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[IMAGE 75]

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Aryl halides are less reactive as compared to alkyl halides as the halogen atom in these compounds is directly attached with benzene ring and cannot be replaced by nucleophiles such as ${\mathrm{OH}}^{-}$, ${\mathrm{NH}}_2^{-}$, etc. In chlorobenzene, the electron pair of chlorine atom is in conjugation with $\pi$-electrons of benzene ring. Thus C - Cl bond acquires partial double bond character and is difficult to break.

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[IMAGE 80]

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[IMAGE 81]

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$m$-Bromoanisole gives only the respective meta substituted aniline. This is a substitution reaction which goes by an elimination-addition pathway.

[IMAGE 87]

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The reaction,

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}+\mathrm{NaCN} ⟶ {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{CN}+\mathrm{NaBr}$

follows ${\mathrm{S}}_{\mathrm{N}}2$ mechanism which is favoured by polar aprotic solvent i.e., N, N' -dimethylformamide (DMF),

[IMAGE 88]

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o-Substituted biphenyls are optically active as both the rings are not in one plane and their mirror images are non-superimposable.

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${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}+\mathrm{KOH} ⟶ {\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2+\mathrm{KBr}+{\mathrm{H}}_2\mathrm{O}$

Saturated compound is converted into unsaturated compound by removal of group of atoms hence, it is an elimination reaction.

[IMAGE 94]--------------

—Br group is replaced by —OH group hence, it is a substitution reaction.

[IMAGE 95]--------------

Addition of ${\mathrm{Br}}_2$ converts an unsaturated compound into a saturated compound hence, it is an addition reaction.

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[IMAGE 96]

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In case of optically active alkyl halides, $S_{N}1$ reaction is accompanied by racemisation. The carbocation formed in the slow step being $s{p}^2$ hybridised is planar and attack of nucleophile may take place from either side resulting in a mixture of products, one having the same configuration and other having inverted configuration.

The isomer corresponding to inversion is present in slight excess because $S_{N}1$ also depends upon the degree of shielding of the front side of the reacting carbon.