NCERT Textbook Organic Chemistry Class 11 PDF

256 chemistry Unit 8 Organic Chemistry – Some Basic Principles and...

256 chemistry Unit 8 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 It also forms covalent bonds with atoms of other elements be able to like hydrogen, oxygen, nitrogen, sulphur, phosphorus understand reasons for and halogens. The resulting compounds are studied tetravalence of carbon and under a separate branch of chemistry called organic shapes of organic molecules; chemistry. This unit incorporates some basic principles write structures of organic and 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 8.1 General Introduction to IUPAC system of nomenclature and also derive their structures Organic compounds are vital for sustaining life on earth from the given names; and include complex molecules like genetic information understand the concept of bearing deoxyribonucleic acid (DNA) and proteins that organic reaction mechanism; constitute essential compounds of our blood, muscles and explain the influence of electronic skin. Organic compounds appear in materials like clothing, displacements on structure and fuels, polymers, dyes and medicines. These are some of reactivity of organic compounds; the important areas of application of these compounds. recognise the types of organic Science of organic chemistry is about two hundred reactions; years old. Around the year 1780, chemists began to learn the techniques of distinguish between organic compounds obtained from purification of organic plants and animals and inorganic compounds prepared compounds; from mineral sources. Berzilius, a Swedish chemist write the chemical reactions proposed that a ‘vital force’ was responsible for the involved in the qualitative formation of organic compounds. However, this notion analysis of organic compounds; was rejected in 1828 when F. Wohler synthesised an understand the principles involved in quantitative analysis organic compound, urea from an inorganic compound, of organic compounds. ammonium cyanate. NH4CNO NH2CONH2 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. Rationalised 2023-24 Unit 8.indd 256 2/24/2023 16:52:17  organic chemistry – some basic principles and techniques 257 The development of electronic theory of Thus, in H2C=CH2 molecule all the atoms covalent bonding ushered organic chemistry must 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. 8.2 TETRAVALENCE OF CARBON: Rotation of one CH2 fragment with respect SHAPES OF ORGANIC COMPOUNDS to other interferes with maximum overlap 8.2.1 The Shapes of Carbon Compounds of p orbitals and, therefore, such rotation The knowledge of fundamental concepts of about 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 and Problem 8.1 the shapes of molecules like methane (CH4), ethene (C2H4), ethyne (C2H2) are explained How many σ and π bonds are present in in terms of the use of sp3, sp2 and sp hybrid each of the following molecules? orbitals by carbon atoms in the respective (a) HC≡CCH=CHCH3 (b) CH2=C=CHCH3 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 8.2 the sp3 hybrid orbital. The sp2 hybrid orbital What is the type of hybridisation of each is intermediate in s character between sp carbon in the following compounds? and sp3 and, hence, the length and enthalpy of the bonds it forms, are also intermediate (a) CH3Cl, (b) (CH3)2CO, (c) CH3CN, between them. The change in hybridisation (d) HCONH2, (e) CH3CH=CHCN affects the electronegativity of carbon. The Solution greater the s character of the hybrid orbitals, the greater is the electronegativity. Thus, a (a) sp3, (b) sp3, sp2, (c) sp3, sp, (d) sp2, (e) carbon atom having an sp hybrid orbital with sp3, sp2, sp2, sp 50% s character is more electronegative than Problem 8.3 that possessing sp2 or sp3 hybridised orbitals. This relative electronegativity is reflected in Write the state of hybridisation of carbon several physical and chemical properties of in the following compounds and shapes the molecules concerned, about which you of each of the molecules. will learn in later units. (a) H2C=O, (b) CH3F, (c) HC≡N. 8.2.2 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. Rationalised 2023-24 Unit 8.indd 257 10/10/2022 10:37:28 AM  258 chemistry 8.3 STRUCTURAL RepresenTATIONS Similarly, CH3CH2CH2CH2CH2CH2CH2CH3 OF organic COMPOUNDs can be further condensed to CH3(CH2)6CH3. 8.3.1 Complete, Condensed and Bond-line For further simplification, organic chemists Structural Formulas use another way of r epr esenting the structures, in which only lines are used. Structures of organic compounds are In this bond-line structural representation represented in several ways. The Lewis of organic compounds, carbon and structure or dot structure, dash structure, hydrogen atoms are not shown and the condensed structure and bond line structural lines representing carbon-carbon bonds are formulas are some of the specific types. The drawn 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., oxygen, examples are represented as follows: nitrogen, sulphur, halogens etc.) may or may not be shown. Thus, ethane (C2H6), ethene (i) 3-Methyloctane can be represented in (C2H4), ethyne (C2H2) and methanol (CH3OH) various forms as: can be represented by the following structural (a) CH3CH2CHCH2CH2CH2CH2CH3 formulas. Such structural representations are | called complete structural formulas. CH3 (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, (a) CH3CHBrCH2CH3 (b) ethane, ethene, ethyne and methanol can be written as: CH3CH3 H2C=CH2 HC≡CH CH3OH (c) Ethane Ethene Ethyne Methanol Rationalised 2023-24 Unit 8.indd 258 10/10/2022 10:37:29 AM  organic chemistry – some basic principles and techniques 259 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 8.6 Problem 8.4 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 8.5 For each of the following compounds, write a condensed formula and also their bond-line formula. (a) HOCH2CH2CH2CH(CH3)CH(CH3)CH3 Rationalised 2023-24 Unit 8.indd 259 10/10/2022 10:37:30 AM  260 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 8.3.2 Three-Dimensional springs in place of sticks. These models are Representation of Organic referred to as ball-and-spring model. The Molecules space-filling model emphasises the relative The three-dimensional (3-D) structure of size of each atom based on its van der Waals organic molecules can be represented on radius. Bonds are not shown in this model. paper by using certain conventions. For It conveys the volume occupied by each atom example, by using solid ( ) and dashed in the 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 bonds Framework model Ball and stick model 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. 8.1 Space filling model Fig. 8.2 Fig. 8.1 Wedge-and-dash representation of CH4 Rationalised 2023-24 Unit 8.indd 260 11/10/2022 15:19:57  organic chemistry – some basic principles and techniques 261 8.4 Classification of Organic (homocyclic). Compounds The existing large number of organic compounds and their ever -increasing numbers has made it necessary to classify them on the basis of their structures. Organic Cyclopropane Cyclohexene Cyclohexane 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 9. 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 C y c l i c o r c l o s e d c h a i n o r r i n g compounds (a) Alicyclic compounds Tropone Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring Rationalised 2023-24 Unit 8.indd 261 11/10/2022 15:20:58  262 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 on the basis of functional groups, into families given to the newly discovered C60 cluster (a or homologous series. form of carbon) noting its structural similarity 8.4.1 Functional Group to the geodesic domes popularised by the famous architect R. Buckminster Fuller. The functional group is an atom or a group Common names are useful and in many of atoms joined to the carbon chain which is cases 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 8.1. group (–CHO) and carboxylic acid group (– COOH) etc. Table 8.1 Common or Trivial Names of Some Organic Compounds 8.4.2 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. 8.5 NOMENCLATURE OF ORGANIC COMPOUNDS Organic chemistry deals with millions of 8.5.1 The IUPAC System of Nomenclature compounds. In order to clearly identify A systematic name of an organic compound them, a systematic method of naming has is generally derived by identifying the parent been 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 Rationalised 2023-24 Unit 8.indd 262 11/10/2022 15:20:58  organic chemistry – some basic principles and techniques 263 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 and hydrogen only are called hydrocarbons. from a saturated hydrocarbon by removing A hydrocarbon is termed saturated if it a hydrogen atom from carbon. Thus, CH4 contains only carbon-carbon single bonds. becomes -CH3 and is called methyl group. An The IUPAC name for a homologous series of alkyl group is named by substituting ‘yl’ for such compounds is alkane. Paraffin (Latin: ‘ane’ in the corresponding alkane. Some alkyl little affinity) was the earlier name given to groups are listed in Table 8.3. these compounds. Unsaturated hydrocarbons Table 8.3 Some Alkyl Groups are those, which contain at least one carbon- carbon double or triple bond. 8.5.2 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 8.2. The alkanes in Table groups. For example, methyl is abbreviated as 8.2 differ from each other by merely the Me, ethyl as Et, propyl as Pr and butyl as Bu. number of -CH2 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 8.2 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 branched chain compound small chains of trivial names. For example, the propyl groups carbon atoms are attached at one or more can either be n-propyl group or isopropyl carbon atoms of the parent chain. The small group. The branched butyl groups are called carbon chains (branches) are called alkyl sec-butyl, isobutyl and tert-butyl group. groups. For example: We also encounter the structural unit, –CH2C(CH3)3, which is called neopentyl group. CH3–CH–CH2–CH3 CH3–CH–CH2–CH–CH3    Nomenclature of branched chain alkanes: CH3 CH2CH3 CH3 We encounter a number of branched chain alkanes. The rules for naming them are given (a)(b) below. Rationalised 2023-24 Unit 8.indd 263 10/10/2022 10:37:31 AM  264 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 or root chain. Selection of parent chain as groups are present then the numbers shown in (II) is not correct because it has are 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 2,4-Dimethylpentane 2,2,4-Trimethylpentane 2. The carbon atoms of the parent chain are numbered to identify the parent alkane H3C H2C CH3 and to locate the positions of the carbon   atoms at which branching takes place due CH3CH2CHCCH2CH2CH3 to the substitution of alkyl group in place 1 2 3 4 5 6 7 of hydrogen atoms. The numbering is CH3 done in such a way that the branched 3-Ethyl-4,4-dimethylheptane carbon atoms get the lowest possible numbers. Thus, the numbering in the 5. If the two substituents are found in above example should be from left to right equivalent positions, the lower number (branching at carbon atoms 2 and 6) and is given to the one coming first in the not from right to left (giving numbers alphabetical listing. Thus, the following 4 and 8 to the carbon atoms at which compound is 3-ethyl-6-methyloctane and branches are attached). not 6-ethyl-3-methyloctane. 1 2 3 4 5 6 7 8 9 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   CH2CH3 CH3 9 8 7 6 5 4 3 2 1 CCCCCCCCC 6. The branched alkyl groups can be   named by following the above mentioned C CC procedures. However, the carbon atom 3. The names of alkyl groups attached of the branch that attaches to the root as a branch are then prefixed to the alkane is numbered 1 as exemplified name of the parent alkane and position below. of the substituents is indicated by the 4 3 2 1 appropriate numbers. If different alkyl CH3–CH–CH2–CH– groups are present, they are listed in alphabetical order. Thus, name for the   compound shown above is: 6-ethyl-2- CH3 CH3 1,3-Dimethylbutyl- methylnonane. [Note: the numbers are Rationalised 2023-24 Unit 8.indd 264 10/10/2022 10:37:31 AM  organic chemistry – some basic principles and techniques 265 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 corresponding straight chain alkane. If side of 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 (not 1-ethyl-3,3-dimethylcyclohexane) is to be done from the end closer to the substituent. Problem 8.7 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 5-sec-Butyl-4-isopropyldecane in equivalent position; lower number is given to the one that comes first in the name according to alphabetical order. 8.5.3 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 5-(2,2-Dimethylpropyl)nonane a unique manner which is usually the site of Rationalised 2023-24 Unit 8.indd 265 10/27/2022 2:13:15 PM  266 chemistry chemical reactivity in an organic molecule. class suffix. In such cases the full name of the Compounds having the same functional parent alkane is written before the class suffix. group undergo similar reactions. For example, For example CH2(OH)CH2(OH) is named as CH 3OH, CH 3CH 2OH, and (CH 3) 2CHOH — ethane–1,2–diol. However, the ending – ne of all having -OH functional group liberate the parent alkane is dropped in the case of hydrogen on reaction with sodium metal. compounds having more than one double or The presence of functional groups enables triple bond; for example, CH2=CH-CH=CH2 is systematisation of organic compounds into named as buta–1,3–diene. different classes. Examples of some functional groups with their prefixes and suffixes along Problem 8.8 with some examples of organic compounds Write the IUPAC names of the compounds possessing these are given in Table 8.4. 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 8.4, the name of the compound The longest chain containing -OH is arrived at. has eight carbon atoms. Hence the In the case of polyfunctional compounds, corresponding saturated hydrocar- one of the functional groups is chosen as the bon is octane. principal functional group and the compound The -OH is on carbon atom 3. In is 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 substituents. Thus, a compound containing of 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. preferred to the hydroxyl group. The longest chain contains 6 carbon For example, HOCH2(CH2)3CH2COCH3 will atoms, hence, parent hydrocarbon is be named as 7-hydroxyheptan-2-one and not hexane. Thus, the systematic name is as 2-oxoheptan -7-ol. Similarly, BrCH2CH=CH2 Hexane-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 Rationalised 2023-24 Unit 8.indd 266 10/10/2022 10:37:32 AM  organic chemistry – some basic principles and techniques 267 Table 8.4 Some Functional Groups and Classes of Organic Compounds Rationalised 2023-24 Unit 8.indd 267 10/10/2022 10:37:32 AM  268 chemistry Solution (iii) Six membered ring containing a Here, two functional groups namely carbon-carbon double bond is implied ketone and carboxylic acid are present. by cyclohexene, which is numbered as The principal functional group is the shown in (I). The prefix 3-nitro means carboxylic acid group; hence the parent that a nitro group is present on C-3. chain will be suffixed with ‘oic’ acid. Thus, complete structural formula of the Numbering of the chain starts from compound is (II). Double bond is suffixed carbon of – COOH functional group. functional group whereas NO2 is prefixed The keto group in the chain at carbon functional group therefore double bond 5 is indicated by ‘oxo’. The longest gets preference over –NO2 group: chain 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 Problem 8.9 to 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: CH3CH(OH) Solution CH2CH2CH2CH2CHO. Carbon atom of – CHO group is included while numbering (i) ‘hexane’ indicates the presence of the carbon chain. 6 carbon atoms in the chain. The functional group chloro is present at carbon 2. Hence, the structure of the 8.5.4 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 benzene compounds, the substituent is in the chain. ‘en’ and ‘ol’ correspond to the functional groups C=C and -OH at placed as prefix to the word benzene as shown carbon atoms 4 and 2 respectively. Thus, in the following examples. However, common the structure is names (written in bracket below) of many substituted benzene compounds are also CH2=CHCH2CH (OH)CH3. universally used. Rationalised 2023-24 Unit 8.indd 268 10/10/2022 10:37:32 AM  organic chemistry – some basic principles and techniques 269 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 by numbering 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) 2-Chloro-4-methylanisole 4-Ethyl-2-methylaniline are used as prefixes to indicate the relative positions 1,2;1,3 and 1,4 respectively. Thus, 1,3-dibr omobenzene (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 considered the lowest locant rule. In some cases, common as substituent, instead of a parent. The name name of benzene derivatives is taken as the for benzene as substituent is phenyl (C6H5-, base compound. also abbreviated as Ph). Rationalised 2023-24 Unit 8.indd 269 10/10/2022 10:37:33 AM  270 chemistry different carbon skeletons, these are referred Problem 8.10 to as chain isomers and the phenomenon is Write the structural formula of: termed as chain isomerism. For example, (a) o-Ethylanisole, (b) p-Nitroaniline, C5H12 represents three compounds: (c) 2,3 - Dibromo -1 - phenylpentane, (d) 4-Ethyl-1-fluoro-2-nitrobenzene. CH3  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 (c) (d) substituent atom or functional group on the carbon skeleton, they are called position 8.6 ISOMERISM isomers and this phenomenon is termed as position isomerism. For example, the The phenomenon of existence of two or more molecular formula C3H 8O represents two compounds possessing the same molecular alcohols: formula but different properties is known as isomerism. Such compounds are called OH as isomers. The following flow chart shows  different types of isomerism. CH3CH2CH2OH CH3−CH-CH3 Propan-1-ol Propan-2-ol 8.6.1 Structural Isomerism Compounds having the same molecular (iii) Functional group isomerism: Two or formula but different structures (manners more compounds having the same molecular in which atoms are linked) are classified as formula but different functional groups structural isomers. Some typical examples are called functional isomers and this of different types of structural isomerism are phenomenon is termed as functional group given below: isomerism. For example, the molecular (i) Chain isomerism: When two or more formula C3H6O represents an aldehyde and compounds have similar molecular formula but a ketone: Isomerism Structural isomerism Stereoisomerism Chain Position Functional Metamerism Geometrical Optical isomerism isomerism group isomerism isomerism isomerism Rationalised 2023-24 Unit 8.indd 270 10/10/2022 10:37:33 AM  organic chemistry – some basic principles and techniques 271 in understanding the reactivity of organic O H compounds and in planning strategy for their   synthesis. CH3−C-CH3 CH3−CH2—C= O In the following sections, we shall learn Propanone Propanal some of the principles that explain how these (iv) Metamerism: It arises due to different reactions take place. alkyl chains on either side of the functional 8.7.1 Fission of a Covalent Bond group in the molecule. For example, C4H10O represents methoxypropane (CH3OC3H7) and A covalent bond can get cleaved either by : (i) ethoxyethane (C2H5OC2H5). heterolytic cleavage, or by (ii) homolytic cleavage. 8.6.2 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 lone stereoisomerism and can be classified as pair and a negative charge. Thus, heterolytic geometrical and optical isomerism. + cleavage of bromomethane will give C H3 and 8.7 F U N D A M E N TA L C O N C E P T S I N Br– as shown below. ORGANIC REACTION MECHANISM In an organic reaction, the organic molecule (also referred as a substrate) reacts with an A species having a carbon atom possessing appropriate attacking reagent and leads to sextext of electrons and a positive charge is the formation of one or more intermediate(s) called a carbocation (earlier called carbonium and finally product(s) ion). The H3 ion is known as a methyl cation or methyl carbonium ion. Carbocations are The general reaction is depicted as follows : classified as primary, secondary or tertiary Attacking depending on whether one, two or three Reagent [Intermediate] Product(s) Organic carbons are directly attached to the positively molecule charged carbon. Some+ other examples of (Substrate) Byproducts carbocations are: CH3C H2 (ethyl cation, a + primary carbocation), (CH3)2C H (isopropyl+ Substrate is that reactant which supplies cation, a secondary carbocation), and (CH3)3C carbon to the new bond and the other reactant (tert-butyl cation, a tertiary carbocation). is called reagent. If both the reactants Carbocations are highly unstable and supply carbon to the new bond then choice reactive species. Alkyl groups directly is arbitrary and in that case the molecule on attached to the positively charged carbon which attention is focused is called substrate. stabilise the carbocations due to inductive In such a reaction a covalent bond and hyperconjugation effects, which you will between two carbon atoms or a carbon and be studying in the sections 8.7.5 and 8.7.9. some other atom is broken and a new bond is The+ observed+ order of carbocation + stability + formed. A sequential account of each step, is: C H3 < CH3CH2 < (CH3)2CH < (CH3)3C. These describing details of electron movement, carbocations have trigonal planar shape energetics during bond cleavage and bond with positively charged carbon+ being sp 2 formation, and the rates of transformation hybridised. Thus, the shape of C H3 may be of reactants into products (kinetics) is considered as being derived from the overlap referred to as reaction mechanism. The of three equivalent C(sp2) hybridised orbitals knowledge of reaction mechanism helps with 1s orbital of each of the three hydrogen Rationalised 2023-24 Unit 8.indd 271 12/13/2022 10:12:45  272 chemistry atoms. Each bond may be represented as curved arrow. Such cleavage results in C(sp 2)–H(1s) sigma bond. The remaining the formation of neutral species (atom or carbon orbital is perpendicular to the group) which contains an unpaired electron. molecular plane and contains no electrons. These species are called free radicals. Like [Fig. 8.3(a)]. carbocations and carbanions, 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 Fig. 8.3 (a) Shape of methyl carbocation tertiary: The heterolytic cleavage can also give a , species in which carbon gets the shared Methyl Ethyl Isopropyl Tert-butyl pair of electrons. For example, when group free free free free Z attached to the carbon leaves without radical radical radical radical Organic reactions, which proceed by homolytic fission are called free radical or electron pair, the methyl anion is homopolar or nonpolar reactions. formed. Such a carbon species carrying a 8.7.2 Substrate and Reagent negative charge on carbon atom is called Ions are generally not formed in the reactions carbanion. Carbon in carbanion is generally of organic compounds. Molecules as such sp3 hybridised and its structure is distorted participate in the reaction. It is convenient to tetrahedron as shown in Fig. 8.3(b). name one reagent as substrate and other as 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 → Fig. 8.3 (b) Shape of methyl carbanion CH2 Br – CH2Br Substrate Reagent Product Carbanions are also unstable and reactive species. The organic reactions which proceed through heterolytic bond cleavage are called (ii) 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 takes place instead of an Nucleophiles and Electrophiles electron pair. The single electron movement Reagents attack the reactive site of the is shown by ‘half-headed’ (fish hook: ) substrate. The reactive site may be electron Rationalised 2023-24 Unit 8.indd 272 10/10/2022 10:37:34 AM  organic chemistry – some basic principles and techniques 273 deficient portion of the molecule (a positive reactive site) e.g., an atom with incomplete Problem 8.11 electron shell or the positive end of the dipole Using curved-arrow notation, show the in the molecule. If the attacking species is formation of reactive intermediates when electron rich, it attacks these sites. If attacking the following covalent bonds undergo species is electron deficient, the reactive site heterolytic cleavage. for it is that part of the substrate molecule (a) CH3–SCH3, (b) CH3–CN, (c) CH3–Cu which can supply electrons, e.g., π electrons Solution in a double bond. 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 8.12 During a polar organic reaction, a Giving justification, categorise the nucleophile attacks an electrophilic centre following molecules/ions as nucleophile of the substrate which is that specific atom or electrophile: or part 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 substrate when the two undergo bonding Nucleophiles: HS–,C2H5O–,(CH3)3N:H2N:– interaction. A curved-arrow notation is used These species have unshared pair of to show the movement of an electron pair electrons, which can be donated and from the nucleophile to the electrophile. Some shared with an electrophile. examples of nucleophiles are the negatively + + + Electrophiles: BF 3 ,C1 H 3 –C =O,N O 2. charged ions with lone pair of electrons such Reactive sites have only six valence as hydroxide (HO– ), cyanide (NC–) ions and electrons; can accept electron pair from carbanions (R3C:–). Neutral molecules such a nucleophile. as etc., can also act as nucleophiles due to the presence of lone Problem 8.13 pair of electrons. Examples of electrophiles Identify electrophilic centre in the + include carbocations (C H 3 ) and neutral following: CH3CH=O, CH3CN, CH3I. molecules having functional groups like Solution carbonyl group (>C=O) or alkyl halides * * A m o n g C H 3 H C = O , H 3 C C ≡N , a n d (R 3C-X, where X is a halogen atom). The * H3C –I, the starred carbon atoms are carbon atom in carbocations has sextet electrophilic centers as they will have configuration; hence, it is electron deficient and can receive a pair of electrons from the partial positive charge due to polarity of nucleophiles. In neutral molecules such as the bond. alkyl halides, due to the polarity of the C-X bond a partial positive charge is generated 8.7.3 Electron Movement in Organic on the carbon atom and hence the carbon Reactions atom becomes an electrophilic centre at which The movement of electrons in organic a nucleophile can attack. reactions can be shown by curved-arrow Rationalised 2023-24 Unit 8.indd 273 11/10/2022 15:21:53  274 chemistry notation. It shows how changes in bonding 8.7.5 Inductive Effect occur due to electronic redistribution during When a covalent bond is formed between the reaction. To show the change in position atoms of different electronegativity, the of a pair of electrons, curved arrow starts electron density is more towards the more from the point from where an electron pair is electronegative atom of the bond. Such a shift shifted and it ends at a location to which the of electron density results in a polar covalent pair of electron 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) (i) from π bond to in which the C–Cl bond is a polar covalent adjacent bond position bond. It is polarised in such a way that the carbon-1 gains some positive charge (δ +) (ii) from π bond to and the chlorine some negative charge (δ–). adjacent atom The fractional electronic charges on the two (iii) from atom to adjacent atoms in a polar covalent bond are denoted bond position by symbol δ (delta) and the shift of electron Movement of single electron is indicated by density is shown by an arrow that points from a single barbed ‘fish hooks’ (i.e. half headed δ+ to δ– end of the polar bond. curved arrow). For example, in transfer of δδ+ δ+ δ− hydroxide ion giving ethanol and in the CH3→CH2→Cl dissociation of chloromethane, the movement 2 1 of electron using curved arrows can be In turn carbon-1, which has developed depicted as follows: partial positive charge (δ + ) draws some electron 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 8.7.4 Electron Displacement Effects in in the adjacent bonds. Such polarisation of Covalent Bonds σ-bond caused by the polarisation of adjacent σ-bond is referred to as the inductive effect. The electron displacement in an organic This effect is passed on to the subsequent molecule may take place either in the ground bonds also but the effect decreases rapidly state under the influence of an atom or a as the number of intervening bonds increases substituent group or in the presence of an and becomes vanishingly small after three appropriate attacking reagent. The electron bonds. The inductive effect is related to the displacements due to the influence of ability of substituent(s) to either withdraw or an atom or a substituent group present in donate electron density to the attached carbon the molecule cause permanent polarlisation atom. Based on this ability, the substitutents of the bond. Inductive effect and resonance can be classified as electron-withdrawing or effects are examples of this type of electron electron donating groups relative to hydrogen. displacements. Temporary electron Halogens and many other groups such as displacement effects are seen in a molecule nitro (- NO2), cyano (- CN), carboxy (- COOH), when a reagent approaches to attack it. ester (COOR), aryloxy (-OAr, e.g. – OC6H5), etc. This type of electron displacement is called are electron-withdrawing groups. On the other electromeric effect or polarisability effect. hand, the alkyl groups like methyl (–CH3) and In the following sections we will learn about ethyl (–CH2–CH3) are usually considered as these types of electronic displacements. electron donating groups. ­ Rationalised 2023-24 Unit 8.indd 274 10/10/2022 10:37:34 AM  organic chemistry – some basic principles and techniques 275 benzene cannot be adequately represented Problem 8.14 by any of these structures, rather it is Which bond is more polar in the following a hybrid of the two structures (I and II) pairs of molecules: (a) H3C-H, H3C-Br called resonance structures. The resonance (b) H 3 C-NH 2 , H 3 C-OH (c) H 3 C-OH, structures (canonical structures or H3C-SH contributing structures) are hypothetical and individually do not represent any Solution real 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 8.15 by nitromethane (CH 3NO 2) which can be In which C–C bond of CH3CH2CH2Br, the represented by two Lewis structures, (I and inductive effect is expected to be the least? II). There are two types of N-O bonds in these 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 8.7.6 Resonance Structure bonds of nitromethane are of the same length (intermediate between a N–O single There are many organic molecules whose bond and a N=O double bond). The actual behaviour cannot be explained by a single Lewis structure of nitromethane is therefore structure. An example is that of a 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 (the resonance hybrid) is lower than that of is inadequate for explaining its Benzene any of the canonical structures. The difference characteristic properties. in 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, resonance stabilisation energy or simply due to C–C single and C=C double bonds. the resonance energy. The more the number However, as determined experimentally of important contributing structures, the benzene has a uniform C–C bond distances more is the resonance energy. Resonance is particularly important when the contributing of 139 pm, a value intermediate between the structures are equivalent in energy. C–C single(154 pm) and C=C double (134 pm) bonds. Thus, the structure of benzene The following rules are applied while writing cannot be represented adequately by the resonance structures: The resonance structures have (i) the same above structure. Further, benzene can be positions of nuclei and (ii) the same number of represented equally well by the energetically unpaired electrons. Among the resonance identical structures I and II. 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 electropositive atom) and more dispersal of theory (Unit 4) the actual structure of charge, is more stable than others. Rationalised 2023-24 Unit 8.indd 275 10/10/2022 10:37:34 AM  276 chemistry Problem 8.16 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 8.7.7 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 the other structures. of 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 8.17 (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 from an atom or substituent group attached of 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. charge II: negative charge on more For 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 8.18 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 Rationalised 2023-24 Unit 8.indd 276 10/10/2022 10:37:35 AM  organic chemistry – some basic principles and techniques