\(CH_{ 3 }Br+\underset { Ethylchloride }{ CH } -CI+KOH(aq)\rightarrow \underset { Ethanol }{ CH_{ 3 } } CH_{ 2 }OH+KCI\)

The reaction is an S_N2 reaction. In this reaction, CN- acts as the nucleophile and attacks the carbon atom to which Br is attacked. CN- is an ambident nucleophile and can attack through both C and N. In this case, it attacks through the C-atom.

Ambident Nucleophile- An anionic nucleophile, which has two nucleophilic centers, or two negative sites is known as an ambident nucleophile. This negative charge is delocalized due to resonance.
Example – Cyanide and Thiocyanate are examples of ambident nucleophiles. 
^-C≡N   ↔   :C=N^-
S=C=N^- ↔  ^-S - C≡ N

(i) 1-butanol
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(a) Vinyl chloride is unreactive in nucleophilic substitution reactions because of double bond character between  bond which is difficult to break.

(b) neo - Pentyl bromide is \({ 1 }^{ \circ }\) halide, forms \({ 1 }^{ \circ }\) carbocation which is less stable, changes \({ 3 }^{ \circ }\) to  carbocation which requires energy, therefore, undergoes nucleophilic substitution reaction slowly.
(c) 3-Bromocyclohexene forms allyi carbocation which is more stable than carbocation formed by 4-bromocyclohexene, therefore, it can undergo hydrolysis faster.

(i) The polyhalogen compound for which Paul Muller was awarded the Nobel Prize is DDT. (Dichlorodiphenyltrichloroethane). The structure of DOT is given below:
(ii) The use of DOT is banned in some countries because:
(a) it is toxic to aquatic life.
(b) it is not degraded rapidly by soil microorganisms due to its complex structure.
(c) it is not metabolized rapidly by animals and deposited in the fatty tissues.
(d) Because of its stability, it accumulates in food chain leading to biomagnification. 
(iii) Environmental protection and sensitivity towards life.

CFC (chlorofluoro carbons) are responsible for the destruction of ozone layer and UV light can reach earth which may cause skin cancer and affect flora and fauna.
These compounds should be banned and their use should be made to minimum extent.

(i) HOCI in presence of an acid generates the reactive electrophile, chloronium ion «r, which attacks benzene to give chlorobenzene (A).
(ii) Chlorobenzene (A) on treatment with NaNH2/liq. NH₃ undergaes-dehydrohalogenation via benzyne to afford aniline (B).
(iii) Aniline (B) on treatment with HBF4 forms the corresponding salt anilinium tetrafluoroborate (C) which on heating with NaNO₂ undergoes Balz-Schiemann reaction through the intermedium formation of benzenediazonium tetrafluoroborate to afford fluorobenzene (D).

tert-Alkyl halides prefer to undergo dehydrohalogenation in presence of a strong base such as Na metal instead of undergoing Wurtz reaction as explained below:
\(\underset{tert-Butyl\ bromide}{(CH_3)_3C-Br}+2Na\longrightarrow\underset{tert-Butylsodium}{(CH_3)_3Na^+}+Na^+Br^-\)
Thus, only rand 2° alkyl halides undergo Wurtz reaction while 3° alkyl halides prefer to undergo dehydrohalogenation to form alkenes.