Organic Chemistry - "Change", Part 7: Hydration of an Alkene to form a Tertiary Alcohol

 Greetings,

This post covers the electrophilic addition of water (hydration) to a branched n-alkene.  The reaction is regioselective in that the -OH group selectively adds to the #2 carbon of the branched n-alkene.  This reaction is also acid-catalyzed and occurs at low temperature (ambient).

Thermodynamics Explanation

Because this reaction is catalytic the activation energy is low compared to the non-catalyzed analogue of the same reaction. A relatively stable 3-degree carbocation is formed at the outset: The carbocation-intermediate formation for this reaction requires less heat energy; therefore occurs at a lower temperature. The learned reader may recall that a reaction intermediate is short-lived because its conversion to the final product requires very little additional energy.

Kinetics Explanation of the Reaction Mechanism

The initial reaction-pathway step, formation of the carbocation, is the slowest part of the overall mechanism. It is this initial step that limits the rate of the reaction - In fact, the overall rate of reaction is equal to the rate of carbocation formation.

A visual representation of the mechanism is provided below.

Keep in mind that the sulfuric acid is an aqueous solution, which explains the source of the water reactant.

The overall reaction, obtained by adding the three mechanism steps is:

(CH3)2CCH2 (Isobutylene) + H2O (Water) → (CH3)3COH (t-Butyl Alcohol)

That's all for this post.  As always, thank you for reading!