Acid-Catalyzed Hydration of Alkenes (Markovnikov Alcohols)

Treat an alkene with aqueous strong acid (H₃O⁺ or dilute H₂SO₄) and water adds across the double bond to give the Markovnikov alcohol. Protonation of the pi bond produces a carbocation; water attacks that cation, and deprotonation regenerates the acid catalyst. Because the key intermediate is a planar carbocation, 1,2-hydride or 1,2-alkyl shifts can occur before capture, and any newly formed stereocenter is produced as a racemate.

Introduction

Dilute mineral acid and water bring alkenes to Markovnikov alcohols through a classical E1-like manifold. Protonation chooses the double bond that gives the most stable carbocation; resonance (allylic or benzylic) overrides simple substitution counts. The carbocation can rearrange (1,2-hydride or 1,2-alkyl migration) if a more stable cation is available. Water traps the carbocation to form an oxonium ion, and deprotonation delivers the neutral alcohol while regenerating H₃O⁺. Because the intermediate carbocation is planar, water attack is not stereospecific—racemic mixtures form when new stereocenters arise.

Common aqueous acids: dilute H₂SO₄ (most common), aqueous H₃O⁺ generated in situ, or H₃PO₄ for acid-sensitive substrates. Avoid HCl or HBr (they deliver hydrohalogenation) and concentrated H₂SO₄ without water (sulfation or dehydration dominates).

In allylic/benzylic systems, the carbocation is resonance-stabilized; water can attack at more than one resonance site, so mixtures of regioisomeric alcohols are common.

Quick Summary

• Reagents: H₃O⁺, dilute H₂SO₄/H₂O, or aqueous H₃PO₄; use water or dilute acid as solvent.
• Outcome: Markovnikov alcohol (OH on the more substituted or resonance-stabilized carbon).
• Mechanism: Protonation → carbocation (optional 1,2-hydride or 1,2-alkyl shift) → H₂O attack → deprotonation regenerates H₃O⁺.
• Stereochemistry: Carbocation capture is not stereospecific; newly formed stereocenters are racemic unless a chiral environment biases attack.
• Competing pathways: E1 elimination (especially at higher temperature/low water), alkyl hydrogen sulfate formation under concentrated H₂SO₄, and acid-triggered polymerization for highly activated alkenes.

Mechanism

The pi bond donates to H⁺, the H-O bond breaks to water, and the carbocation forms on the more substituted/resonance-stabilized carbon. This step sets Markovnikov selectivity. This carbocation-forming step is rate-determining; subsequent water capture and deprotonation are faster.

Rearrangements happen after protonation and before nucleophilic capture. Hydride shifts are favored when a tertiary or resonance-stabilized site is adjacent; methyl or larger alkyl shifts occur when they deliver a markedly more stable cation.

Water is the nucleophile. Because the carbocation is planar, attack can come from either face, so new stereocenters form as racemic mixtures unless an existing stereocenter or chiral medium biases approach.

Water, sulfate, or another weak base removes the proton from oxonium, restoring the acid catalyst. Under concentrated H₂SO₄ and limited water, alkyl hydrogen sulfate (ROSO₃H) can form and persist until aqueous workup, which hydrolyzes it to the alcohol.

The completed product highlights the Markovnikov alcohol formed after any rearrangements and deprotonation.

Water attack on a planar carbocation is not stereospecific; expect racemic mixtures when a new stereocenter forms.

Mechanistic Checklist

• Protonation places H on the carbon that generates the more stable carbocation (Markovnikov rule). Resonance (allylic/benzylic) overrides simple substitution counts.
• Consider 1,2-hydride or 1,2-alkyl shifts at the carbocation stage; migrate only if stability increases. Ring expansion (e.g., cyclobutyl → cyclopentyl) is common.
• Water attack forms an oxonium; deprotonation restores H₃O⁺ and yields the alcohol.
• Stereochemistry: racemic when a new stereocenter forms; pre-existing stereocenters can deliver diastereomeric mixtures.
• Watch for competing pathways: E1 elimination under heat/low water, alkyl hydrogen sulfate with concentrated H₂SO₄, or cationic polymerization for highly activated alkenes (e.g., isobutene, styrene).

Worked Examples

Multiple Alkenes & Selectivity

Protonation targets the double bond that produces the most stable carbocation. Tertiary, benzylic, or allylic cations win; resonance stabilization outranks simple degree. When two sites are close in energy, expect mixtures. Conjugated systems (allylic/benzylic alkenes) can give resonance-delocalized cations, so water may attack at more than one position—predict mixtures unless one resonance form is clearly superior. Small rings often expand (cyclobutyl → cyclopentyl) via 1,2-alkyl shifts to relieve strain.

Practical Tips & Pitfalls

• Use dilute acid: Aqueous H₂SO₄ (≈1-5% v/v) or H₃PO₄; concentrated H₂SO₄ promotes ROSO₃H formation or dehydration.
• Temperature control: Higher temperatures and low water activity favor E1 elimination; cool, dilute conditions favor hydration.
• Polymerization risk: Highly activated alkenes (isobutene, styrene) can polymerize under strong acid—run cold and dilute or switch to oxymercuration.
• Functional-group sensitivity: Acetals, epoxides, and other acid-labile groups may react or decompose; choose milder methods when needed.
• Safety: Mixing concentrated H₂SO₄ with water is strongly exothermic—add acid to water with cooling and PPE. Concentrated acid can dehydrate alcohol products if not quenched promptly.

Exam-Style Summary

• Protonation → carbocation → (possible 1,2 shift) → water attack → deprotonation.
• Markovnikov OH placement because the more stable carbocation forms.
• Rearrangements occur at the carbocation stage; look for hydride or alkyl migrations.
• Products are racemic when new stereocenters appear.
• Competing pathways: E1 elimination (hot, low water) and sulfation/dehydration under concentrated H₂SO₄.
• Alternative when rearrangements must be avoided: oxymercuration-demercuration (Markovnikov, no rearrangement) or hydroboration-oxidation (anti-Markovnikov).

Interactive Toolbox

• Practice hydration vs. elimination calls in the Reaction Solver.
• Use the Mechanism Solver to visualize potential hydride/methyl shifts before water capture.
• Review alternative hydration routes: Oxymercuration-demercuration and Hydroboration-oxidation for anti-Markovnikov outcomes.

FAQ / Exam Notes

• Will rearrangements occur? Yes whenever a 1,2-hydride or 1,2-alkyl shift generates a more stable carbocation. Hydride > alkyl in migratory aptitude.
• How do I favor hydration over elimination? Keep the mixture dilute in acid and water, run cool, and avoid extended heat. Switch to oxymercuration if rearrangement-free hydration is needed.
• Why do I sometimes see sulfate esters? Concentrated H₂SO₄ or limited water forms ROSO₃H; aqueous workup hydrolyzes to the alcohol.
• What acids are commonly used? Dilute H₂SO₄ or H₃PO₄; HCl/HBr produce hydrohalogenation instead, and HNO₃/HClO₃ are oxidizing/nitrating.
• Polymerization concerns? Yes for highly activated alkenes; run cold/dilute or pick a different method.
• Related guides: Oxymercuration (Markovnikov, no rearrangement), Hydroboration-oxidation (anti-Markovnikov), Acid-catalyzed ether formation.