Organic Chemistry Past Paper PDF

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

334 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 compounds 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. structur e 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 or ganic 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. 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 335 The development of electronic theory of Thus, in H2C=CH2 molecule all the atoms must covalent bonding ushered organic chemistry be in the same plane. The p orbitals are into its modern shape. mutually parallel and both the p orbitals are perpendicular to the plane of the molecule. 12.2 TETRAVALENCE OF CARBON: Rotation of one CH2 fragment with respect to SHAPES OF ORGANIC COMPOUNDS other interferes with maximum overlap of p 12.2.1 The Shapes of Carbon Compounds orbitals and, therefore, such rotation about The knowledge of fundamental concepts of carbon-carbon double bond (C=C) is molecular structure helps in understanding restricted. The electron charge cloud of the π and predicting the properties of organic bond is located above and below the plane of compounds. You have already learnt theories bonding atoms. This results in the electrons of valency and molecular structure in Unit 4. being easily available to the attacking Also, you already know that tetravalence of reagents. In general, π bonds provide the most carbon and the formation of covalent bonds reactive centres in the molecules containing by it are explained in terms of its electronic multiple bonds. configuration and the hybridisation of s and p orbitals. It may be recalled that formation Problem 12.1 and the shapes of molecules like methane (CH 4 ), ethene (C 2 H 4), ethyne (C 2H 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. Solution Hybridisation influences the bond length and bond enthalpy (strength) in compounds. (a) σC – C: 4; σC–H : 6; πC=C :1; π C≡C:2 The sp hybrid orbital contains more s (b) σC – C: 3; σC–H: 6; πC=C: 2. character and hence it is closer to its nucleus and forms shorter and stronger bonds than Problem 12.2 the sp3 hybrid orbital. The sp2 hybrid orbital What is the type of hybridisation of each is intermediate in s character between sp and carbon in the following compounds? sp3 and, hence, the length and enthalpy of the bonds it forms, are also intermediate between (a) CH3Cl, (b) (CH3)2CO, (c) CH3CN, them. The change in hybridisation affects the (d) HCONH2, (e) CH3CH=CHCN electronegativity of carbon. The greater the s Solution character of the hybrid orbitals, the greater is the electronegativity. Thus, a carbon atom (a) sp3, (b) sp3, sp2, (c) sp3, sp, (d) sp2, (e) having an sp hybrid orbital with 50% s sp3, sp2, sp2, sp character is more electronegative than that Problem 12.3 possessing sp2 or sp3 hybridised orbitals. This relative electronegativity is reflected in several Write the state of hybridisation of carbon physical and chemical properties of the in the following compounds and shapes molecules concerned, about which you will of each of the molecules. learn in later units. (a) H2C=O, (b) CH3F, (c) HC≡N. 12.2.2 Some Characteristic Features of π Solution Bonds (a) sp2 hybridised carbon, trigonal planar; In a π (pi) bond formation, parallel orientation (b) sp3 hybridised carbon, tetrahedral; (c) of the two p orbitals on adjacent atoms is sp hybridised carbon, linear. necessary for a proper sideways overlap. 2022-23 336 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 formulas. 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 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 337 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 2022-23 338 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 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 339 12.4 CLASSIFICATION OF ORGANIC (homocyclic). COMPOUNDS The existing large number of organic compounds and their ever -incr easing numbers has made it necessary to classify them on the basis of their structures. Organic Cyclopropane Cyclohexane Cyclohexene compounds are broadly classified as follows: Sometimes atoms other than carbon are also present in the ring (heterocylic). Tetrahydrofuran given below is an example of this type of compound: Tetrahydrofuran These exhibit some of the properties similar to those of aliphatic compounds. (b) 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 I. Acyclic or open chain compounds atom in the ring. Such compounds are called hetrocyclic aromatic compounds. Some of the These compounds are also called as aliphatic examples of various types of aromatic compounds and consist of straight or compounds are: branched chain compounds, for example: Benzenoid aromatic compounds CH3CH3 Ethane Isobutane Benzene Aniline Naphthalene Non-benzenoid compound Acetaldehyde Acetic acid II Cyclic or closed chain or ring compounds (a) Alicyclic compounds Alicyclic (aliphatic cyclic) compounds contain Tropone carbon atoms joined in the form of a ring 2022-23 340 CHEMISTRY Heterocyclic aromatic compounds so because it is found in citrus fruits and the 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 Furan Thiophene Pyridine names are followed even today. For example, Organic compounds can also be classified Buckminsterfullerene is a common name given on the basis of functional groups, into families to the newly discovered C60 cluster (a form of or homologous series. carbon) noting its structural similarity to the geodesic domes popularised by the famous 12.4.1 Functional Group architect R. Buckminster Fuller. Common The functional group is an atom or a group of names are useful and in many cases atoms joined to the carbon chain which is indispensable, particularly when the responsible for the characteristic chemical alternative systematic names are lengthy and properties of the organic compounds. The complicated. Common names of some organic examples are hydroxyl group (–OH), aldehyde compounds are given in Table 12.1. group (–CHO) and carboxylic acid group Table 12.1 Common or Trivial Names of Some (–COOH) etc. Organic Compounds 12.4.2 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. It is also possible that a compound contains two or more identical or different functional groups. This gives rise to polyfunctional compounds. 12.5 NOMENCLATURE OF ORGANIC COMPOUNDS Organic chemistry deals with millions of 12.5.1 The IUPAC System of Nomenclature compounds. In order to clearly identify them, a A systematic name of an organic compound is systematic method of naming has been generally derived by identifying the parent developed and is known as the IUPAC hydrocarbon and the functional group(s) (International Union of Pure and Applied attached to it. See the example given below. Chemistry) system of nomenclature. In this systematic nomenclature, the names are 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 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 341 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 Abbreviations are used for some alkyl given in Table 12.2. The alkanes in Table 12.2 groups. For example, methyl is abbreviated as differ from each other by merely the number Me, ethyl as Et, propyl as Pr and butyl as Bu. of -CH 2 groups in the chain. They are The alkyl groups can be branched also. Thus, homologues of alkane series. propyl and butyl groups can have branched Table 12.2 IUPAC Names of Some Unbranched 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- Branched chain hydrocarbons: In a Common branched groups have specific trivial branched chain compound small chains of names. For example, the propyl groups can carbon atoms are attached at one or more either be n-propyl group or isopropyl group. carbon atoms of the parent chain. The small The branched butyl groups are called sec- carbon chains (branches) are called alkyl butyl, isobutyl and tert-butyl group. We 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. 2022-23 342 CHEMISTRY 1. First of all, the longest carbon chain in separated from the groups by hyphens the molecule is identified. In the example and there is no break between methyl (I) given below, the longest chain has nine and 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 H3C H2C 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 procedures. However, the carbon atom of as a branch are then prefixed to the the branch that attaches to the root name of the parent alkane and position alkane is numbered 1 as exemplified of the 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- 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 343 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 chains. 3-Ethyl-1,1-dimethylcyclohexane After selection of the chain, numbering is (not 1-ethyl-3,3-dimethylcyclohexane) to be done from the end closer to the substituent. Problem 12.7 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 2022-23 344 CHEMISTRY chemical reactivity in an organic molecule. suffix. In such cases the full name of the Compounds having the same functional group parent alkane is written before the class suffix. undergo similar reactions. For example, For example CH2(OH)CH2(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 of functional groups enables systematisation or triple bond; for example, CH2=CH-CH=CH2 of organic compounds into different classes. is named as buta–1,3–diene. Examples of some functional groups with their prefixes and suffixes 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 as hence suffix becomes ‘dione’. The two 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 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 345 Table 12.4 Some Functional Groups and Classes of Organic Compounds 2022-23 346 CHEMISTRY Solution (iii) Six membered ring containing a Here, two functional groups namely carbon-carbon double bond is implied by cyclohexene, which is numbered as ketone and carboxylic acid are present. The principal functional group is the shown in (I). The prefix 3-nitro means that a nitro group is present on C-3. Thus, carboxylic acid group; hence the parent chain will be suffixed with ‘oic’ acid. complete structural formula of the compound is (II). Double bond is suffixed Numbering of the chain starts from carbon of – COOH functional group. The functional group whereas NO2 is prefixed functional group therefore double bond keto group in the chain at carbon 5 is 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. formula 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 placed as prefix to the word benzene as functional groups C=C and -OH at carbon atoms 4 and 2 respectively. Thus, the shown in the following examples. However, structure is common names (written in bracket below) of many substituted benzene compounds CH2=CHCH2CH (OH)CH3. are also universally used. 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 347 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 parent. name of benzene derivatives is taken as the The name for benzene as substituent is phenyl base compound. (C6H5-, also abbreviated as Ph). 2022-23 348 CHEMISTRY different carbon skeletons, these are referred to Problem 12.10 as chain isomers and the phenomenon is termed Write the structural formula of: as chain isomerism. For example, C 5H 12 (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 differ in the position of substituent (c) (d) atom or functional group on the carbon skeleton, they are called position isomers and 12.6 ISOMERISM this phenomenon is termed as position isomerism. For example, the molecular The phenomenon of existence of two or more formula C3H8O represents two alcohols: compounds possessing the same molecular 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 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 349 O H understanding the reactivity of organic y x 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 10O 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. +The formed. A sequential account of each step, observed+ order of carbocation + stability + is: C H3 describing details of electron movement, < CH 3 CH 2 < (CH 3 ) 2CH < (CH 3) 3 C. 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 2022-23 350 CHEMISTRY with 1s orbital of each of the three hydrogen headed’ (fish hook: ) curved arrow. Such atoms. Each bond may be represented as cleavage results in the formation of neutral C(sp 2)–H(1s) sigma bond. The remaining species (atom or group) which contains an carbon orbital is perpendicular to the unpaired electron. These species are called free molecular plane and contains no electrons. radicals. Like carbocations and carbanions, [Fig. 12.3(a)]. free radicals are also very reactive. A homolytic cleavage can be shown as: Alkyl free radical Alkyl radicals are classified as primary, secondary, or tertiary. Alkyl radical stability increases as we proceed from primary to tertiary: Fig. 12.3 (a) Shape of methyl carbocation The heterolytic cleavage can also give a species , in which carbon gets the shared pair of Methyl Ethyl Isopropyl Tert-butyl electrons. For example, when group Z free free free free radical radical radical radical attached to the carbon leaves without Organic reactions, which proceed by homolytic fission are called free radical or homopolar or nonpolar reactions. electron pair, the methyl anion is 12.7.2 Substrate and Reagent formed. Such a carbon species carrying a Ions are generally not formed in the reactions of negative charge on carbon atom is called organic compounds. Molecules as such carbanion. Carbon in carbanion is generally participate in the reaction. It is convenient to sp3 hybridised and its structure is distorted name one reagent as substrate and other as tetrahedron as shown in Fig. 12.3(b). reagent. In general, a molecule whose carbon is involved in new bond formation is called substrate and the other one is called reagent. When carbon-carbon bond is formed, the choice of naming the reactants as substrate and reagent is arbitrary and depends on molecule under observation. Example: (i) CH2 = CH2 + Br2 CH2 Br – CH2Br Substrate Reagent Product Fig. 12.3 (b) Shape of methyl carbanion Carbanions are also unstable and reactive species. The organic reactions which proceed (ii) through heterolytic bond cleavage are called ionic or heteropolar or just polar reactions. In homolytic cleavage, one of the electrons of the shared pair in a covalent bond goes with each of the bonded atoms. Thus, in homolytic cleavage, the movement of a single electron Nucleophiles and Electrophiles takes place instead of an electron pair. The Reagents attack the reactive site of the substrate. single electron movement is shown by ‘half- The reactive site may be electron deficient 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 351 portion of the molecule (a positive reactive site) Problem 12.11 e.g., an atom with incomplete electron shell or the positive end of the dipole in the molecule. If Using curved-arrow notation, show the formation of reactive intermediates when the attacking species is electron rich, it attacks the following covalent bonds undergo these sites. If attacking species is electron heterolytic cleavage. deficient, the reactive site for it is that part of the substrate molecule which can supply electrons, (a) CH3–SCH3, (b) CH3–CN, (c) CH3–Cu e.g., π electrons in a double bond. Solution A reagent that brings an electron pair to the reactive site is called a nucleophile (Nu:) i.e., nucleus seeking and the reaction is then called nucleophilic. A reagent that takes away an electron pair from reactive site is called electrophile (E+) i.e., electron seeking and the reaction is called electrophilic. Problem 12.12 During a polar organic reaction, a Giving justification, categorise the nucleophile attacks an electrophilic centre of following molecules/ions as nucleophile the substrate which is that specific atom or part or electrophile: of the substrate which is electron deficient. Similarly, the electrophiles attack at nucleophilic centre, which is the electron rich centre of the substrate. Thus, the electrophiles receive electron pair from the Solution Nucleophiles: HS ,C2H5O ,( CH3 )3 N:,H2N: substrate when the two undergo bonding − − − interaction. A curved-arrow notation is used These species have unshared pair of to show the movement of an electron pair from electrons, which can be donated and the nucleophile to the electrophile. Some shared with an electrophile. examples of nucleophiles are the negatively + + + charged ions with lone pair of electrons such E l e c t r o p h i l e s : BF3 ,Cl,CH3 −C = O,NO2. – – as hydroxide (HO ), cyanide (NC ) ions and – Reactive sites have only six valence carbanions (R3C: ). Neutral molecules such electrons; can accept electron pair from as etc., can also act as a nucleophile. nucleophiles due to the presence of lone pair of electrons. Examples of electrophiles Problem 12.13 + include carbocations ( C H ) and neutral Identify electrophilic centre in the 3 molecules having functional groups like following: CH3CH=O, CH3CN, CH3I. carbonyl group (>C=O) or alkyl halides Solution (R 3C-X, where X is a halogen atom). The * * Among CH 3 HC =O, H 3 C C ≡N, and carbon atom in carbocations has sextet * H3C –I, the starred carbon atoms are configuration; hence, it is electron deficient and can receive a pair of electrons from the electrophilic centers as they will have nucleophiles. In neutral molecules such as partial positive charge due to polarity of alkyl halides, due to the polarity of the C-X the bond. bond a partial positive charge is generated 12.7.3 Electron Movement in Organic on the carbon atom and hence the carbon atom Reactions becomes an electrophilic centre at which a nucleophile can attack. The movement of electrons in organic reactions can be shown by curved-arrow notation. It 2022-23 352 CHEMISTRY shows how changes in bonding occur due to 12.7.5 Inductive Effect electronic redistribution during the reaction. When a covalent bond is formed between atoms To show the change in position of a pair of of different electronegativity, the electron electrons, curved arrow starts from the point density is more towards the more from where an electron pair is shifted and it electronegative atom of the bond. Such a shift ends at a location to which the pair of electron of electron density results in a polar covalent may move. bond. Bond polarity leads to various electronic Presentation of shifting of electron pair is effects in organic compounds. given below : Let us consider cholorethane (CH3CH2Cl) in which the C–Cl bond is a polar covalent (i) from π bond to bond. It is polarised in such a way that the adjacent bond position + carbon-1 gains some positive charge (δ ) and – (ii) from π bond to the chlorine some negative charge (δ ). The adjacent atom fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol (iii) from atom to adjacent δ (delta) and the shift of electron density is + – bond position shown by an arrow that points from δ to δ end of the polar bond. Movement of single electron is indicated by + + − δδ δ δ a single barbed ‘fish hooks’ (i.e. half headed CH3 →CH2→ →Cl curved arrow). For example, in transfer of 2 1 hydroxide ion giving ethanol and in the dissociation of chloromethane, the movement In turn carbon-1, which has developed + of electron using curved arrows can be partial positive charge (δ ) draws some electron depicted as follows: density towards it from the adjacent C-C bond. + Consequently, some positive charge (δδ ) + develops on carbon-2 also, where δδ symbolises relatively smaller positive charge as compared to that on carbon – 1. In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarisation of σ- 12.7.4 Electron Displacement Effects in bond caused by the polarisation of adjacent Covalent Bonds σ-bond is referred to as the inductive effect. This effect is passed on to the subsequent The electron displacement in an organic bonds also but the effect decreases rapidly as molecule may take place either in the ground the number of intervening bonds increases and state under the influence of an atom or a becomes vanishingly small after three bonds. substituent group or in the presence of an The inductive effect is related to the ability of appropriate attacking reagent. The electron substituent(s) to either withdraw or donate displacements due to the influence of electron density to the attached carbon atom. an atom or a substituent group present in the Based on this ability, the substitutents can be molecule cause permanent polarlisation of the classified as electron-withdrawing or electron bond. Inductive effect and resonance effects are donating groups relative to hydrogen. Halogens examples of this type of electron displacements. and many other groups such as nitro (- NO2), Temporary electron displacement effects are cyano (- CN), carboxy (- COOH), ester (COOR), seen in a molecule when a reagent approaches aryloxy (-OAr, e.g. – OC6H5), etc. are electron- to attack it. This type of electron displacement withdrawing groups. On the other hand, the is called electromeric effect or polarisability alkyl groups like methyl (–CH3) and ethyl effect. In the following sections we will learn (–CH2–CH3) are usually considered as electron about these types of electronic displacements. donating groups. 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 353 be adequately represented by any of these Problem 12.14 structures, rather it is a hybrid of the two Which bond is more polar in the following structures (I and II) called resonance pairs of molecules: (a) H3C-H, H3C-Br structures. The resonance structures (b) H 3 C-NH 2, H 3 C-OH (c) H 3C-OH, (canonical structures or contributing H3C-SH structures) are hypothetical and individually do not represent any real Solution molecule. They contribute to the actual (a) C–Br, since Br is more electronegative structure in proportion to their stability. than H, (b) C–O, (c) C–O Another example of resonance is provided Problem 12.15 by nitromethane (CH3NO 2) which can be represented by two Lewis structures, (I and II). In which C–C bond of CH3CH2CH2Br, the There are two types of N-O bonds in these inductive effect is expected to be the least? structures. Solution Magnitude of inductive effect diminishes as the number of intervening bonds increases. Hence, the effect is least in the bond between carbon-3 and hydrogen. However, it is known that the two N–O bonds 12.7.6 Resonance Structure of nitromethane are of the same length (intermediate between a N–O single bond There are many organic molecules whose and a N=O double bond). The actual behaviour cannot be explained by a single structure of nitromethane is therefore a Lewis structure. An example is that of resonance hybrid of the two canonical benzene. Its cyclic structure forms I and II. containing alternating C–C single The energy of actual structure of the molecule and C=C double bonds shown is (the resonance hybrid) is lower than that of any inadequate for explaining its Benzene of the canonical structures. The difference in characteristic properties. energy between the actual structure and the As per the above representation, benzene lowest energy resonance structure is called the should exhibit two different bond lengths, due resonance stabilisation energy or simply to C–C single and C=C double bonds. However, the resonance energy. The more the number as determined experimentally benzene has a of important contributing structures, the more uniform C–C bond distances of 139 pm, a is the resonance energy. Resonance is value intermediate between the C–C single(154 particularly important when the contributing pm) and C=C double (134 pm) bonds. Thus, structures are equivalent in energy. the structure of benzene cannot be represented The following rules are applied while writing adequately by the above structure. Further, resonance structures: benzene can be represented equally well by the The resonance structures have (i) the same positions of nuclei and (ii) the same number of energetically identical structures I and II. 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 on more Therefore, according to the resonance theory electropositive atom) and more dispersal of (Unit 4) the actual structure of benzene cannot charge, is more stable than others. 2022-23 354 CHEMISTRY Problem 12.16 Solution Write resonance structures of CH3COO – The two structures are less important and show the movement of electrons by contributors as they involve charge curved arrows. separation. Additionally, structure I contains a carbon atom with an Solution incomplete octet. First, write the structure and put unshared pairs of valence electrons on 12.7.7 Resonance Effect appropriate atoms. Then draw the arrows The resonance effect is defined as ‘the polarity one at a time moving the electrons to get produced in the molecule by the interaction of the other structures. two π-bonds or between a π-bond and lone pair of electrons present on an adjacent atom’. The effect is transmitted through the chain. There are two types of resonance or mesomeric effect designated as R or M effect. Problem 12.17 (i) Positive Resonance Effect (+R effect) Write resonance structures of In this effect, the transfer of electrons is away CH2=CH–CHO. Indicate relative stability of from an atom or substituent group attached the contributing structures. to the conjugated system. This electron displacement makes certain positions in the Solution molecule of high electron densities. This effect in aniline is shown as : Stability: I > II > III (ii) Negative Resonance Effect (- R effect) [I: Most stable, more number of covalent This effect is observed when the transfer of bonds, each carbon and oxygen atom has electrons is towards the atom or substituent an octet and no separation of opposite group attached to the conjugated system. For charge II: negative charge on more example in nitrobenzene this electron electronegative atom and positive charge displacement can be depicted as : on more electropositive atom; III: does not contribute as oxygen has positive charge and carbon has negative charge, hence least stable]. Problem 12.18 Explain why the following two structures, I and II cannot be the major contributors The atoms or substituent groups, which to the real structure of CH3COOCH3. represent +R or –R electron displacement effects are as follows : +R effect: – halogen, –OH, –OR, –OCOR, –NH2, –NHR, –NR2, –NHCOR, – R effect: – COOH, –CHO, >C=O, – CN, –NO2 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 355 The presence of alternate single and double system or to an atom with an unshared bonds in an open chain or cyclic system is p orbital. The σ electrons of C—H bond of the termed as a conjugated system. These systems alkyl group enter into partial conjugation with often show abnormal behaviour. The examples the attached unsaturated system or with the are 1,3- butadiene, aniline and nitrobenzene unshared p orbital. Hyperconjugation is a etc. In such systems, the π-electrons are permanent effect. delocalised and the system develops polarity. To understand hyperconjugation effect, let + 12.7.8 Electromeric Effect (E effect) us take an example of CH3 CH2 (ethyl cation) It is a temporary effect. The organic in which the positively charged carbon atom compounds having a multiple bond (a double has an empty p orbital. One of the C-H bonds or triple bond) show this effect in the presence of the methyl group can align in the plane of of an attacking reagent only. It is defined as this empty p orbital and the electrons the complete transfer of a shared pair of constituting the C-H bond in plane with this p π-electrons to one of the atoms joined by a orbital can then be delocalised into the empty multiple bond on the demand of an attacking p orbital as depicted in Fig. 12.4 (a). reagent. The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented by E and the shifting of the electrons is shown by a curved arrow ( ). There are two distinct types of electromeric effect. (i) Positive Eelctromeric Effect (+E effect) In this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example : Fig. 12.4(a) Orbital diagram showing hyperconjugation in ethyl cation This type of overlap stabilises the carbocation because electron density from the (ii) Negative Electromeric Effect (–E effect) In this adjacent σ bond helps in dispersing the positive effect the π - electrons of the multiple bond are charge. transferred to that atom to which the attacking reagent does not get attached. For example: When inductive and electromeric effects operate in opposite directions, the electomeric effect predominates. In general, greater the number of alkyl 12.7.9 Hyperconjugation groups attached to a positively charged carbon Hyperconjugation is a general stabilising atom, the greater is the hyperconjugation interaction. It involves delocalisation of interaction and stabilisation of the cation. σ electrons of C—H bond of an alkyl group Thus, we have the following relative stability directly attached to an atom of unsaturated of carbocations : 2022-23 356 CHEMISTRY Problem 12.19 + Explain why (CH3)3C is more stable than + + CH3 CH2 and C H3 is the least stable cation. Hyperconjugation is also possible in Solution alkenes and alkylarenes. + Hyperconjugation interaction in (CH3)3C is Delocalisation of electrons by + + greater than in CH C H as the (CH3)3C hyperconjugation in the case of alkene can be 3 2+ depicted as in Fig. 12.4(b). has nine C-H bonds. In C H3 , vacant p orbital is perpendicular to the plane in which C-H bonds lie; hence cannot + overlap with it. Thus, C H lacks 3 hyperconjugative stability. 12.7.10 Types of Organic Reactions and Mechanisms Organic reactions can be classified into the Fig. 12.4(b) Orbital diagram showing following categories: hyperconjugation in propene (i) Substitution reactions (ii) Addition reactions There are various ways of looking at the (iii) Elimination reactions hyperconjugative effect. One of the way is to (iv) Rearrangement reactions regard C—H bond as possessing partial ionic You will be studying these reactions in character due to resonance. Unit 13 and later in class XII. 12.8 METHODS OF PURIFICATION OF ORGANIC COMPOUNDS Once an organic compound is extracted from a natural source or synthesised in the laboratory, it is essential to purify it. Various methods used for the purification of organic compounds are based on the nature of the compound and the impurity present in it. The common techniques used for purification are as follows : (i) Sublimation (ii) Crystallisation (iii) Distillation (iv) Differential extraction and (v) Chromatography Finally, the purity of a compound is ascertained by determining its melting or boiling point. Most of the pure compounds have sharp melting points and boiling points. 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 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 357 of chromatographic and spectroscopic carefully. On boiling, the vapours of lower techniques. boiling component are formed first. The 12.8.1 Sublimation vapours are condensed by using a condenser and the liquid is collected in a receiver. The You have learnt earlier that on heating, some vapours of higher boiling component form later solid substances change from solid to vapour and the liquid can be collected separately. state without passing through liquid state. The purification technique based on the above principle is known as sublimation and is used to separate sublimable compounds from non- sublimable impurities. 12.8.2 Crystallisation This is one of the most commonly used techniques for the purification of solid organic compounds. It is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. The impure compound is dissolved in a solvent in which it is sparingly soluble at room temperature but appreciably soluble at higher temperature. The solution is concentrated to get a nearly saturated solution. On cooling the solution, pure compound crystallises out and is removed by filtration. The filtrate (mother liquor) contains impurities and small quantity of the compound. If the compound is highly soluble in one solvent and very little soluble Fig.12.5 Simple distillation. The vapours of a in another solvent, crystallisation can be substance formed are condensed and satisfactorily carried out in a mixture of these the liquid is collected in conical flask. solvents. Impurities, which impart colour to the solution are removed by adsorbing over Fractional Distillation: If the difference in activated charcoal. Repeated crystallisation boiling points of two liquids is not much, simple becomes necessary for the purification of distillation cannot be used to separate them. compounds containing impurities of The vapours of such liquids are formed within comparable solubilities. the same temperature range and are condensed 12.8.3 Distillation simultaneously. The technique of fractional This important method is used to separate (i) distillation is used in such cases. In this volatile liquids from nonvolatile impurities and technique, vapours of a liquid mixture are (ii) the liquids having sufficient difference in passed through a fractionating column before their boiling points. Liquids having different condensation. The fractionating column is boiling points vaporise at different fitted over the mouth of the round bottom flask temperatures. The vapours are cooled and the (Fig.12.6, page 358). liquids so formed are collected separately. Vapours of the liquid with higher boiling Chloroform (b.p 334 K) and aniline (b.p. 457 point condense before the vapours of the liquid K) are easily separated by the technique of with lower boiling point. The vapours rising distillation (Fig 12.5). The liquid mixture is up in the fractionating column become richer taken in a round bottom flask and heated in more volatile component. By the time the 2022-23 358 CHEMISTRY column is called a theoretical plate. Commercially, columns with hundreds of plates are available. One of the technological applications of fractional distillation is to separate different fractions of crude oil in petroleum industry. Distillation under reduced pressure: This method is used to purify liquids having very high boiling points and those, which decompose at or below their boiling points. Such liquids are made to boil at a temperature lower than their normal boiling points by reducing the pressure on their surface. A liquid boils at a temperature at which its vapour pressure is equal to the external pressure. The pressure is reduced with the help of a water pump or vacuum pump (Fig.12.8). Glycerol can be Fig.12.6 Fractional distillation. The vapours of lower boiling separated from spent-lye in fraction reach the top of the column first followed by soap industry by using this vapours of higher boiling fractions. technique. vapours reach to the top of the fractionating column, these are rich in the more volatile component. Fractionating columns are available in various sizes and designs as shown in Fig.12.7. A fractionating column provides many surfaces for heat exchange between the ascending vapours and the descending condensed liquid. Some of the condensing liquid in the fractionating column obtains heat from the ascending vapours and revaporises. The vapours thus become richer in low boiling component. The vapours of low boiling component ascend to the top of the column. On reaching the top, the vapours become pure in low boiling component and pass through the condenser and the pure liquid is collected in a receiver. After a series of successive distillations, the remaining liquid in the distillation flask gets enriched in high boiling component. Each successive condensation and vaporisation unit in the fractionating Fig.12.7 Different types of fractionating columns. 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 359 Fig.12.8 Distillation under reduced pressure. A liquid boils at a temperature below its vapour pressure by reducing the pressure. Steam Distillation: This technique is 12.8.4 Differential Extraction applied to separate substances which are When an organic compound is present in an steam volatile and are immiscible with water. aqueous medium, it is separated by shaking In steam distillation, steam from a steam it with an organic solvent in which it is more generator is passed through a heated flask soluble than in water. The organic solvent and containing the liquid to be distilled. The the aqueous solution should be immiscible mixture of steam and the volatile organic with each other so that they form two distinct compound is condensed and collected. The layers which can be separated by separatory compound is later separated from water funnel. The organic solvent is later removed by using a separating funnel. In steam distillation or by evaporation to get back distillation, the liquid boils when the sum the compound. Differential extraction is carried of vapour pressures due to the organic out in a separatory funnel as shown in liquid (p 1 ) and that due to water (p 2 ) Fig. 12.10 (Page 360). If the organic compound becomes equal to the atmospheric pressure is less soluble in the organic solvent, a very (p), i.e. p =p 1+ p 2. Since p1 is lower than p, large quantity of solvent would be required to the organic liquid vaporises at lower extract even a very small quantity of the temperature than its boiling point. compound. The technique of continuous Thus, if one of the substances in the extraction is employed in such cases. In this mixture is water and the other, a water technique same solvent is repeatedly used for insoluble substance, then the mixture will boil extraction of the compound. close to but below, 373K. A mixture of water 12.8.5 Chromatography and the substance is obtained which can be Chromatography is an important technique separated by using a separating funnel. extensively used to separate mixtures into their Aniline is separated by this technique from components, purify compounds and also to aniline – water mixture (Fig.12.9, Page 360). test the purity of compounds. The name 2022-23 360 CHEMISTRY Fig.12.9 Steam distillation. Steam volatile component volatilizes, the vapours con- 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, or a gas is allowed to move slowly over the stationary phase. The components of the mixture get gradually separated from one another. The moving phase is called the mobile phase. Based on the principle involved, chromatography is classified into different categories. Two of these are: (a) Adsorption chromatography, and (b) Partition chromatography. Fig.12.10 Differential extraction. Extraction of com- a) Adsorption Chromatography: Adsor- pound takes place based on difference ption chromatography is based on the fact that in solubility different compounds are adsorbed on an adsorbent to different degrees. Commonly used adsorbents are silica gel and alumina. are two main types of chromatographic When a mobile phase is allowed to move techniques based on the principle of differential over a stationary phase (adsorbent), the adsorption. components of the mixture move by varying (a) Column chromatography, and distances over the stationary phase. Following (b) Thin layer chromatography. 2022-23 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES 361 Column Chromatography: Column plate is then placed in a closed jar containing chromatography involves separation of a the eluant (Fig. 12.12a). As the eluant rises mixture over a column of adsorbent up the plate, the components of the mixture (stationary phase) packed in a glass tube. The move up along with the eluant to different column is fitted with a s