Acid Catalyzed Hydration: Mechanism & Rearrangements | Chemca

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Acid Catalyzed Hydration of Alkenes

By Chemca Editorial Team • Last Updated: January 2026 • 9 min read

Acid Catalyzed Hydration is the electrophilic addition of water to an alkene in the presence of an acid catalyst (usually dilute $H_2SO_4$) to form an alcohol. This reaction follows Markovnikov's Rule and is characterized by the formation of a Carbocation intermediate.

1. General Reaction

The acid acts as a catalyst, providing protons to initiate the reaction. Water acts as the nucleophile.

$$ R-CH=CH_2 + H_2O \xrightarrow{H^+} \underbrace{R-CH(OH)-CH_3}_{\text{Markovnikov Alcohol}} $$

Conditions:

• Reagent: Dilute Sulfuric Acid ($H_2SO_4$) or Phosphoric Acid ($H_3PO_4$).
• Regiochemistry: Markovnikov Addition.
• Intermediate: Carbocation ($C^+$).

2. Detailed Mechanism

The mechanism involves three distinct steps.

Step 1: Protonation (Rate Determining Step)

The pi-electrons of the alkene attack the proton ($H^+$) supplied by the acid. The proton attaches to the carbon with more hydrogens (Markovnikov's rule) to form the most stable carbocation.

$$ R-CH=CH_2 + H^+ \xrightarrow{\text{Slow}} R-\overset{+}{C}H-CH_3 $$

Step 2: Nucleophilic Attack

Water acts as a nucleophile and attacks the electron-deficient carbocation.

$$ R-\overset{+}{C}H-CH_3 + H_2O \rightarrow R-CH(\overset{+}{O}H_2)-CH_3 $$

Step 3: Deprotonation

A water molecule abstracts a proton from the oxonium ion to yield the alcohol and regenerate the acid catalyst.

$$ R-CH(\overset{+}{O}H_2)-CH_3 \rightarrow R-CH(OH)-CH_3 + H^+ $$

3. Rearrangements (The Critical Concept)

Exam Alert!

Since a Carbocation is formed, the reaction is subject to rearrangements (Hydride shift, Methyl shift, Ring expansion) to form a more stable carbocation before water attacks.

Example: Hydration of 3,3-Dimethyl-1-butene

Expected: 3,3-Dimethyl-2-butanol (Simple Markovnikov).
Actual Major Product: 2,3-Dimethyl-2-butanol (Rearranged).

Mechanism:

1. Formation of $2^\circ$ carbocation: $(CH_3)_3C-\overset{+}{C}H-CH_3$.
2. 1,2-Methyl Shift: A methyl group migrates to form a more stable $3^\circ$ carbocation: $(CH_3)_2\overset{+}{C}-CH(CH_3)_2$.
3. Water attacks the $3^\circ$ carbocation.

4. Stereochemistry

The carbocation intermediate is planar ($sp^2$ hybridized). The nucleophile (water) can attack from either the top or the bottom face with equal probability.

• If a chiral center is generated, a Racemic Mixture (pair of enantiomers) is obtained.
• The reaction is Not Stereoselective.

5. Comparison

Feature	Acid Catalyzed Hydration	Oxymercuration-Demercuration
Regiochemistry	Markovnikov	Markovnikov
Rearrangement	Possible (Carbocation)	No (Mercurinium Ion)
Intermediate	Carbocation	Cyclic Ion

Acid Hydration Quiz

Test your concepts on Markovnikov addition and Rearrangements. 10 MCQs with explanations.

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