The treatment of alkyl chlorides with aqueous $K O H$ leads to the formation of alcohols but in the presence of alcoholic $K O H$ , alkenes are major products. Explain.

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Solution

$R - C l + K O H (aq) \to R - O H + K C l$

The ionization of aqueous KOH produces hydroxide ions which are strong nucleophiles. Hence, alkyl chlorides undergo substitution to form alcohol.

$R - C H_{2} - C H_{2} - C l + K O H (alc) \to R - C H = C H_{2} + K C l + H_{2} O$

Alcoholic KOH solution gives alkoxide ion which is a strong base. It abstracts $β$ hydrogen atom of alkyl chloride. A molecule of HCl is eliminated and an alkene is formed.
Note: The basicity of hydroxide ion is much lower than the basicity of alkoxide ion as hydroxide ion is significantly hydrated in aqueous solution.
Hence, hydroxide ion cannot abstract $β$ hydrogen atom of alkyl chloride.

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