Nomenclature of organic compounds

How are organic compounds named?

In plain words: a name such as 6-methyloctan-3-ol reads like an address. “Oct” says the main chain has eight carbons, “an” that its carbon–carbon bonds are single, “ol” that it is an alcohol with OH on carbon 3, and “6-methyl” that a CH₃ branch sits on carbon 6. Learn the order of the parts and a handful of rules, and the simple names in this lesson can be turned back into connectivity. A name without stereodescriptors may leave stereochemistry unspecified.

Common and IUPAC names

Before the IUPAC system, compounds were named after their source or a property: citric acid is found in citrus fruits, and formic acid in red ants (Latin formica, ant). These trivial or common names are still used, especially when the systematic name is long; buckminsterfullerene, for the C₆₀ cluster of carbon, was named after the architect R. Buckminster Fuller, whose geodesic domes it resembles (NCERT page 262). Some common names from NCERT Table 8.1 (page 262):

Naming alkanes

Straight chains. Acyclic hydrocarbons with only single bonds are alkanes, a saturated family; saturated rings are cycloalkanes (paraffin is an older term for alkanes, meaning little affinity). Unsaturated hydrocarbons contain at least one C=C or C≡C (NCERT page 263). A straight-chain alkane is named by a prefix for the number of carbons and the ending -ane (NCERT Section 8.5.2, Table 8.2, page 263). The prefixes for one to four carbons come from trivial names; from five onwards they come from number words (pent- to oct- and dec- are Greek; non- is Latin):

NCERT also lists icosane (C₂₀H₄₂) and triacontane (C₃₀H₆₂).

Alkyl groups. Removing one hydrogen from an alkane gives an alkyl group, named by changing -ane to -yl: methyl (–CH₃, Me), ethyl (–CH₂CH₃, Et), propyl (Pr), butyl (Bu) (NCERT Table 8.3, page 263). Branched alkyl groups have trivial names (page 263):

• isopropyl, (CH₃)₂CH–;
• sec-butyl, CH₃CH₂CH(CH₃)–;
• isobutyl, (CH₃)₂CHCH₂–;
• tert-butyl, (CH₃)₃C–;
• neopentyl, (CH₃)₃CCH₂–.

Branched alkanes (NCERT rules, pages 264–265):

1. Find the longest carbon chain: it is the parent (root) chain. In NCERT’s example the longest chain has nine carbons, so the parent is nonane, not an eight-carbon chain.
2. Number the chain from the end that gives the branched carbons the lowest possible numbers: 2 and 6, not 4 and 8.
3. Write the alkyl groups as prefixes with their locants, in alphabetical order, numbers separated from words by hyphens and with no break before the parent: 6-ethyl-2-methylnonane.
4. Identical groups are counted with di-, tri-, tetra-…, and their locants separated by commas: 2,4-dimethylpentane, 2,2,4-trimethylpentane. The multiplying prefixes are ignored in alphabetical order: 3-ethyl-4,4-dimethylheptane (e before m). Inside a bracketed (complex) substituent, though, the whole name counts, di- included: (2,2-dimethylbutyl) is alphabetised under d.
5. If two groups are in equivalent positions, the one first in alphabetical order gets the lower number: 3-ethyl-6-methyloctane, not 6-ethyl-3-methyloctane.
6. A branched substituent is numbered from the carbon attached to the parent chain and written in brackets: 1,3-dimethylbutyl-; 5-(2-ethylbutyl)-3,3-dimethyldecane. Trivial prefixes such as isopropyl are allowed only when the group carries no further substituents (page 265).

For alphabetical order, iso- and neo- count as part of the name of the alkyl group, but sec- and tert- do not: 5-sec-butyl-4-isopropyldecane (b before i). If two chains are equally long, choose the one with more side chains (page 265).

Rings. A saturated monocyclic compound takes the prefix cyclo-: cyclopentane, cyclohexane. With substituents, the rules above apply: 1-methyl-3-propylcyclohexane (alphabetical order decides which gets 1), and 3-ethyl-1,1-dimethylcyclohexane, not 1-ethyl-3,3-dimethylcyclohexane (the set 1,1,3 is lower than 1,3,3) (page 265).

Compounds with functional groups

A functional group is usually the site of chemical reactivity: CH₃OH, CH₃CH₂OH and (CH₃)₂CHOH all release hydrogen with sodium because of their –OH group (NCERT Section 8.5.3, pages 265–266). To name such a compound (page 266):

• identify the functional group, which fixes the suffix;
• for the simple acyclic examples here, choose a parent containing the maximum number of principal functional groups, then the longest suitable chain and number it so that the functional group gets the lowest possible number;
• if there are several functional groups, the one highest in priority is the principal functional group and gives the suffix; the others are named as prefixes;
• after the principal group, the C=C and C≡C bonds taken together get the lowest locants; only if both ends tie does the double bond get the lower number. Branches come last: CH₃CH=CHCH₂C≡CH is hex-4-en-1-yne, not hex-2-en-5-yne.

The order of decreasing seniority for some characteristic functional groups (page 266) is shown below. Multiple bonds follow characteristic-group locants; they are numbered together, with C=C preferred only when the multiple-bond locants tie:

–COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂

–R, C₆H₅–, halogens, –NO₂ and alkoxy (–OR) are always written as prefixes. So HOCH₂(CH₂)₃CH₂COCH₃ is 7-hydroxyheptan-2-one (ketone over alcohol), and BrCH₂CH=CH₂ is 3-bromoprop-1-ene (page 266).

With two or more groups of the same kind, di-, tri-… go before the class suffix and the full alkane name is kept: CH₂(OH)CH₂(OH) is ethane-1,2-diol. For more than one double or triple bond, the alkane name loses its -ne (butane → buta-) before -diene or -diyne: CH₂=CH–CH=CH₂ is buta-1,3-diene (page 266).

Suffixes and prefixes for the main classes, from NCERT Table 8.4 (page 267):

When a group is not the principal one, it is written as a prefix: hydroxy- (–OH), oxo- (>C=O; formyl- or oxo- for –CHO), cyano- (–CN), carboxy- (–COOH), amino- (–NH₂). In the substitutive naming system used here, halides, ethers and nitro compounds are expressed with prefixes: halo- (1-bromobutane), alkoxy- (ethoxyethane) and nitro- (1-nitrobutane).

The carbon of –CHO, –COOH, –CN and similar groups at the end of a chain is counted as carbon 1 of the chain: butanal is CH₃CH₂CH₂CHO, with four carbons. NCERT’s example for sulphonic acids, CH₃SO₃H, is called methylsulphonic acid in Table 8.4; its IUPAC name is methanesulphonic acid.

Substituted benzene compounds

For substituted benzenes the substituent is put as a prefix to benzene: methylbenzene (toluene), methoxybenzene (anisole), nitrobenzene and bromobenzene; the common names are used universally too (NCERT Section 8.5.4, pages 268–269). NCERT also lists aminobenzene (aniline). Aniline is a retained preferred IUPAC name, with benzenamine a systematic alternative.

• Two substituents: number the ring so that they get the lowest numbers: 1,3-dibromobenzene, not 1,5-. In the trivial system, the positions 1,2, 1,3 and 1,4 are called ortho (o-), meta (m-) and para (p-): m-dibromobenzene (page 269).
• Three or more: o-, m- and p- cannot be used. Use the lowest set of locants: 1-chloro-2,4-dinitrobenzene (1,2,4 beats 1,3,4); 2-chloro-1-methyl-4-nitrobenzene. Only when two numberings give the same set does the substituent first in alphabetical order get the lower number: 1-bromo-3-chloro-5-nitrobenzene.
• Common name as the base: the substituent of the base compound is carbon 1: 2-chloro-4-methylanisole, 4-ethyl-2-methylaniline, 3,4-dimethylphenol.
• When benzene is attached to a chain carrying a functional group, it is a substituent called phenyl, C₆H₅– (Ph) (page 269).

Solved examples

Common mistakes to avoid

Quick revision summary

• Name = locants and prefixes (alphabetical) + word root (chain length) + -ane/-ene/-yne + suffix of the principal functional group.
• Longest chain (with the functional group); lowest locants; alphabetical order breaks ties; di-, tri-, sec-, tert- ignored in alphabetising, iso- and neo- counted.
• Priority: –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂; halogens, –NO₂, –OR, –R and C₆H₅– only as prefixes.
• Benzene: lowest locants; o-, m-, p- = 1,2-, 1,3-, 1,4-; phenyl = C₆H₅–.

Questions students ask

What are the IUPAC names of acetone, acetic acid and ethyl alcohol?

The IUPAC name of acetone, CH₃COCH₃, is propanone (in full, propan-2-one); acetic acid, CH₃COOH, has the systematic alternative ethanoic acid, while acetic acid itself is the retained preferred IUPAC name; and ethyl alcohol, C₂H₅OH, is ethanol. Each IUPAC name is read from the structure, ‘prop’ and ‘eth’ counting three and two carbons and the endings -one, -oic acid and -ol naming the ketone, acid and alcohol groups, whereas a common name records history: ‘acetic’ comes from Latin acetum, vinegar. The same rules turn formaldehyde, HCHO, into methanal and chloroform, CHCl₃, into trichloromethane.

What is the IUPAC name of benzene?

Benzene is itself the IUPAC name of C₆H₆: IUPAC keeps it as the name of the parent ring, and its derivatives are named from it, as chlorobenzene and nitrobenzene are. IUPAC also keeps phenol for C₆H₅OH (rather than benzenol), aniline for C₆H₅NH₂, and benzoic acid and benzaldehyde for C₆H₅COOH and C₆H₅CHO. Toluene is also a retained preferred IUPAC name; methylbenzene is an acceptable systematic alternative. Anisole is retained for general nomenclature, while methoxybenzene is its preferred systematic name.

How do you name an ester in the IUPAC system?

An ester, RCOOR′, is named in two words: first the alkyl group R′ that came from the alcohol, then the acid part with -oic acid changed to -oate, so CH₃COOC₂H₅ is ethyl ethanoate (common name ethyl acetate) and HCOOCH₃ is methyl methanoate. The carbon of the C=O group belongs to the acid part and is counted in its chain: CH₃CH₂CH₂COOC₂H₅, made from butanoic acid and ethanol, is ethyl butanoate.

How are ethers named in the IUPAC system?

For a simple acyclic ether, R–O–R′, the substitutive name is an alkoxyalkane: the larger alkyl group gives the parent alkane, and the smaller one, together with the oxygen, becomes an alkoxy prefix such as methoxy- (CH₃O–) or ethoxy- (C₂H₅O–). So CH₃OCH₂CH₂CH₃ is 1-methoxypropane (common name methyl propyl ether) and CH₃OC₂H₅ is methoxyethane (ethyl methyl ether). An alkoxy group never takes a suffix, so when the molecule has a group that does, the alkoxy group stays a prefix: CH₃OCH₂CH₂OH is 2-methoxyethan-1-ol.

Which chapter of Class 11 covers IUPAC nomenclature of organic compounds?

IUPAC nomenclature of organic compounds is NCERT Section 8.5, Nomenclature of Organic Compounds (pages 262–270), in Unit 8, Organic Chemistry – Some Basic Principles and Techniques, of the Class 11 Chemistry book, Part II. Unit 14 of JEE Main 2027 and of NEET (UG) 2026, Some Basic Principles of Organic Chemistry, lists it as ‘Nomenclature (Trivial and IUPAC)’, and the rules are used again in Unit 9, Hydrocarbons, and in the Class 12 units on haloalkanes, alcohols, carbonyl compounds and amines. The IUPAC naming of coordination compounds is a separate topic, in the Class 12 unit Coordination Compounds.

What is the best way to learn IUPAC nomenclature?

The best way to learn IUPAC nomenclature is step by step: first the word roots for one to ten carbons (meth-, eth-, prop-, but- … dec-), then naming alkanes by the longest chain, lowest locants and alphabetical order, then the suffixes and priority order of functional groups, and last benzene derivatives. Practise both ways, naming a given structure and drawing the structure from a name, since a sufficiently specified name fixes the connectivity, with additional stereodescriptors needed when stereochemistry matters.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which is the correct IUPAC name in each pair (NCERT Exercise 8.5)? (a) 2,2-dimethylpentane or 2-dimethylpentane; (b) 2,4,7-trimethyloctane or 2,5,7-trimethyloctane; (c) 2-chloro-4-methylpentane or 4-chloro-2-methylpentane; (d) but-3-yn-1-ol or but-4-ol-1-yne.

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   (a) 2,2-Dimethylpentane: every methyl needs its own locant. (b) 2,4,7-Trimethyloctane: 2,4,7 is lower than 2,5,7 at the first difference. (c) 2-Chloro-4-methylpentane: the locants 2,4 tie, so chloro, first alphabetically, gets 2. (d) But-3-yn-1-ol: the –OH, which gives the suffix, gets the lowest number.

2. Q2. Name Cl₂CHCH₂OH (NCERT Exercise 8.4 (f)).

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   Alcohol on a two-carbon chain, numbered from the OH carbon: C1 carries OH, C2 carries two Cl. Name: 2,2-dichloroethan-1-ol (often written 2,2-dichloroethanol).

3. Q3. Write the condensed formulas of (a) 2,2,4-trimethylpentane and (b) hexanedial (from NCERT Exercise 8.7), and name the functional groups.

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   (a) (CH₃)₃CCH₂CH(CH₃)₂, an alkane with no functional group. (b) OHC(CH₂)₄CHO, six carbons with an aldehyde group (–CHO) at each end.