Nomenclature of haloalkanes and haloarenes

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NCERT Class 12 Chemistry Chapter 6 Section 6.2

Quick answer

Common names of haloalkanes give the alkyl group and then the halide, as in n-propyl bromide, while IUPAC names treat them as halosubstituted hydrocarbons with a locant, as in 1-bromopropane. For benzene with one halogen both names agree; with two, the common system uses o-, m- and p- and IUPAC uses 1,2-, 1,3- and 1,4-. Dihalides with both halogens on one carbon are gem-dihalides (alkylidene halides) and those on adjacent carbons vic-dihalides (alkylene dihalides); IUPAC names both as dihaloalkanes, such as 1,1- and 1,2-dichloroethane.

With your textbook

NCERT Chemistry Part II (Class 12), Chapter 6 “Haloalkanes and Haloarenes”, Section 6.2 “Nomenclature”, pages 161–163.

Open the official NCERT chapter (PDF, 4.3 MB, ncert.nic.in) (opens in a new tab)

Checked against the 2026-27 reprint on 19 Oct 2026.

How are haloalkanes and haloarenes named?

Naming haloalkanes and haloarenes

Haloalkanes have common names built from the alkyl group and the halide, and IUPAC names that treat them as hydrocarbons with halogen substituents (NCERT Section 6.2, page 161). NCERT puts the IUPAC rule in one line:

In the IUPAC system of nomenclature, alkyl halides are named as halosubstituted hydrocarbons.

NCERT Chemistry Part II (Class 12), Section 6.2, page 161

In plain words: the common name is two words, “alkyl + halide” (ethyl bromide), while the IUPAC name reads like a substituted hydrocarbon, “locant + halo + parent” (bromoethane). The parent chain, its numbering and the alphabetical order of substituents follow the rules of IUPAC nomenclature from Class 11.

Common and IUPAC names

Common names name the alkyl group, then the halide: n-propyl bromide, isopropyl chloride (NCERT page 161).
IUPAC names use fluoro, chloro, bromo or iodo as a prefix with a locant: 1-bromopropane, 2-chloropropane (NCERT page 161).
Benzene derivatives. For one halogen on benzene the common and IUPAC names are the same (bromobenzene). For two halogens the common system uses o-, m- and p-, while IUPAC uses the locants 1,2-, 1,3- and 1,4-: m-dibromobenzene is 1,3-dibromobenzene, and sym-tribromobenzene is 1,3,5-tribromobenzene (NCERT page 161).

NCERT’s examples, common name first, then IUPAC (NCERT page 161, and Table 6.1 on page 162):

CH₃CH₂CH₂Br: n-propyl bromide; 1-bromopropane
(CH₃)₂CHCl: isopropyl chloride; 2-chloropropane
(CH₃)₂CHCH₂Cl: isobutyl chloride; 1-chloro-2-methylpropane
CH₃CH₂CH(Cl)CH₃: sec-butyl chloride; 2-chlorobutane
(CH₃)₃CCH₂Br: neo-pentyl bromide; 1-bromo-2,2-dimethylpropane
(CH₃)₃CBr: tert-butyl bromide; 2-bromo-2-methylpropane
CH₂=CHCl: vinyl chloride; chloroethene
CH₂=CHCH₂Br: allyl bromide; 3-bromopropene
CH₃CH₂CH₂F: n-propyl fluoride; 1-fluoropropane
CH₃C₆H₄Cl with Cl and CH₃ side by side: o-chlorotoluene; 1-chloro-2-methylbenzene or 2-chlorotoluene
C₆H₅CH₂Cl: benzyl chloride; chlorophenylmethane
CH₂Cl₂, CHCl₃, CHBr₃, CCl₄: methylene chloride, chloroform, bromoform, carbon tetrachloride; dichloromethane, trichloromethane, tribromomethane, tetrachloromethane

Geminal and vicinal dihalides

Dihaloalkanes with the same halogen are named in the common system as alkylidene or alkylene dihalides (NCERT page 161).
Geminal (gem) dihalides have both halogens on the same carbon; their common names are alkylidene halides. CH₃CHCl₂ is ethylidene chloride, 1,1-dichloroethane (NCERT page 162).
Vicinal (vic) dihalides have the halogens on adjacent carbons; their common names are alkylene dihalides. ClCH₂CH₂Cl is ethylene dichloride, 1,2-dichloroethane (NCERT page 162).
In the IUPAC system both are named as dihaloalkanes (NCERT page 162).

Solved examples

Example 1: The eight isomers of C<sub>5</sub>H<sub>11</sub>Br (NCERT Example 6.1)

Name the eight structural isomers of C₅H₁₁Br and classify each as 1°, 2° or 3°.

Answer (NCERT pages 162–163):

CH₃CH₂CH₂CH₂CH₂Br: 1-bromopentane, 1°
CH₃CH₂CH₂CH(Br)CH₃: 2-bromopentane, 2°
CH₃CH₂CH(Br)CH₂CH₃: 3-bromopentane, 2°
(CH₃)₂CHCH₂CH₂Br: 1-bromo-3-methylbutane, 1°
(CH₃)₂CHCHBrCH₃: 2-bromo-3-methylbutane, 2°
(CH₃)₂CBrCH₂CH₃: 2-bromo-2-methylbutane, 3°
CH₃CH₂CH(CH₃)CH₂Br: 1-bromo-2-methylbutane, 1°
(CH₃)₃CCH₂Br: 1-bromo-2,2-dimethylpropane, 1°

The three C₅ skeletons, pentane, 2-methylbutane and 2,2-dimethylpropane, have 3, 4 and 1 distinct carbons for the bromine (the central carbon of 2,2-dimethylpropane has no H): 3 + 4 + 1 = 8 (reasoned).

Example 2: Lowest locants, then the alphabet

Name (i) (CH₃)₂CHCH₂CHClCH₂CH₃ and (ii) CH₃CH₂CH(CH₃)CH₂CHBrCH₂CH₃.

Answer: (i) Six-carbon chain: {2, 4} from the isopropyl end, {3, 5} from the other. The lowest set wins even though Cl gets the higher number, because a halogen has no priority over an alkyl group: 4-chloro-2-methylhexane. (ii) {3, 5} from either end. On a tie, the prefix first in alphabetical order gets the lower number: 3-bromo-5-methylheptane (reasoned from NCERT Class 11 Chemistry, Unit 8).

Example 3: Naming bromoalkenes (NCERT Example 6.2)

Name CH₃CH=CHCH(Br)CH₃, CH₂=C(CH₃)CH(Br)CH₃ and CH₂=C(CH₃)CH₂Br.

Answer: Halogens are only prefixes, so the C=C takes the lowest locant first: 4-bromopent-2-ene, 3-bromo-2-methylbut-1-ene, 3-bromo-2-methylpropene. NCERT’s other three are 4-bromo-3-methylpent-2-ene, 1-bromo-2-methylbut-2-ene and 1-bromobut-2-ene (NCERT page 163). So allyl bromide is 3-bromopropene, not 1-bromoprop-2-ene.

Common mistakes to avoid

Common mistake

Reading the -ene in common names

Ethylene dichloride, ClCH₂CH₂Cl, has no double bond: it is 1,2-dichloroethane. Methylene chloride, CH₂Cl₂, has both Cl on one carbon, even though its name follows the alkylene pattern of vic-dihalides (NCERT page 162, Table 6.1).

Common mistake

Confusing gem and vic

Gem means both halogens on one carbon and vic on neighbouring carbons, wherever they are: CH₃CCl₂CH₃ (2,2-dichloropropane) is gem and CH₃CHClCHClCH₃ (2,3-dichlorobutane) is vic (reasoned from NCERT page 162).

Quick revision summary

Common names: alkyl group + halide (n-propyl bromide); IUPAC: locant + halo + parent (1-bromopropane).
Benzene: one halogen, the same name both ways; two halogens, o-, m-, p- (common) or 1,2-, 1,3-, 1,4- (IUPAC).
gem-dihalides (same carbon): alkylidene halides; vic-dihalides (adjacent carbons): alkylene dihalides; IUPAC: dihaloalkanes (1,1- and 1,2-dichloroethane).
C_₅H_₁₁Br has 8 structural isomers: four 1°, three 2° and one 3°.

Formula sheet

CH_₃CHCl_₂: ethylidene chloride = 1,1-dichloroethane
ClCH_₂CH_₂Cl: ethylene dichloride = 1,2-dichloroethane

Questions students ask

What are the IUPAC names of chloroform and iodoform?

The IUPAC name of chloroform, CHCl₃, is trichloromethane, and that of iodoform, CHI₃, is triiodomethane (NCERT Table 6.1, page 162, and Section 6.8.3, page 187). The parent is methane and tri- counts the three halogen atoms; no locants are needed, because a one-carbon chain has only one position. When the halogens differ, they are cited in alphabetical order, so freon 12, CCl₂F₂, is dichlorodifluoromethane (reasoned from the naming rules of NCERT Class 11 Chemistry, Unit 8).

What is the IUPAC name of DDT?

With ethane as the parent, DDT, (ClC₆H₄)₂CH–CCl₃, is named 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane, a systematic name beyond the NCERT text that PubChem lists among its synonyms; bis- replaces di- because 4-chlorophenyl is itself a substituted group (reasoned). PubChem’s own computed IUPAC name takes benzene as the parent instead, so names of both styles are in use. NCERT gives only the common name, p,p’-dichlorodiphenyltrichloroethane (NCERT page 188).

Why does benzyl chloride have two IUPAC names?

Benzyl chloride, C₆H₅CH₂Cl, can be named from either part of the molecule: chlorophenylmethane, the name in NCERT Table 6.1 (page 162), takes methane as the parent with chloro and phenyl prefixes, while (chloromethyl)benzene takes benzene as the parent with a chloromethyl group. Beyond the NCERT text, PubChem lists both names, and its own computed IUPAC name, chloromethylbenzene, uses the benzene parent. In an NCERT-based answer, write chlorophenylmethane, the name the book uses.

What is the IUPAC name of p-chlorotoluene?

The IUPAC name of p-chlorotoluene, with Cl and CH₃ opposite each other on the ring, is 1-chloro-4-methylbenzene, also written 4-chlorotoluene; m-chlorotoluene is 1-chloro-3-methylbenzene, or 3-chlorotoluene (reasoned from NCERT Table 6.1, page 162, which names the ortho isomer 1-chloro-2-methylbenzene or 2-chlorotoluene). With benzene as the parent, chloro and methyl are both prefixes and get the same pair of locants whichever group is counted first, so the alphabet decides and chloro takes 1. With toluene as the parent, the ring carbon that carries CH₃ is C-1.

Are haloalkanes and alkyl halides the same?

Yes, haloalkanes and alkyl halides are the same compounds, R–X, named in two styles: “alkyl halide” follows the common-name pattern of alkyl group then halide, as in methyl iodide, and “haloalkane” the IUPAC pattern of a halo prefix on the alkane, as in iodomethane (NCERT pages 159 and 161). Haloarenes and aryl halides are likewise one family, and NCERT calls them all organohalogen compounds (page 159). Beyond the NCERT text, halogenoalkane is a longer form of the same name (Wikipedia, “Haloalkane”).

Practice questions

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

Q1. Write structures of (i) 2-chloro-3-methylpentane, (ii) 1,4-dibromobut-2-ene and (iii) 1-chloro-4-ethylcyclohexane. (NCERT Intext Question 6.1, parts (i), (iv) and (ii).)

Show answerHide answer

(i) CH₃CHClCH(CH₃)CH₂CH₃. (ii) BrCH₂CH=CHCH₂Br. (iii) A cyclohexane ring with Cl on one carbon and C₂H₅ on the carbon opposite it (positions 1 and 4) (reasoned).
Q2. Give the common and IUPAC names of CH₃CH₂CH(Br)CH₃ and CH₂=CHCH₂Cl.

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sec-Butyl bromide, 2-bromobutane; allyl chloride, 3-chloropropene (reasoned from NCERT Table 6.1).