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NCERT Class XII Chemistry
Chapter - Haloalkanes and Haloarenes
Questions with Solutions

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Question : 20
Total: 31
The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain.
Solution:  
Formation of alcohols from the reaction between alkyl chlorides and aqueous KOH is an example of simple nucleophilic substitution.
CH3−CH2Cl+KOH
H2O
––––––––▶
CH3−CH2−OH+KCl
But when aqueous KOH is replaced by alcoholic KOH, alkenes are formed instead of alcohols due to elimination of HCl from an alkyl halide.
CH3−CH2Cl+KOH––––––––▶CH2=CH2
This can be exaplained if we consider the size of the nucleophile in the two reactions. In the aqueous medium the N
⊝
u
 is
⊝
O
H which is relatively small while in the alcoholic medium the N
⊝
u
 is C2
⊝
H
5 which is bulky in nature.
Thebulky N
⊝
u
 will always find it easier to abstract a proton than to attack a tetravalent carbon to produce a substitution product.
If C2
⊝
H
5 was to attack the carbon carrying halogen, steric repulsions would hinder the attack and prevent substitution product.
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