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Question
Why is sulphuric acid not used during the reaction of alcohols with KI?
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Solution
\[\ce{2KI + 2H2SO4 -> 2KHSO4 + 2HI}\]
\[\ce{2HI + H2SO4 -> 2H2O + I2 + SO2}\]
In these reactions, H2SO4 is an oxidising agent. It oxidises the HI formed during the reaction to I2 and prevents the formation of alkyl halide by the reaction of HI and alcohol. To overcome this problem, phosphoric acid (H3PO4) is used instead of H2SO4, which provides HI for the reaction and does not give I2 like H2SO4.
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\[\begin{array}{cc}
\ce{H3C}\phantom{..................................}\\
\backslash\phantom{...........................}\\
\ce{CH - CH2 - CH3}\\
/\phantom{...........................}\\
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