Nomenclature, Classification & Isomerism

CHEClassification & Preparation

This subtopic fixes the vocabulary of the chapter — how halogen compounds are classified, named by IUPAC, and how they show structural and optical isomerism. Getting these conventions right protects easy marks across every other subtopic.

Number of optical isomers

\text{isomers} = 2^n

n

= number of dissimilar chiral (asymmetric) carbons; fewer if a meso form exists

Structural isomers of

C_4H_9Br

C_4H_9Br:\; 1\text{-bromobutane},\; 2\text{-bromobutane},\; 1\text{-bromo-2-methylpropane},\; 2\text{-bromo-2-methylpropane}

four isomers; only 2-bromobutane has a chiral carbon and is optically active

Classify by the carbon bearing X: $1^\circ$ (that C is attached to one other carbon), $2^\circ$ (two), $3^\circ$ (three).
By position relative to unsaturation: allylic (X on the carbon next to $C{=}C$ ), vinylic (X on the $C{=}C$ carbon), benzylic (X on the carbon next to a ring), aryl (X on the ring carbon).
IUPAC: treat the halogen as a substituent (prefix fluoro / chloro / bromo / iodo), apply the lowest-locant rule, and list substituents alphabetically.
Count of halogens: mono-, di-, tri-, poly-halogen compounds.
A molecule with a chiral carbon (four different groups) shows optical isomerism.
Dihalides: gem- (both X on the same carbon) versus vic- (on adjacent carbons).
$(CH_3)_3C\text{-}CH_2Br$ is 1-bromo-2,2-dimethylpropane (neopentyl bromide); it is a primary halide because the C bearing Br is joined to only one other carbon.
Allyl, vinyl, benzyl and aryl are derived from the parent group: $CH_2{=}CHCl$ is vinyl/chloroethene, $C_6H_5CH_2Br$ is benzyl bromide, $CH_2{=}CHCH_2Cl$ is allyl chloride, $C_6H_5Br$ is bromobenzene (aryl).
The longest chain containing the carbon attached to the halogen is the parent; number it to give the halogen (with other substituents) the lowest set of locants.
Structural isomerism in haloalkanes can be chain isomerism (e.g. 1-bromobutane vs 1-bromo-2-methylpropane) or position isomerism (e.g. 1-bromobutane vs 2-bromobutane).
An $sp^2$ (vinylic, aryl) carbon cannot be a chiral centre; optical activity needs an $sp^3$ carbon with four different groups.
Common-name conventions still appear: methylene chloride ( $CH_2Cl_2$ ), chloroform ( $CHCl_3$ ), carbon tetrachloride ( $CCl_4$ ).

Class	The C–X carbon is…	Example	Reactivity note
Primary ( $1^{\circ}$ )	attached to one other carbon	$\mathrm{CH_3CH_2CH_2Cl}$	Favours $S_N2$
Secondary ( $2^{\circ}$ )	attached to two other carbons	$\mathrm{(CH_3)_2CHCl}$	Either mechanism
Tertiary ( $3^{\circ}$ )	attached to three other carbons	$\mathrm{(CH_3)_3CCl}$	Favours $S_N1$
Vinylic	$sp^2$ , part of a $\mathrm{C{=}C}$	$\mathrm{CH_2{=}CHCl}$	Very unreactive — resonance
Allylic	$sp^3$ , next to a $\mathrm{C{=}C}$	$\mathrm{CH_2{=}CHCH_2Cl}$	Very reactive — stable carbocation
Aryl	$sp^2$ , in the ring	$\mathrm{C_6H_5Cl}$	Very unreactive — resonance

Classification is fixed by the carbon directly bonded to the halogen — nothing else about the molecule matters.

Where the marks go

Calling neopentyl bromide tertiary because of the bulky $(CH_3)_3C$ group — it is primary; classification depends only on the carbon directly bearing the halogen.
Numbering the chain from the wrong end and giving the halogen a higher locant than necessary.
Confusing gem- and vic-dihalides, or allylic with vinylic (allylic X is next to, not on, the double bond).
Declaring 1-chlorobutane or 2-chloropropane chiral; neither has four different groups on one carbon, so neither is optically active.
Listing substituents by size or position rather than alphabetically when assigning locants in an IUPAC name.

How the board asks it

Conversioniupac naming of haloalkanes with lowest-locant and alphabetical rules
Write the IUPAC name of $(CH_3)_3C\text{-}CH_2Br$ and of $CH_2{=}CH\text{-}CH_2Cl$ , and draw the structure of $1$ -bromo- $4$ -sec-butyl- $2$ -methylbenzene.
Identify / classifyprimary/secondary/tertiary and allylic/vinylic/benzylic/aryl classification
Classify each of $C_6H_5CH_2Br$ , $CH_2{=}CHCl$ , $(CH_3)_3CBr$ and $C_6H_5Br$ as a $1^\circ$ , $2^\circ$ or $3^\circ$ halide and as allylic, vinylic, benzylic or aryl.
Identify / classifystructural isomers and chain versus position isomerism
Draw the structures and give the IUPAC names of all the structural isomers of $C_4H_9Br$ , and state which pair are chain isomers and which are position isomers.
Distinguishgem- versus vic-dihalides and allylic versus vinylic positions
Distinguish between a gem-dihalide and a vic-dihalide, and between an allylic and a vinylic halide, giving one structural example of each.
Give reasonsclassification fixed by the carbon directly bonded to the halogen
Account for the fact that neopentyl bromide, $(CH_3)_3C\text{-}CH_2Br$ , is classified as a primary halide even though it carries a bulky tert-butyl group.
Give reasonsoptical activity needs an sp3 carbon with four different groups
Identify which of $1$ -chlorobutane and $2$ -chlorobutane is optically active, and justify your choice in terms of a chiral carbon.

Practise this topic

NextPreparation & Conversions

Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.