds are called I. Acyclic or open chain compounds hetrocyclic aromatic compounds. Some of the examples of various types of aromatic These compounds are also called as aliphatic compounds are: compounds and consist of straight or branched chain compounds, for example: Benzenoid aromatic compounds CH3CH3 Ethane Isobutane Benzene Aniline Naphthalene Non-benzenoid compound Acetaldehyde Acetic acid II Alicyclic or closed chain or ring compounds Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring Tropolone 332 CHEMISTRY Heterocyclic aromatic compounds acid found in red ant is named formic acid since the Latin word for ant is formica. These names are traditional and are considered as trivial or common names. Some common names are followed even today. For example, Furan Thiophene Pyridine Buckminsterfullerene is a common name Organic compounds can also be classified given to the newly discovered C60 cluster on the basis of functional groups, into families (a form of carbon) noting its structural or homologous series. similarity to the geodesic domes popularised by the famous architect R. Buckminster Functional Group Fuller. Common names are useful and in The functional group may be defined as an many cases indispensable, particularly when atom or group of atoms joined in a specific the alternative systematic names are lengthy manner which is responsible for the and complicated. Common names of some characteristic chemical properties of the organic compounds are given in Table 12.1. organic compounds. The examples are Table 12.1 Common or Trivial Names of Some hydroxyl group (–OH), aldehyde group (–CHO) Organic Compounds and carboxylic acid group (–COOH) etc. Homologous Series A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in molecular formula by a –CH2 unit. There are a number of homologous series of organic compounds. Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids, amines etc. 12.5 NOMENCLATURE OF ORGANIC COMPOUNDS Organic chemistry deals with millions of compounds. In order to clearly identify them, a systematic method of naming has been 12.5.1 The IUPAC System of Nomenclature developed and is known as the IUPAC A systematic name of an organic compound (International Union of Pure and Applied is generally derived by identifying the parent Chemistry) system of nomenclature. In this hydrocarbon and the functional group(s) systematic nomenclature, the names are attached to it. See the example given below. correlated with the structure such that the reader or listener can deduce the structure from the name. Before the IUPAC system of nomenclature, however, organic compounds were assigned names based on their origin or certain properties. For instance, citric acid is named so because it is found in citrus fruits and the ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 333 By further using prefixes and suffixes, the In order to name such compounds, the names parent name can be modified to obtain the of alkyl groups are prefixed to the name of actual name. Compounds containing carbon parent alkane. An alkyl group is derived from and hydrogen only are called hydrocarbons. A a saturated hydrocarbon by removing a hydrocarbon is termed saturated if it contains hydrogen atom from carbon. Thus, CH4 only carbon-carbon single bonds. The IUPAC becomes -CH3 and is called methyl group. An name for a homologous series of such alkyl group is named by substituting ‘yl’ for compounds is alkane. Paraffin (Latin: little ‘ane’ in the corresponding alkane. Some alkyl affinity) was the earlier name given to these groups are listed in Table 12.3. compounds. Unsaturated hydrocarbons are Table 12.3 Some Alkyl Groups those, which contain at least one carbon- carbon double or triple bond. 12.5.2 IUPAC Nomenclature of Alkanes Straight chain hydrocarbons: The names of such compounds are based on their chain structure, and end with suffix ‘-ane’ and carry a prefix indicating the number of carbon atoms present in the chain (except from CH4 to C4H10, where the prefixes are derived from trivial names). The IUPAC names of some straight chain saturated hydrocarbons are given in Table 12.2. The alkanes in Table 12.2 Abbreviations are used for some alkyl differ from each other by merely the number groups. For example, methyl is abbreviated of -CH 2 groups in the chain. They are as Me, ethyl as Et, propyl as Pr and butyl as homologues of alkane series. Bu. The alkyl groups can be branched also. Thus, propyl and butyl groups can have Table 12.2 IUPAC Names of Some Unbranched branched structures as shown below. Saturated Hydrocarbons CH3-CH- CH3-CH2-CH- CH3-CH-CH2- ⏐ ⏐ ⏐ CH3 CH3 CH3 Isopropyl- sec-Butyl- Isobutyl- CH3 CH3 ⏐ ⏐ CH3-C- CH3-C-CH2- ⏐ ⏐ CH3 CH3 tert-Butyl- Neopentyl- Common branched groups have specific Branched chain hydrocarbons: In a trivial names. For example, the propyl groups branched chain compound small chains of can either be n-propyl group or isopropyl carbon atoms are attached at one or more group. The branched butyl groups are called carbon atoms of the parent chain. The small sec-butyl, isobutyl and tert-butyl group. We carbon chains (branches) are called alkyl also encounter the structural unit, groups. For example: –CH2C(CH3)3, which is called neopentyl group. CH3–CH–CH2–CH3 CH3–CH–CH2–CH–CH3 Nomenclature of branched chain alkanes: ⏐ ⏐ ⏐ We encounter a number of branched chain CH3 CH2CH3 CH3 alkanes. The rules for naming them are given (a) (b) below. 334 CHEMISTRY 1. First of all, the longest carbon chain in separated from the groups by hyphens and the molecule is identified. In the example there is no break between methyl and (I) given below, the longest chain has nine nonane.] carbons and it is considered as the parent 4. If two or more identical substituent groups or root chain. Selection of parent chain as are present then the numbers are shown in (II) is not correct because it has separated by commas. The names of only eight carbons. identical substituents are not repeated, instead prefixes such as di (for 2), tri (for 3), tetra (for 4), penta (for 5), hexa (for 6) etc. are used. While writing the name of the substituents in alphabetical order, these prefixes, however, are not considered. Thus, the following compounds are named as: CH3 CH3 CH3 CH3 ⏐ ⏐ ⏐ ⏐ CH3-CH-CH2-CH-CH3 CH3⎯C⎯CH2⎯CH⎯CH3 1 2 3 4 5 1 2⏐ 3 4 5 CH3 2. The carbon atoms of the parent chain are numbered to identify the parent alkane and 2,4-Dimethylpentane 2,2,4-Trimethylpentane to locate the positions of the carbon atoms H 3 C H2 C CH3 at which branching takes place due to the ⏐ ⏐ substitution of alkyl group in place of CH3⎯CH2⎯CH⎯C⎯CH2⎯CH2⎯CH3 hydrogen atoms. The numbering is done 1 2 3 ⏐4 5 6 7 in such a way that the branched carbon CH3 atoms get the lowest possible numbers. Thus, the numbering in the above example 3-Ethyl-4,4-dimethylheptane should be from left to right (branching at 5. If the two substituents are found in carbon atoms 2 and 6) and not from right to left (giving numbers 4 and 8 to the equivalent positions, the lower number is carbon atoms at which branches are given to the one coming first in the attached). alphabetical listing. Thus, the following compound is 3-ethyl-6-methyloctane and 1 2 3 4 5 6 7 8 9 not 6-ethyl-3-methyloctane. C ⎯ C ⎯ C ⎯ C ⎯ C ⎯ C ⎯C ⎯ C ⎯ C 1 2 3 4 5 6 7 8 ⏐ ⏐ CH3 — CH2—CH—CH2—CH2—CH—CH2 —CH3 C C⎯C ⏐ ⏐ 9 8 7 6 5 4 3 2 1 CH2CH3 CH3 C⎯ C⎯C⎯C⎯C⎯C⎯C⎯C⎯C ⏐ ⏐ 6. The branched alkyl groups can be named C C⎯C by following the above mentioned 3. The names of alkyl groups attached as a procedures. However, the carbon atom of branch are then prefixed to the name of the branch that attaches to the root the parent alkane and position of the alkane is numbered 1 as exemplified substituents is indicated by the below. appropriate numbers. If different alkyl 4 3 2 1 groups are present, they are listed in CH3–CH–CH2–CH– alphabetical order. Thus, name for the ⏐ ⏐ compound shown above is: 6-ethyl-2- CH3 CH3 methylnonane. [Note: the numbers are 1,3-Dimethylbutyl- ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 335 The name of such branched chain alkyl group Cyclic Compounds: A saturated monocyclic is placed in parenthesis while naming the compound is named by prefixing ‘cyclo’ to the compound. While writing the trivial names of corresponding straight chain alkane. If side substituents’ in alphabetical order, the chains are present, then the rules given above prefixes iso- and neo- are considered to be are applied. Names of some cyclic compounds the part of the fundamental name of alkyl are given below. group. The prefixes sec- and tert- are not considered to be the part of the fundamental name. The use of iso and related common prefixes for naming alkyl groups is also allowed by the IUPAC nomenclature as long as these are not further substituted. In multi- substituted compounds, the following rules may aso be remembered: If there happens to be two chains of equal size, then that chain is to be selected which contains more number of side 3-Ethyl-1,1-dimethylcyclohexane chains. (not 1-ethyl-3,3-dimethylcyclohexane) After selection of the chain, numbering is to be done from the end closer to the Problem 12.7 substituent. Structures and IUPAC names of some hydrocarbons are given below. Explain why the names given in the parentheses are incorrect. 2,5,6- Trimethyloctane [and not 3,4,7-Trimethyloctane] 5-(2-Ethylbutyl)-3,3-dimethyldecane [and not 5-(2,2-Dimethylbutyl)-3-ethyldecane] 3-Ethyl-5-methylheptane [and not 5-Ethyl-3-methylheptane] Solution (a) Lowest locant number, 2,5,6 is lower than 3,5,7, (b) substituents are in 5-sec-Butyl-4-isopropyldecane equivalent position; lower number is given to the one that comes first in the name according to alphabetical order. 12.5.3 Nomenclature of Organic Compounds having Functional Group(s) A functional group, as defined earlier, is an atom or a group of atoms bonded together in a 5-(2,2-Dimethylpropyl)nonane unique manner which is usually the site of 336 CHEMISTRY chemical reactivity in an organic molecule. suffix. In such cases the full name of the parent Compounds having the same functional group alkane is written before the class suffix. For undergo similar reactions. For example, example CH 2 (OH)CH 2 (OH) is named as CH3OH, CH3CH2OH, and (CH3)2CHOH — all ethane–1,2–diol. However, the ending – ne of having -OH functional group liberate hydrogen the parent alkane is dropped in the case of on reaction with sodium metal. The presence compounds having more than one double or of functional groups enables systematisation triple bond; for example, CH2=CH-CH=CH2 is of organic compounds into different classes. named as buta–1,3–diene. Examples of some functional groups with their prefixes and suf fixes along with some Problem 12.8 examples of organic compounds possessing Write the IUPAC names of the compounds these are given in Table 12.4. i-iv from their given structures. First of all, the functional group present in the molecule is identified which determines the choice of appropriate suffix. The longest chain of carbon atoms containing the functional group is numbered in such a way that the functional group is attached at the Solution carbon atom possessing lowest possible The functional group present is an number in the chain. By using the suffix as alcohol (OH). Hence the suffix is ‘-ol’. given in Table 12.4, the name of the compound The longest chain containing -OH has is arrived at. eight carbon atoms. Hence the In the case of polyfunctional compounds, corresponding saturated hydrocarbon one of the functional groups is chosen as the is octane. principal functional group and the compound is The -OH is on carbon atom 3. In then named on that basis. The remaining addition, a methyl group is attached functional groups, which are subordinate at 6th carbon. functional groups, are named as substituents Hence, the systematic name of this using the appropriate prefixes. The choice of compound is 6-Methyloctan-3-ol. principal functional group is made on the basis of order of preference. The order of decreasing priority for some functional groups is: -COOH, –SO3H, -COOR (R=alkyl group), COCl, -CONH2, -CN,-HC=O, >C=O, -OH, -NH2, >C=CC=O), hence suffix ‘-one’. Presence of substituents. Thus, a compound containing two keto groups is indicated by ‘di’, both an alcohol and a keto group is named hence suffix becomes ‘dione’. The two as hydroxyalkanone since the keto group is keto groups are at carbons 2 and 4. The preferred to the hydroxyl group. longest chain contains 6 carbon atoms, For example, HOCH2(CH2)3CH2COCH3 will be hence, parent hydrocarbon is hexane. named as 7-hydroxyheptan-2-one and not as Thus, the systematic name is Hexane- 2-oxoheptan -7-ol. Similarly, BrCH2CH=CH2 2,4-dione. is named as 3-bromoprop-1-ene and not 1- bromoprop-2-ene. If more than one functional group of the same type are present, their number is indicated by adding di, tri, etc. before the class ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 337 Table 12.4 Some Functional Groups and Classes of Organic Compounds 338 CHEMISTRY Solution (iii) Six membered ring containing a Here, two functional groups namely carbon-carbon double bond is implied by ketone and carboxylic acid are present. cyclohexene, which is numbered as The principal functional group is the shown in (I). The prefix 3-nitro means that carboxylic acid group; hence the parent a nitro group is present on C-3. Thus, chain will be suffixed with ‘oic’ acid. complete structural formula of the Numbering of the chain starts from compound is (II). Double bond is suffixed carbon of – COOH functional group. The functional group whereas NO2 is prefixed keto group in the chain at carbon 5 is functional group therefore double bond indicated by ‘oxo’. The longest chain gets preference over –NO2 group: including the principal functional group has 6 carbon atoms; hence the parent hydrocarbon is hexane. The compound is, therefore, named as 5-Oxohexanoic acid. (iv) ‘1-ol’ means that a -OH group is Solution present at C-1. OH is suffixed functional The two C=C functional groups are group and gets preference over C=C present at carbon atoms 1 and 3, while bond. Thus the structure is as shown the C≡C functional group is present at in (II): carbon 5. These groups are indicated by suffixes ‘diene’ and ‘yne’ respectively. The longest chain containing the functional groups has 6 carbon atoms; hence the parent hydrocarbon is hexane. The name of compound, therefore, is Hexa-1,3- dien-5-yne. (v) ‘heptanal’ indicates the compound to Problem 12.9 be an aldehyde containing 7 carbon Derive the structure of (i) 2-Chlorohexane, atoms in the parent chain. The (ii) Pent-4-en-2-ol, (iii) 3- Nitrocyclohexene, ‘6-hydroxy’ indicates that -OH group is (iv) Cyclohex-2-en-1-ol, (v) 6-Hydroxy- present at carbon 6. Thus, the structural heptanal. for mula of the compound is: Solution CH3CH(OH)CH2CH2CH2CH2CHO. Carbon atom of –CHO group is included while (i) ‘hexane’ indicates the presence of numbering the carbon chain. 6 carbon atoms in the chain. The functional group chloro is present at carbon 2. Hence, the structure of the 12.5.4 Nomenclature of Substituted compound is CH3CH2CH2CH2CH(Cl)CH3. Benzene Compounds (ii) ‘pent’ indicates that parent For IUPAC nomenclature of substituted hydrocarbon contains 5 carbon atoms in benzene compounds, the substituent is the chain. ‘en’ and ‘ol’ correspond to the functional groups C=C and -OH at placed as prefix to the word benzene as carbon atoms 4 and 2 respectively. Thus, shown in the following examples. However, the structure is common names (written in bracket below) of many substituted benzene compounds CH2=CHCH2CH (OH)CH3. are also universally used. ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 339 Substituent of the base compound is assigned number1 and then the direction of numbering is chosen such that the next substituent gets the lowest number. The substituents appear in the name in Methylbenzene Methoxybenzene Aminobenzene alphabetical order. Some examples are given (Toluene) (Anisole) (Aniline) below. Nitrobenzene Bromobenzene 1-Chloro-2,4-dinitrobenzene (not 4-chloro,1,3-dinitrobenzene) If benzene ring is disubstituted, the position of substituents is defined b y n u m b e r i n g the carbon atoms of the ring such that the substituents are located at the lowest numbers possible.For example, the compound(b) is named as 1,3-dibromobenzene and not as 1,5-dibromobenzene. 2-Chloro-1-methyl-4-nitrobenzene (not 4-methyl-5-chloro-nitrobenzene) (a) (b) (c) 1,2-Dibromo- 1,3-Dibromo- 1,4-Dibromo- benzene benzene benzene In the trivial system of nomenclature the terms ortho (o), meta (m) and para (p) are used 2-Chloro-4-methylanisole 4-Ethyl-2-methylaniline as prefixes to indicate the relative positions 1,2- ;1,3- and 1,4- respectively. Thus, 1,3-dibromobenzene (b) is named as m-dibromobenzene (meta is abbreviated as m-) and the other isomers of dibromobenzene 1,2-(a) and 1,4-(c), are named as ortho (or just o-) and para (or just p-)-dibromobenzene, respectively. For tri - or higher substituted benzene 3,4-Dimethylphenol derivatives, these prefixes cannot be used and the compounds are named by identifying When a benzene ring is attached to an substituent positions on the ring by following alkane with a functional group, it is the lowest locant rule. In some cases, common considered as substituent, instead of a name of benzene derivatives is taken as the parent. The name for benzene as substituent base compound. is phenyl (C6H5-, also abbreviated as Ph). 340 CHEMISTRY different carbon skeletons, these are referred Problem 12.10 to as chain isomers and the phenomenon is Write the structural formula of: termed as chain isomerism. For example, C5H12 (a) o-Ethylanisole, (b) p-Nitroaniline, represents three compounds: (c) 2,3 - Dibromo -1 - phenylpentane, CH3 (d) 4-Ethyl-1-fluoro-2-nitrobenzene. ⏐ Solution CH3CH2CH2CH2CH3 CH3−CHCH2CH3 Pentane Isopentane (2-Methylbutane) CH3 ⏐ CH3⎯ C⎯ CH3 (a) (b) ⏐ CH3 Neopentane (2,2-Dimethylpropane) (ii) Position isomerism: When two or more compounds dif fer in the position of (c) (d) substituent atom or functional group on the carbon skeleton, they are called position 12.6 ISOMERISM isomers and this phenomenon is termed as position isomerism. For example, the The phenomenon of existence of two or more molecular formula C 3H 8O represents two compounds possessing the same molecular alcohols: formula but different properties is known as OH isomerism. Such compounds are called as ⏐ isomers. The following flow chart shows CH3CH2CH2OH CH3−CH-CH3 different types of isomerism. Propan-1-ol Propan-2-ol 12.6.1 Structural Isomerism Compounds having the same molecular (iii) Functional group isomerism: Two or formula but different structures (manners in more compounds having the same molecular which atoms are linked) are classified as formula but different functional groups are structural isomers. Some typical examples of called functional isomers and this different types of structural isomerism are given phenomenon is termed as functional group below: isomerism. For example, the molecular (i) Chain isomerism: When two or more formula C3H6O represents an aldehyde and a compounds have similar molecular formula but ketone: Isomerism Structural isomerism Stereoisomerism Chain Position Functional Metamerism Geometrical Optical isomerism isomerism group isomerism isomerism isomerism ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 341 O H understanding the reactivity of organic   compounds and in planning strategy for their CH3−C-CH3 CH3−CH2—C= O synthesis. Propanone Propanal In the following sections, we shall learn some of the principles that explain how these (iv) Metamerism: It arises due to different alkyl reactions take place. chains on either side of the functional group 12.7.1 Fission of a Covalent Bond in the molecule. For example, C 4 H 10 O represents methoxypropane (CH3OC3H7) and A covalent bond can get cleaved either by : (i) ethoxyethane (C2H5OC2H5). heterolytic cleavage, or by (ii) homolytic cleavage. 12.6.2 Stereoisomerism In heterolytic cleavage, the bond breaks The compounds that have the same in such a fashion that the shared pair of constitution and sequence of covalent bonds electrons remains with one of the fragments. but differ in relative positions of their atoms After heterolysis, one atom has a sextet or groups in space are called stereoisomers. electronic structure and a positive charge and This special type of isomerism is called as the other, a valence octet with at least one stereoisomerism and can be classified as lone pair and a negative charge. Thus, geometrical and optical isomerism. heterolytic cleavage of bromomethane will give  12.7 FUNDAMENTAL CONCEPTS IN CH 3 and Br– as shown below. ORGANIC REACTION MECHANISM In an organic reaction, the organic molecule (also referred as a substrate) reacts with an appropriate attacking reagent and leads to the A species having a carbon atom possessing formation of one or more intermediate(s) and sextext of electrons and a positive charge is finally product(s) called a carbocation (earlier called carbonium  ion). The C H3 ion is known as a methyl cation The general reaction is depicted as follows : or methyl carbonium ion. Carbocations are Attacking classified as primary, secondary or tertiary Reagent [Intermediate] Product(s) depending on whether one, two or three Organic molecule carbons are directly attached to the positively (Substrate) Byproducts charged carbon. Some+ other examples of carbocations are: CH3C H2 (ethyl + cation, a Substrate is that reactant which supplies primary carbocation), (CH3)2C H (isopropyl+ carbon to the new bond and the other reactant cation, a secondary carbocation), and (CH3)3C is called reagent. If both the reactants supply (tert-butyl cation, a tertiary carbocation). carbon to the new bond then choice is Carbocations are highly unstable and reactive arbitrary and in that case the molecule on species. Alkyl groups directly attached to the which attention is focused is called substrate. positively charged carbon stabilise the In such a reaction a covalent bond carbocations due to inductive and between two carbon atoms or a carbon and hyperconjugation effects, which you will be some other atom is broken and a new bond is studying in the sections 12.7.5 and 12.7.9. formed. A sequential account of each step, The + observed+ order of carbocation + stability + is: describing details of electron movement, C H3 < CH3CH2 < (CH3)2CH < (CH3)3C. These energetics during bond cleavage and bond carbocations have trigonal planar shape with formation, and the rates of transformation positively charged carbon +being sp 2 of reactants into products (kinetics) is hybridised. Thus, the shape of C H3 may be referred to as reaction mechanism. The considered as being derived from the overlap knowledge of reaction mechanism helps in of three equivalent C(sp2) hybridised orbitals 342 CHEMISTRY with 1s orbital of each of the three hydrogen Alkyl radicals are classified as primary, atoms. Each bond may be represented as secondary, or tertiary. Alkyl radical stability C(sp 2)–H(1s) sigma bond. The remaining increases as we proceed from primary to carbon orbital is perpendicular to the tertiary: molecular plane and contains no electrons. (Fig. 12.3). , Methyl Ethyl Isopropyl Tert-butyl free free free free radical radical radical radical Organic reactions, which proceed by homolytic fission are called free radical or homopolar or nonpolar reactions. 12.7.2 Nucleophiles and Electrophiles A reagent that brings an electron pair is called Fig. 12.3 Shape of methyl cation a nucleophile (Nu:) i.e., nucleus seeking and the reaction is then called nucleophilic. A The heterolytic cleavage can also give a reagent that takes away an electron pair is species in which carbon gets the shared pair called electrophile (E+) i.e., electron seeking of electrons. For example, when group Z and the reaction is called electrophilic. attached to the carbon leaves without During a polar organic reaction, a nucleophile attacks an electrophilic centre of the substrate which is that specific atom or electron pair, the methyl anion is part of the electrophile that is electron deficient. Similarly, the electrophiles attack at formed. Such a carbon species carrying a nucleophilic centre, which is the electron negative charge on carbon atom is called rich centre of the substrate. Thus, the carbanion. Carbanions are also unstable and electrophiles receive electron pair from reactive species. The organic reactions which nucleophile when the two undergo bonding proceed through heterolytic bond cleavage are interaction. A curved-arrow notation is used called ionic or heteropolar or just polar to show the movement of an electron pair from reactions. the nucleophile to the electrophile. Some In homolytic cleavage, one of the examples of nucleophiles are the negatively electrons of the shared pair in a covalent bond charged ions with lone pair of electrons such – – goes with each of the bonded atoms. Thus, in as hydroxide (HO ), cyanide (NC ) ions and – homolytic cleavage, the movement of a single carbanions (R3C: ). Neutral molecules such electron takes place instead of an electron as etc., can also act as pair. The single electron movement is shown nucleophiles due to the presence of lone pair by ‘half-headed’ (fish hook: ) curved arrow. of electrons. Examples of electrophiles Such cleavage results in the formation of  neutral species (atom or group) which include carbocations ( C H 3 ) and neutral contains an unpaired electron. These species molecules having functional groups like are called free radicals. Like carbocations carbonyl group (>C=O) or alkyl halides and carbanions, free radicals are also (R 3C-X, where X is a halogen atom). The very reactive. A homolytic cleavage can be carbon atom in carbocations has sextet shown as: configuration; hence, it is electron deficient and can receive a pair of electrons from the nucleophiles. In neutral molecules such as Alkyl alkyl halides, due to the polarity of the C-X free radical bond a partial positive charge is generated ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 343 on the carbon atom and hence the carbon 12.7.3 Electron Movement in Organic atom becomes an electrophilic centre at Reactions which a nucleophile can attack. The movement of electrons in organic reactions can be shown by curved-arrow Problem 12.11 notation. It shows how changes in bonding Using curved-arrow notation, show the occur due to electronic redistribution during formation of reactive intermediates when the reaction. To show the change in position the following covalent bonds undergo of a pair of electrons, curved arrow starts from heterolytic cleavage. the point from where an electron pair is shifted (a) CH3–SCH3, (b) CH3–CN, (c) CH3–Cu and it ends at a location to which the pair of electron may move. Solution Presentation of shifting of electron pair is given below : (i) from π bond to adjacent bond position (ii) from π bond to adjacent atom Problem 12.12 (iii) from atom to adjacent Giving justification, categorise the bond position following molecules/ions as nucleophile or electrophile: Movement of single electron is indicated by a single barbed ‘fish hooks’ (i.e. half headed curved arrow). For example, in transfer of hydroxide ion giving ethanol and in the dissociation of chloromethane, the movement Solution of electron using curved arrows can be depicted as follows: Nucleophiles: HS ,C2H5O , CH3 3 N:,H2N:    These species have unshared pair of electrons, which can be donated and shared with an electrophile.    E l e c t r o p h i l e s : BF3,Cl,CH3 C  O,NO2. Reactive sites have only six valence 12.7.4 Electron Displacement Effects in electrons; can accept electron pair from Covalent Bonds a nucleophile. The electron displacement in an organic molecule may take place either in the ground Problem 12.13 state under the influence of an atom or a Identify electrophilic centre in the substituent group or in the presence of an following: CH3CH=O, CH3CN, CH3I. appropriate attacking reagent. The electron Solution displacements due to the influence of * * an atom or a substituent group present in Among CH 3 HC =O, H 3 C C ≡N, and * the molecule cause permanent polarlisation H 3C –I, the starred carbon atoms are electrophilic centers as they will have of the bond. Inductive ef fect and resonance effects are examples of this type of partial positive charge due to polarity of electron displacements. Temporary electron the bond. displacement effects are seen in a molecule 344 CHEMISTRY when a reagent approaches to attack it. This nitro (- NO2), cyano (- CN), carboxy (- COOH), type of electron displacement is called ester (-COOR), aryloxy (-OAr, e.g. – OC6H5), electromeric effect or polarisability effect. In etc. are electron-withdrawing groups. On the the following sections we will learn about these other hand, the alkyl groups like methyl types of electronic displacements. (–CH 3) and ethyl (–CH 2–CH 3) are usually considered as electron donating groups. 12.7.5 Inductive Effect When a covalent bond is formed between Problem 12.14 atoms of different electronegativity, the Which bond is more polar in the following electron density is more towards the more pairs of molecules: (a) H3C-H, H3C-Br electronegative atom of the bond. Such a shift (b) H 3 C-NH 2 , H 3 C-OH (c) H 3 C-OH, of electron density results in a polar covalent H3C-SH bond. Bond polarity leads to various electronic effects in organic compounds. Solution Let us consider cholorethane (CH3CH2Cl) (a) C–Br, since Br is more electronegative in which the C–Cl bond is a polar covalent than H, (b) C–O, (c) C–O bond. It is polarised in such a way that the Problem 12.15 + carbon-1 gains some positive charge (δ ) and – In which C–C bond of CH3CH2CH2Br, the the chlorine some negative charge (δ ). The fractional electronic charges on the two atoms inductive effect is expected to be the in a polar covalent bond are denoted by least? symbol δ (delta) and the shift of electron Solution density is shown by an arrow that points from + – Magnitude of inductive effect diminishes δ to δ end of the polar bond. + + − as the number of intervening bonds δδ δ δ increases. Hence, the effect is least in the CH3 ⎯→⎯CH2⎯→⎯ ⎯→⎯Cl bond between carbon-3 and hydrogen. 2 1 In turn carbon-1, which has developed 12.7.6 Resonance Structure + partial positive charge (δ ) draws some There are many organic molecules whose electron density towards it from the adjacent behaviour cannot be explained by a single C-C bond. Consequently, some positive charge + + Lewis structure. An example is that of (δδ ) develops on carbon-2 also, where δδ benzene. Its cyclic structure symbolises relatively smaller positive charge containing alternating C–C single as compared to that on carbon – 1. In other words, the polar C – Cl bond induces polarity and C=C double bonds shown is in the adjacent bonds. Such polarisation of inadequate for explaining its Benzene σ-bond caused by the polarisation of adjacent characteristic properties. σ-bond is referred to as the inductive effect. As per the above representation, benzene This effect is passed on to the subsequent should exhibit two different bond lengths, due bonds also but the effect decreases rapidly to C–C single and C=C double bonds. However, as the number of intervening bonds increases as determined experimentally benzene has a and becomes vanishingly small after three uniform C–C bond distances of 139 pm, a bonds. The inductive effect is related to the value inter mediate between the C–C ability of substituent(s) to either withdraw or single(154 pm) and C=C double (134 pm) donate electron density to the attached carbon bonds. Thus, the structure of benzene cannot atom. Based on this ability, the substitutents be represented adequately by the above can be classified as electron-withdrawing or structure. Further, benzene can be electron donating groups relative to hydrogen. represented equally well by the energetically Halogens and many other groups such as identical structures I and II. ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 345 unpaired electrons. Among the resonance structures, the one which has more number of covalent bonds, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any Therefore, according to the resonance theory on more electropositive atom) and more (Unit 4) the actual structure of benzene dispersal of charge, is more stable than others. cannot be adequately represented by any of these structures, rather it is a hybrid of the Problem 12.16 two structures (I and II) called resonance – Write resonance structures of CH3COO structures. The resonance structures and show the movement of electrons by (canonical structures or contributing curved arrows. structures) are hypothetical and Solution individually do not represent any real molecule. They contribute to the actual First, write the structure and put structure in proportion to their stability. unshared pairs of valence electrons on Another example of resonance is provided appropriate atoms. Then draw the arrows by nitromethane (CH 3NO 2) which can be one at a time moving the electrons to get represented by two Lewis structures, (I and the other structures. II). There are two types of N-O bonds in these structures. Problem 12.17 Write resonance structures of CH2=CH–CHO. Indicate relative stability of However, it is known that the two N–O the contributing structures. bonds of nitromethane are of the same length (intermediate between a N–O single Solution bond and a N=O double bond). The actual structure of nitromethane is therefore a resonance hybrid of the two canonical forms I and II. The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures. The difference in energy between the actual structure and the lowest energy resonance structure is called the resonance Stability: I > II > III stabilisation energy or simply the [I: Most stable, more number of covalent resonance energy. The more the number of bonds, each carbon and oxygen atom has important contributing structures, the more an octet and no separation of opposite is the resonance energy. Resonance is charge II: negative charge on more particularly important when the contributing electronegative atom and positive charge structures are equivalent in energy. on more electropositive atom; III: does The following rules are applied while writing not contribute as oxygen has positive resonance structures: charge and carbon has negative charge, The resonance structures have (i) the same hence least stable]. positions of nuclei and (ii) the same number of 346 CHEMISTRY The atoms or substituent groups, which Problem 12.18 represent +R or –R electron displacement Explain why the following two structures, effects are as follows : I and II cannot be the major contributors to the real structure of CH3COOCH3. +R effect: – halogen, –OH, –OR, –OCOR, –NH2, –NHR, –NR2, –NHCOR, – R effect: – COOH, –CHO, >C=O, – CN, –NO2 The presence of alternate single and double bonds in an open chain or cyclic Solution system is termed as a conjugated system. The two structures are less important These systems often show abnor mal contributors as they involve charge behaviour. The examples are 1,3- butadiene, separation. Additionally, structure I aniline and nitrobenzene etc. In such systems, contains a carbon atom with an the π-electrons are delocalised and the system develops polarity. incomplete octet. 12.7.8 Electromeric Effect (E effect) 12.7.7 Resonance Effect It is a temporary ef fect. The organic The resonance effect is defined as ‘the polarity compounds having a multiple bond (a double produced in the molecule by the interaction or triple bond) show this effect in the presence of two π-bonds or between a π-bond and lone of an attacking reagent only. It is defined as pair of electrons present on an adjacent atom’. the complete transfer of a shared pair of The effect is transmitted through the chain. π-electrons to one of the atoms joined by a There are two types of resonance or multiple bond on the demand of an attacking mesomeric effect designated as R or M effect. reagent. The effect is annulled as soon as the (i) Positive Resonance Effect (+R effect) attacking reagent is removed from the domain of the reaction. It is represented by E and the In this effect, the transfer of electrons is away shifting of the electrons is shown by a curved from an atom or substituent group attached to the conjugated system. This electron arrow ( ). There are two distinct types of displacement makes certain positions in the electromeric effect. molecule of high electron densities. This effect (i) Positive Eelctromeric Effect (+E effect) In in aniline is shown as : this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example : (ii) Negative Resonance Effect (- R effect) This effect is observed when the transfer of (ii) Negative Electromeric Effect (–E effect) In electrons is towards the atom or substituent this effect the π - electrons of the multiple group attached to the conjugated system. For bond are transferred to that atom to which example in nitrobenzene this electron the attacking reagent does not get attached. displacement can be depicted as : For example: ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 347 When inductive and electromeric effects In general, greater the number of alkyl operate in opposite directions, the electomeric groups attached to a positively charged carbon effect predominates. atom, the greater is the hyperconjugation interaction and stabilisation of the cation. 12.7.9 Hyperconjugation Thus, we have the following relative stability Hyperconjugation is a general stabilising of carbocations : interaction. It involves delocalisation of σ electrons of C—H bond of an alkyl group directly attached to an atom of unsaturated system or to an atom with an unshared p orbital. The σ electrons of C—H bond of the alkyl group enter into partial conjugation with Hyperconjugation is also possible in the attached unsaturated system or with the alkenes and alkylarenes. unshared p orbital. Hyperconjugation is a Delocalisation of electrons by permanent effect. hyperconjugation in the case of alkene can To understand hyperconjugation effect, let be depicted as in Fig. 12.4(b).  us take an example of CH3 CH2 (ethyl cation) in which the positively charged carbon atom has an empty p orbital. One of the C-H bonds of the methyl group can align in the plane of this empty p orbital and the electrons constituting the C-H bond in plane with this p orbital can then be delocalised into the empty p orbital as depicted in Fig. 12.4 (a). Fig. 12.4(b) Orbital diagram showing hyperconjugation in propene There are various ways of looking at the hyperconjugative effect. One of the way is to regard C—H bond as possessing partial ionic character due to resonance. Fig. 12.4(a) Orbital diagram showing hyperconjugation in ethyl cation This type of overlap stabilises the carbocation because electron density from the adjacent σ bond helps in dispersing the positive charge. 348 CHEMISTRY The hyperconjugation may also be New methods of checking the purity of an regarded as no bond resonance. organic compound are based on different types of chromatographic and spectroscopic Problem 12.19 + techniques. Explain why (CH3)3C is more stable than + + 12.8.1 Sublimation CH3CH2 and C H3 is the least stable cation. You have learnt earlier that on heating, some solid substances change from solid to vapour Solution + state without passing through liquid state. Hyperconjugation interaction in (CH3)3C The purification technique based on the above + is greater than in CH C H as the principle is known as sublimation and is used + 3 2 + (CH3)3C has nine C-H bonds. In C H3 , to separate sublimable compounds from non- sublimable impurities. vacant p orbital is perpendicular to the plane in which C-H bonds lie; hence 12.8.2 Crystallisation + cannot overlap with it. Thus, C H lacks This is one of the most commonly used 3 hyperconjugative stability. techniques for the purification of solid organic compounds. It is based on the difference in 12.7.10 Types of Organic Reactions and the solubilities of the compound and the Mechanisms impurities in a suitable solvent. The impure Organic reactions can be classified into the compound is dissolved in a solvent in which following categories: it is sparingly soluble at room temperature (i) Substitution reactions but appreciably soluble at higher temperature. The solution is concentrated to (ii) Addition reactions get a nearly saturated solution. On cooling (iii) Elimination reactions the solution, pure compound crystallises out (iv) Rearrangement reactions and is removed by filtration. The filtrate You will be studying these reactions in (mother liquor) contains impurities and small Unit 13 and later in class XII. quantity of the compound. If the compound is highly soluble in one solvent and very little 12.8 METHODS OF PURIFICATION OF soluble in another solvent, crystallisation can ORGANIC COMPOUNDS be satisfactorily carried out in a mixture of Once an organic compound is extracted from these solvents. Impurities, which impart a natural source or synthesised in the colour to the solution are removed by laboratory, it is essential to purify it. Various adsorbing over activated charcoal. Repeated methods used for the purification of organic crystallisation becomes necessary for the compounds are based on the nature of the purification of compounds containing compound and the impurity present in it. impurities of comparable solubilities. The common techniques used for 12.8.3 Distillation purification are as follows : This important method is used to separate (i) (i) Sublimation volatile liquids from nonvolatile impurities and (ii) Crystallisation (ii) the liquids having sufficient difference in (iii) Distillation their boiling points. Liquids having different (iv) Differential extraction and boiling points vaporise at dif ferent (v) Chromatography temperatures. The vapours are cooled and the Finally, the purity of a compound is liquids so formed are collected separately. ascertained by determining its melting or Chloroform (b.p 334 K) and aniline (b.p. 457 boiling point. Most of the pure compounds K) are easily separated by the technique of have sharp melting points and boiling points. distillation (Fig 12.5). The liquid mixture is ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 349 taken in a round bottom flask and heated carefully. On boiling, the vapours of lower boiling component are formed first. The vapours are condensed by using a condenser and the liquid is collected in a receiver. The vapours of higher boiling component for m later and the liquid can be collected separately. Fractional Distillation: If the difference in boiling points of two liquids is not much, simple distillation cannot be used to separate them. The vapours of such liquids are formed within the same temperature range and are condensed simultaneously. The technique of fractional distillation is used in such cases. In this technique, vapours of a liquid mixture are passed through a fractionating column before Fig.12.5 Simple distillation. The vapours of a substance condensation. The fractionating formed are condensed and the liquid is collected column is fitted over the mouth of the in conical flask. round bottom flask (Fig.12.6). Fig.12.6 Fractional distillation. The vapours of lower boiling fraction reach the top of the column first followed by vapours of higher boiling fractions. 350 CHEMISTRY Vapours of the liquid with higher boiling theoretical plate. Commercially, columns point condense before the vapours of the with hundreds of plates are available. liquid with lower boiling point. The vapours One of the technological applications of rising up in the fractionating column become fractional distillation is to separate different richer in more volatile component. By the time fractions of crude oil in petroleum industry. the vapours reach to the top of the Distillation under reduced pressure: This fractionating column, these are rich in the method is used to purify liquids having very more volatile component. Fractionating high boiling points and those, which columns are available in various sizes and decompose at or below their boiling points. designs as shown in Fig.12.7. A fractionating Such liquids are made to boil at a temperature column provides many surfaces for heat lower than their normal boiling points by exchange between the ascending vapours reducing the pressure on their surface. A and the descending condensed liquid. Some liquid boils at a temperature at which its of the condensing liquid in the fractionating vapour pressure is equal to the external column obtains heat from the ascending pressure. The pressure is reduced with the vapours and revaporises. The vapours thus help of a water pump or vacuum pump become richer in low boiling component. The (Fig.12.8). Glycerol can be separated from vapours of low boiling component ascend to spent-lye in soap industry by using this the top of the column. On reaching the top, technique. the vapours become pure in low boiling component and pass through the condenser Steam Distillation: This technique is and the pure liquid is collected in a receiver. applied to separate substances which are After a series of successive distillations, the steam volatile and are immiscible with remaining liquid in the distillation flask gets water. In steam distillation, steam from a enriched in high boiling component. Each steam generator is passed through a heated successive condensation and vaporisation flask containing the liquid to be distilled. unit in the fractionating column is called a The mixture of steam and the volatile organic compound is condensed and collected. The compound is later separated from water using a separating funnel. In steam distillation, the liquid boils when the sum of vapour pressures due to the organic liquid (p 1 ) and that due to water (p 2 ) becomes equal to the atmospheric pressure (p), i.e. p =p 1 + p 2. Since p 1 is lower than p, the organic liquid vaporises at lower temperature than its boiling point. Thus, if one of the substances in the mixture is water and the other, a water insoluble substance, then the mixture will boil close to but below, 373K. A mixture of water and the substance is obtained which can be separated by using a separating funnel. Aniline is separated by this technique from aniline – water mixture (Fig.12.9). 12.8.4 Differential Extraction When an organic compound is present in an Fig.12.7 Different types of fractionating columns. aqueous medium, it is separated by shaking ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 351 Fig.12.8 Distillation under reduced pressure. A liquid boils at a temperature below its vapour pressure by reducing the pressure. Fig.12.9 Steam distillation. Steam volatile component volatilizes, the vapours condense in the condenser and the liquid collects in conical flask. 352 CHEMISTRY it with an organic solvent in which it is more mixture get gradually separated from one soluble than in water. The organic solvent and another. The moving phase is called the mobile the aqueous solution should be immiscible phase. with each other so that they form two distinct Based on the principle involved, layers which can be separated by separatory chromatography is classified into different funnel. The organic solvent is later removed categories. Two of these are: by distillation or by evaporation to get back (a) Adsorption chromatography, and the compound. Differential extraction is (b) Partition chromatography. carried out in a separatory funnel as shown in Fig. 12.10. If the organic compound is less a) Adsorption Chr omatography: Adsor- ption chromatography is based on the fact that different compounds are adsorbed on an adsorbent to different degrees. Commonly used adsorbents are silica gel and alumina. When a mobile phase is allowed to move over a stationary phase (adsorbent), the components of the mixture move by varying distances over the stationary phase. Following are two main types of chromatographic techniques based on the principle of differential adsorption. (a) Column chromatography, and (b) Thin layer chromatography. Column Chromatography: Column chromatography involves separation of a Fig.12.10 Differential extraction. Extraction of com- mixture over a column of adsorbent pound takes place based on difference (stationary phase) packed in a glass tube. The in solubility column is fitted with a stopcock at its lower soluble in the organic solvent, a very large end (Fig. 12.11). The mixture adsorbed on quantity of solvent would be required to extract even a very small quantity of the compound. The technique of continuous extraction is employed in such cases. In this technique same solvent is repeatedly used for extraction of the compound. 12.8.5 Chromatography Chromatography is an important technique extensively used to separate mixtures into their components, purify compounds and also to test the purity of compounds. The name chromatography is based on the Greek word chroma, for colour since the method was first used for the separation of coloured substances found in plants. In this technique, the mixture of substances is applied onto a stationary phase, which may be a solid or a liquid. A pure solvent, a mixture of solvents, Fig.12.11 Column chromatography. Different or a gas is allowed to move slowly over the stages of separation of components stationary phase. The components of the of a mixture. ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 353 adsorbent is placed on the top of the adsorbent eluant rises up the plate, the components of column packed in a glass tube. An appropriate the mixture move up along with the eluant to eluant which is a liquid or a mixture of liquids different distances depending on their degree is allowed to flow down the column slowly. of adsorption and separation takes place. The Depending upon the degree to which the relative adsorption of each component of the compounds are adsorbed, complete separation mixture is expressed in ter ms of its takes place. The most readily adsorbed retardation factor i.e. Rf value (Fig.12.12 b). substances are retained near the top and others Distance moved by the substance from base line (x) come down to various distances in the column Rf = Distance moved by the solvent from base line (y) (Fig.12.11). The spots of coloured compounds are visible Thin Layer Chromatography: Thin layer on TLC plate due to their original colour. The chromatography (TLC) is another type of spots of colourless compounds, which are adsorption chromatography, which involves invisible to the eye but fluoresce in ultraviolet separation of substances of a mixture over a light, can be detected by putting the plate under thin layer of an adsorbent coated on glass ultraviolet light. Another detection technique is plate. A thin layer (about 0.2mm thick) of an to place the plate in a covered jar containing a adsorbent (silica gel or alumina) is spread over few crystals of iodine. Spots of compounds, a glass plate of suitable size. The plate is which adsorb iodine, will show up as brown known as thin layer chromatography plate or spots. Sometimes an appropriate reagent may chromaplate. The solution of the mixture to also be sprayed on the plate. For example, amino be separated is applied as a small spot about acids may be detected by spraying the plate with 2 cm above one end of the TLC plate. The ninhydrin solution (Fig.12.12b). glass plate is then placed in a closed jar containing the eluant (Fig. 12.12a). As the Partition Chromatography: Partition chromatography is based on continuous differential partitioning of components of a mixture between stationary and mobile phases. Paper chromatography is a type of partition chromatography. In paper chromatography, a special quality paper known as chromatography paper is used. Chromatography paper contains water trapped in it, which acts as the stationary phase. A strip of chromatography paper spotted Fig.12.12 (a) Thin layer chromatography. at the base with the solution of the mixture is Chromatogram being developed. suspended in a suitable solvent or a mixture of solvents (Fig. 12.13). This solvent acts as the mobile phase. The solvent rises up the paper by capillary action and f