Chemistry Nomenclature of organic compounds

### Topics to be covered

=> Nomenclature of organic compounds
=> IUPAC nomenclature of alkanes
=> Nomenclature of organic compounds having functional groups
=> Nomenclature of substituted benzene compounds

### NOMENCLATURE OF ORGANIC COMPOUNDS

Before the IUPAC system of nomenclature, however, organic compounds were assigned names based on their origin or certain properties.

For instance, citric acid is named so because it is found in citrus fruits and the acid found in red ant is named formic acid since the Latin word for ant is color{brown}("𝑓𝑜𝑟𝑚𝑖𝑐𝑎.") These names are traditional and are considered as color{brown}("𝑡𝑟𝑖𝑣𝑖𝑎𝑙") or common names.

Buckminsterfullerene is a common name given to the newly discovered color{red}(C_(60)) cluster (a form of carbon) noting its structural similarity to the geodesic domes popularised by the famous architect R. Buckminster Fuller.

star color{red}("The IUPAC System of Nomenclature")
A systematic name of an organic compound is generally derived by identifying the parent hydrocarbon and the functional group(s) attached to it.

color{green}("Compounds containing carbon and hydrogen only are called hydrocarbons.")
color{maroon}("A hydrocarbon is termed saturated if it contains only carbon-carbon single bonds.")
color{green}("The IUPAC name for a homologous series of such compounds is alkane. Paraffin (Latin: little affinity) was the earlier name given to these compounds.")
color{maroon}(" Unsaturated hydrocarbons are those, which contain at least one carboncarbon double or triple bond.")

### 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 color{red}(CH_4) to color{red}(C_4H_(10)), where the prefixes are derived from trivial names).

𝐁𝐫𝐚𝐧𝐜𝐡𝐞𝐝 𝐜𝐡𝐚𝐢𝐧 𝐡𝐲𝐝𝐫𝐨𝐜𝐚𝐫𝐛𝐨𝐧𝐬: In a branched chain compound small chains of carbon atoms are attached at one or more carbon atoms of the parent chain. The small carbon chains (branches) are called alkyl groups.

In order to name such compounds, the names of alkyl groups are prefixed to the name of parent alkane. An alkyl group is derived from a saturated hydrocarbon by removing a hydrogen atom from carbon. An alkyl group is named by substituting color{brown}("‘𝑦𝑙’") for color{brown}("‘𝑎𝑛𝑒’") in the corresponding alkane.

Abbreviations are used for some alkyl groups. For example, methyl is abbreviated as color{red}("Me"), ethyl as color{red}("Et"), propyl as color{red}("Pr") and butyl as color{red}("Bu"). The alkyl groups can be branched also.

Common branched groups have specific trivial names. For example, the propyl groups can either be n-propyl group or isopropyl group. The branched butyl groups are called sec-butyl, isobutyl and tert-butyl group. We also encounter the structural unit, color{red}(–CH_2C(CH_3)_3), which is called neopentyl group.

star color{red}("Nomenclature of branched chain alkanes:")

We encounter a number of branched chain alkanes. The rules for naming them are given below.

1. First of all, the 𝐥𝐨𝐧𝐠𝐞𝐬𝐭 𝐜𝐚𝐫𝐛𝐨𝐧 𝐜𝐡𝐚𝐢𝐧 𝐢𝐧 𝐭𝐡𝐞 𝐦𝐨𝐥𝐞𝐜𝐮𝐥𝐞 𝐢𝐬 𝐢𝐝𝐞𝐧𝐭𝐢𝐟𝐢𝐞𝐝. In the example (I) given below, the longest chain has nine carbons and it is considered as the parent or root chain. Selection of parent chain as shown in (II) is not correct because it has only eight carbons.

2.The carbon atoms of the parent chain are numbered to identify the parent alkane and to locate the positions of the carbon atoms at which branching takes place due to the substitution of alkyl group in place of hydrogen atoms. 𝐓𝐡𝐞 𝐧𝐮𝐦𝐛𝐞𝐫𝐢𝐧𝐠 𝐢𝐬 𝐝𝐨𝐧𝐞 𝐢𝐧 𝐬𝐮𝐜𝐡 𝐚 𝐰𝐚𝐲 𝐭𝐡𝐚𝐭 𝐭𝐡𝐞 𝐛𝐫𝐚𝐧𝐜𝐡𝐞𝐝 𝐜𝐚𝐫𝐛𝐨𝐧 𝐚𝐭𝐨𝐦𝐬 𝐠𝐞𝐭 𝐭𝐡𝐞 𝐥𝐨𝐰𝐞𝐬𝐭 𝐩𝐨𝐬𝐬𝐢𝐛𝐥𝐞 𝐧𝐮𝐦𝐛𝐞𝐫𝐬. Thus, the numbering in the above example should be from left to right (branching at carbon atoms 2 and 6) and not from right to left (giving numbers 4 and 8 to the carbon atoms at which branches are attached).

3.The names of alkyl groups attached as a branch are then prefixed to the name of the parent alkane and position of the substituents is indicated by the appropriate numbers. If different alkyl groups are present, they are listed in alphabetical order. Thus, name for the compound shown above is: 6-ethyl-2- methylnonane. [Note: the numbers are separated from the groups by hyphens and there is no break between methyl and nonane.]

4. If two or more identical substituent groups are present then the numbers are separated by commas. The names of 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:

5. If the two substituents are found in equivalent positions, the 𝐥𝐨𝐰𝐞𝐫 𝐧𝐮𝐦𝐛𝐞𝐫 𝐢𝐬 𝐠𝐢𝐯𝐞𝐧 𝐭𝐨 𝐭𝐡𝐞 𝐨𝐧𝐞 𝐜𝐨𝐦𝐢𝐧𝐠 𝐟𝐢𝐫𝐬𝐭 𝐢𝐧 𝐭𝐡𝐞 𝐚𝐥𝐩𝐡𝐚𝐛𝐞𝐭𝐢𝐜𝐚𝐥 𝐥𝐢𝐬𝐭𝐢𝐧𝐠. Thus, the following compound is 3-ethyl-6-methyloctane and not 6-ethyl-3-methyloctane.

6. The branched alkyl groups can be named by following the above mentioned procedures. However, the 𝐜𝐚𝐫𝐛𝐨𝐧 𝐚𝐭𝐨𝐦 𝐨𝐟 𝐭𝐡𝐞 𝐛𝐫𝐚𝐧𝐜𝐡 𝐭𝐡𝐚𝐭 𝐚𝐭𝐭𝐚𝐜𝐡𝐞𝐬 𝐭𝐨 𝐭𝐡𝐞 𝐫𝐨𝐨𝐭 𝐚𝐥𝐤𝐚𝐧𝐞 is numbered 1 as exemplified below.

The name of such branched chain alkyl group is placed in parenthesis while naming the compound. While writing the trivial names of substituents’ 𝐢𝐧 𝐚𝐥𝐩𝐡𝐚𝐛𝐞𝐭𝐢𝐜𝐚𝐥 𝐨𝐫𝐝𝐞𝐫, 𝐭𝐡𝐞 𝐩𝐫𝐞𝐟𝐢𝐱𝐞𝐬 𝐢𝐬𝐨- 𝐚𝐧𝐝 𝐧𝐞𝐨- 𝐚𝐫𝐞 𝐜𝐨𝐧𝐬𝐢𝐝𝐞𝐫𝐞𝐝 𝐭𝐨 𝐛𝐞 𝐭𝐡𝐞 𝐩𝐚𝐫𝐭 𝐨𝐟 𝐭𝐡𝐞 𝐟𝐮𝐧𝐝𝐚𝐦𝐞𝐧𝐭𝐚𝐥 𝐧𝐚𝐦𝐞 𝐨𝐟 𝐚𝐥𝐤𝐲𝐥 𝐠𝐫𝐨𝐮𝐩. 𝐓𝐡𝐞 𝐩𝐫𝐞𝐟𝐢𝐱𝐞𝐬 𝐬𝐞𝐜- 𝐚𝐧𝐝 𝐭𝐞𝐫𝐭- 𝐚𝐫𝐞 𝐧𝐨𝐭 considered to be the part of the fundamental name. The use of color{brown}("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 multisubstituted 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.

• After selection of the chain, numbering is to be done from the end closer to the substituent.

𝐂𝐲𝐜𝐥𝐢𝐜 𝐂𝐨𝐦𝐩𝐨𝐮𝐧𝐝𝐬: A saturated monocyclic compound is named by prefixing ‘cyclo’ to the corresponding straight chain alkane. If side chains are present, then the rules given above are applied. Names of some cyclic compounds are given below
Q 3214323259

Structures and IUPAC names of some hydrocarbons are given below. Explain why the names given in the parentheses are incorrect.

Solution:

(a) Lowest locant number, 2,5,6 is lower than 3,5,7,
(b) substituents are in equivalent position; lower number is given to the one that comes first in the name according to alphabetical order.

### Nomenclature of Organic Compounds having Functional Group(s)

Compounds having the same functional group undergo similar reactions. For example, color{red}(CH_3OH, CH_3CH_2OH), and color{red}((CH_3)_2CHOH) — all having color{red}(-OH) functional group liberate hydrogen on reaction with sodium metal.

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 𝐜𝐚𝐫𝐛𝐨𝐧 𝐚𝐭𝐨𝐦 𝐩𝐨𝐬𝐬𝐞𝐬𝐬𝐢𝐧𝐠 𝐥𝐨𝐰𝐞𝐬𝐭 𝐩𝐨𝐬𝐬𝐢𝐛𝐥𝐞 𝐧𝐮𝐦𝐛𝐞𝐫 𝐢𝐧 𝐭𝐡𝐞 𝐜𝐡𝐚𝐢𝐧. In the case of polyfunctional compounds, one of the functional groups is chosen as the color{brown}("𝑝𝑟𝑖𝑛𝑐𝑖𝑝𝑎𝑙 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛𝑎𝑙") group and the compound is then named on that basis. The remaining functional groups, which are subordinate functional groups, are named as substituents using the appropriate prefixes. The choice of principal functional group is made on the basis of order of preference.

𝐓𝐡𝐞 𝐨𝐫𝐝𝐞𝐫 𝐨𝐟 𝐝𝐞𝐜𝐫𝐞𝐚𝐬𝐢𝐧𝐠 𝐩𝐫𝐢𝐨𝐫𝐢𝐭𝐲 𝐟𝐨𝐫 𝐬𝐨𝐦𝐞 𝐟𝐮𝐧𝐜𝐭𝐢𝐨𝐧𝐚𝐥 𝐠𝐫𝐨𝐮𝐩𝐬 𝐢𝐬 :

color{red}(-COOH, –SO_3H, -COOR \ \ \ \ \ \ (R="alkyl group"), COCl, -CONH_2, -CN,-HC=O, > C=O, -OH, -NH_2, > C=C <, -C=C- .)

The color{red}(–R, C_6H_5-), halogens color{red}((F, Cl, Br, I), –NO_2), alkoxy color{red}((–OR)) etc. are always prefix substituents. Thus, a compound containing both an alcohol and a keto group is named as hydroxyalkanone since the keto group is preferred to the hydroxyl group. For example, color{red}(HOCH_2(CH_2)_3CH_2COCH_3) will be named as 7-hydroxyheptan-2-one and not as 2-oxoheptan -7-ol. Similarly, color{red}(BrCH_2CH=CH_2) 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 suffix. In such cases the full name of the parent alkane is written before the class suffix. For example color{red}(CH_2(OH)CH_2(OH)) is named as ethane 1,2–diol. However, the ending – ne of the parent alkane is dropped in the case of compounds having more than one double or triple bond; for example, color{red}(CH_2=CH-CH=CH_2) is named as buta–1,3–diene.

Q 3254423354

Write the IUPAC names of the compounds i-iv from their given structures.

Solution:

(I) • The functional group present is an alcohol (OH). Hence the suffix is ‘-ol’.
• The longest chain containing -OH has eight carbon atoms. Hence the corresponding saturated hydrocarbon is octane.
• The -OH is on carbon atom 3. In addition, a methyl group is attached at 6th carbon. Hence, the systematic name of this compound is 6-Methyloctan-3-ol.

(II) The functional group present is ketone (> C=O), hence suffix ‘-one’. Presence of two keto groups is indicated by ‘di’, hence suffix becomes ‘dione’. The two keto groups are at carbons 2 and 4. The longest chain contains 6 carbon atoms, hence, parent hydrocarbon is hexane. Thus, the systematic name is Hexane- 2,4-dione.

(III) Here, two functional groups namely ketone and carboxylic acid are present. The principal functional group is the carboxylic acid group; hence the parent chain will be suffixed with ‘oic’ acid. Numbering of the chain starts from carbon of – COOH functional group. The keto group in the chain at carbon 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) The two C=C functional groups are present at carbon atoms 1 and 3, while the C≡C functional group is present at 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.
Q 3274423356

Derive the structure of
(i) 2-Chlorohexane,
(ii) Pent-4-en-2-ol,
(iii) 3- Nitrocyclohexene,
(iv) Cyclohex-2-en-1-ol,
(v) 6-Hydroxyheptanal.

Solution:

i) ‘hexane’ indicates the presence of 6 carbon atoms in the chain. The functional group chloro is present at carbon 2. Hence, the structure of the compound is CH_3CH_2CH_2CH_2CH(Cl)CH_3.

(ii) ‘pent’ indicates that parent hydrocarbon contains 5 carbon atoms in the chain. ‘en’ and ‘ol’ correspond to the functional groups C=C and -OH at carbon atoms 4 and 2 respectively. Thus, the structure is CH_2=CHCH_2CH (OH)CH_3.

(iii) Six membered ring containing a carbon-carbon double bond is implied by cyclohexene, which is numbered as shown in (I). The prefix 3-nitro means that a nitro group is present on C-3. Thus, complete structural formula of the compound is (II). Double bond is suffixed functional group whereas NO_2 is prefixed functional group therefore double bond gets preference over –NO_2 group:

(iv) ‘1-ol’ means that a -OH group is present at C-1. OH is suffixed functional group and gets preference over C=C bond. Thus the structure is as shown in (II):

(v) ‘heptanal’ indicates the compound to be an aldehyde containing 7 carbon atoms in the parent chain. The ‘6-hydroxy’ indicates that -OH group is present at carbon 6. Thus, the structural formula of the compound is: CH_3CH(OH)CH_2CH_2CH_2CH_2CHO. Carbon atom of –CHO group is included while numbering the carbon chain.

### Nomenclature of Substituted Benzene Compounds

For IUPAC nomenclature of substituted benzene compounds, the substituent is placed as prefix to the word benzene as shown in the following examples.

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.

In the trivial system of nomenclature the 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 derivatives, these prefixes cannot be used and the compounds are named by identifying substituent positions on the ring by following the lowest locant rule. In some cases, common name of benzene derivatives is taken as the base compound.

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 alphabetical order. Some examples are given below.

When a benzene ring is attached to an alkane with a functional group, it is considered as substituent, instead of a parent. The name for benzene as substituent is phenyl (color{red}(C_6H_5-), also abbreviated as Ph ).
Q 3204423358

Write the structural formula of:
(a) o-Ethylanisole, (b) p-Nitroaniline,
(c) 2,3 - Dibromo -1 - phenylpentane,
(d) 4-Ethyl-1-fluoro-2-nitrobenzene.

Solution: