Significance of conditions for hydration of alkene

Question

Here is what I have concluded (correct me if I am wrong) In hydration of alkenes we react alkenes(non polar) with sulphuric acid first(as direct reaction of water is not possible)~the cold and concentrated conditions favours maximum production of alkyl hydrogen sulphate without oxidizing alkenes.Then we react alkyl hydrogensulfate with boiling water (perhaps boiling water makes hydrogen sulphate a better LG?) and thus water reacts with it forming h30+ and OH- which then attacks on alkyl carbocation forming alcohol. –

Answer 1

Hi Mia,

Acid-catalyzed hydration of alkenes is best done in cold (or room temp.) conditions. This is because hydration is an addition reaction that decreases entropy, so by lowering T, we disfavor the entropy term of the Gibbs Free Energy equation: ΔG = ΔH – TΔS.

The mechanism is slightly different than how you describe it:

(1) Dilute acid reacts with itself: H₂SO₄ + H₂O –––> HSO₄^– + H₃O⁺

(2) The alkene (base) deprotonates H₃O⁺, forming the most stable carbocation intermediate and H₂O.

(3) H₂O (weak nucleophile) then returns to attack the carbocation (strong electrophile), forming a racemic mixture of protonated alcohols.

(4) Remaining H₂O (weak base) is then used to deprotonate the protonated alcohol, to form the neutral alcohol product.

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Conversely, concentrated sulfuric acid at high temperatures favors elimination products.

Hope that provides some clarity!