Reactions of alcohol with sulfuric acid?

Question

I knew two chemical reactions of alcohol with sulfuric acid 1. CH3CH2OH + H2SO4 -> CH2CH2 Here product is having a double bond (ethene) and this reaction happens at 443 K temperature. 2. CH3CH2OH + H2SO4 -> C2H5OC2H5 Here product is ether an happens at 413 K temperature. But today I came across another reaction. 3. C2H5OH + H2SO4 -> C2H5OSO2OH + H2O Now my question is when will this third reaction happen? Any specific temperature? The book from which I found this reaction doesn't describe anything. Just says: Alcohols react with inorganic acids (except halogen acids) to form inorganic esters.

Answer 1

The alkene formation after treatment with sulfuric acid at high temperatures is major product. In ambient or at least lower temperatures, even when elimination product (alkene) is desired and major product, an oxygen atom in sulfuric acid can act as a nucleophile producing the C2H5OSO2OH product but in very low yield (minor product--very minor). This happens after sulfuric acid donates a proton to the hydroxide on the alcohol causing water to be the leaving group, leaving behind a carbocation. The now negatively charged oxygen on sulfuric acid’s conjugate base acts as a nucleophile, substituting onto the carbocation via SN1. This is more likely with with secondary carbons, since primary carbons are highly unlikely to become cations, but, IN YOUR REACTION, it is possible that C2H5OH (primary carbon) went through a concerted mechanism (SN2) where the nucleophile attacked the carbon before the water acted as a leaving group, which does indeed require high temperatures for the higher than usual transition state. This is possible, but the C2H5OSO2OH would be in extremely low yield.