8.17 Complete each synthesis by giving missing starting material, reagent or products

8.2 Name the following compounds according to IUPAC system of nomenclature: (i) CH3CH(CH3)CH2CH2CHO (ii) CH3CH2COCH(C2H5)CH2CH2Cl (iii) CH3CH=CHCHO (iv) CH3COCH2COCH3 (v) CH3CH(CH3)CH2C(CH3)2COCH3 (vi) (CH3)3CCH2COOH (vii) OHCC6H4CHO-p

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 Reprint 2025-26 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 alkylgiven 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: We encounter a number of branched chain CH3 CH2CH3 CH3 alkanes. The rules for naming them are given (a) (b) below. Reprint 2025-26 264 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-Trimethylpentane2. 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 1 2 3 4 5 6 7 8 C  C  C  C  C  C  C  C  C   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 Reprint 2025-26 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 N o m e n c l a t u r e o f O r g a n i c 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 Reprint 2025-26 266 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 CH3OH, CH3CH2OH, and (CH3)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 Solutionthat the functional group is attached at the 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=C<, -C≡C- . Solution The –R, C6H5-, halogens (F, Cl, Br, I), –NO2, The functional group present is ketone alkoxy (–OR) etc. are always prefix (>C=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 Reprint 2025-26 organic chemistry – some basic principles and techniques 267 Table 8.4 Some Functional Groups and Classes of Organic Compounds Reprint 2025-26 268 chemistry Solution (iii) Six membered ring containing a carbon-carbon double bond is implied Here, two functional groups namely by cyclohexene, which is numbered as ketone and carboxylic acid are present. shown in (I). The prefix 3-nitro means The principal functional group is the that a nitro group is present on C-3. carboxylic acid group; hence the parent Thus, complete structural formula of the chain will be suffixed with ‘oic’ acid. compound is (II). Double bond is suffixed Numbering of the chain starts from functional group whereas NO2 is prefixed carbon of – COOH functional group. functional group therefore double bond The keto group in the chain at carbon gets preference over –NO2 group: 5 is indicated by ‘oxo’. The longest 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 placed as prefix to the word benzene as shown the functional groups C=C and -OH at in the following examples. However, common carbon atoms 4 and 2 respectively. Thus, names (written in bracket below) of many the structure is substituted benzene compounds are also CH2=CHCH2CH (OH)CH3. universally used. Reprint 2025-26 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 2-Chloro-4-methylanisole 4-Ethyl-2-methylanilinethe terms ortho (o), meta (m) and para (p) are used 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 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). Reprint 2025-26 270 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