9 min readConcept sheet1-mark MCQ and assertion-reason on classifying a halide; 2-mark questions on IUPAC names

Classification and nomenclature

Haloalkanes carry the halogen on an sp³ carbon; haloarenes carry it on an aromatic ring carbon. Classify them by the number of halogens and by the carbon that holds X.

HaloalkaneR-X: a halogen (X = F, Cl, Br, I) on an sp³ carbon of an alkyl group, e.g. CH₃CH₂Cl

HaloareneAr-X: a halogen on an sp² carbon of an aromatic ring, e.g. C₆H₅Cl

Allylic and benzylic halidesX on an sp³ carbon next to a C=C (CH₂=CHCH₂Cl) or next to a ring (C₆H₅CH₂Cl)

Vinylic halideX directly on a carbon of a C=C (sp²), e.g. CH₂=CHCl

What they are

Replace a hydrogen atom of an aliphatic hydrocarbon by a halogen and you get a haloalkane (alkyl halide). Replace a hydrogen on the ring of an aromatic hydrocarbon and you get a haloarene (aryl halide). The halogen is written X. Hybridisation tells you the shape around a carbon. An sp³ carbon is tetrahedral, with four single bonds. An sp² carbon is flat, as in a double bond or a benzene ring. In a haloalkane X sits on an sp³ carbon; in a haloarene it sits on an sp² carbon of the ring.

Ways to classify

Type	Where X sits	Example
Mono-, di-, tri-, polyhalogen	One, two, three or more X atoms	CH₃Cl, CH₂Cl₂, CHCl₃
Primary (1°) alkyl halide	sp³ carbon joined to one other carbon	CH₃CH₂CH₂Br
Secondary (2°) alkyl halide	sp³ carbon joined to two other carbons	(CH₃)₂CHBr
Tertiary (3°) alkyl halide	sp³ carbon joined to three other carbons	(CH₃)₃CBr
Allylic halide	sp³ carbon next to a C=C	CH₂=CH-CH₂Cl
Benzylic halide	sp³ carbon next to an aromatic ring	C₆H₅CH₂Cl
Vinylic halide	sp² carbon of a C=C	CH₂=CH-Cl
Aryl halide	sp² carbon of an aromatic ring	C₆H₅Cl
Gem-dihalide (alkylidene halide)	Both X on the same carbon	CH₃CHCl₂
Vic-dihalide (alkylene dihalide)	X on two neighbouring carbons	ClCH₂CH₂Cl

Common and IUPAC names

Formula	Common name	IUPAC name
CH₃CH₂CH₂CH₂Br	n-Butyl bromide	1-Bromobutane
CH₃CH₂CH(Cl)CH₃	sec-Butyl chloride	2-Chlorobutane
(CH₃)₂CHCl	Isopropyl chloride	2-Chloropropane
(CH₃)₂CHCH₂Cl	Isobutyl chloride	1-Chloro-2-methylpropane
(CH₃)₃CCl	tert-Butyl chloride	2-Chloro-2-methylpropane
CH₂=CHCl	Vinyl chloride	Chloroethene
CH₂=CHCH₂Br	Allyl bromide	3-Bromoprop-1-ene
C₆H₅CH₂Cl	Benzyl chloride	Chlorophenylmethane
CH₂Cl₂	Methylene chloride	Dichloromethane
CHCl₃	Chloroform	Trichloromethane
CCl₄	Carbon tetrachloride	Tetrachloromethane
CH₃CHCl₂	Ethylidene chloride	1,1-Dichloroethane
ClCH₂CH₂Cl	Ethylene dichloride	1,2-Dichloroethane
C₆H₄Cl₂ (Cl at 1 and 4)	p-Dichlorobenzene	1,4-Dichlorobenzene

Writing an IUPAC name

1
Pick the longest carbon chain that carries the halogen.
2
Number the chain so the substituents (halogens and alkyl groups alike) get the lowest locants (position numbers). If the chain has a C=C, the double bond gets the lowest number first.
3
If both ends give the same locants, the substituent that comes first in alphabetical order gets the lower number.
4
Write the prefixes (bromo, chloro, fluoro, iodo, methyl) in alphabetical order, each with its locant, then the parent name. Di- and tri- are ignored when you alphabetise.
5★
Example: CH₃CH(Cl)CH(CH₃)CH₃. The chain has 4 carbons. Either end gives locants 2 and 3, so chloro (c comes before m) takes 2. Name: 2-chloro-3-methylbutane.
6
Haloarenes: number the ring for the lowest locants. C₆H₄Cl₂ with Cl at 1 and 4 is 1,4-dichlorobenzene (common name p-dichlorobenzene).

★ key step

Allylic and benzylic vs vinylic and aryl

Allylic and benzylicX is on an sp³ carbon that sits next to a C=C or a ring. These halides are very reactive, because the carbocation they form is stabilised by resonance.

Vinylic and arylX is directly on an sp² carbon of the C=C or the ring. The C-X bond is stronger, and these halides resist substitution.

Ask one question: is the carbon that holds X itself part of the double bond or ring? If yes, it is vinylic or aryl.

In real life

Chloroethene (vinyl chloride) is the building block of PVC, used in the water pipes, wire coatings and floor mats found in most Indian homes.

Exam angle

2 marks: “Write the IUPAC names of (i) (CH₃)₃CCl and (ii) CH₂=CHCH₂Br.” (i) Three-carbon chain with Cl and CH₃ on C-2: 2-chloro-2-methylpropane. (ii) Number from the C=C end so the double bond gets locant 1: 3-bromoprop-1-ene.

1 marks: “Classify C₆H₅CH₂Br as an allylic, benzylic, vinylic or aryl halide.” Benzylic: Br is on an sp³ carbon that is attached to the benzene ring.

2 marks: “What are gem-dihalides and vic-dihalides? Give one example of each with its IUPAC name.” Gem: both halogens on the same carbon, CH₃CHCl₂, 1,1-dichloroethane. Vic: halogens on neighbouring carbons, ClCH₂CH₂Cl, 1,2-dichloroethane.

Remember it

On sp³ next door: allylic or benzylic. On sp² itself: vinylic or aryl. Gem means the same carbon (like twins); vic means neighbours.