Electrophilic addition — acid-catalyzed
Acid-catalyzed hydration of an alkene
How the Acid-catalyzed hydration of an alkene works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O. Conditions: 25–100 °C; heat and excess water favour the alcohol. Bonds formed: C–H; C–O; O–H; bonds broken: C=C π. Stereochemistry: Planar carbocation: racemic; Markovnikov regiochemistry. Rate and selectivity: Protonation of the alkene is rate-determining.
At a glance
Step 1: π bond protonated
The alkene's π electrons attack a proton from the acid catalyst, forming the more stable (more substituted) carbocation — this is Markovnikov selectivity playing out through carbocation stability.
Step 2: Water attacks the cation
Water's oxygen lone pair attacks that carbocation directly, forming a protonated alcohol (oxonium ion).
Step 3: Deprotonation
Loss of a proton, regenerating the acid catalyst, gives the Markovnikov alcohol — mechanistically the exact reverse of acid-catalyzed alcohol dehydration to an alkene.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- Markovnikov
- Markovnikov’s rule. In HX addition, H goes to the carbon with more H’s so the more stable cation forms.
How it compares with related reactions
| Acid-catalyzed hydration of an alkene | Br₂ addition to an alkene (vicinal dihalide) | HBr adds across ethylene (Markovnikov) | |
|---|---|---|---|
| Reagents | Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O | Br₂ | HBr (gas or aq.) |
| Conditions | 25–100 °C; heat and excess water favour the alcohol | CH₂Cl₂ or CCl₄, 25 °C, dark | CH₂Cl₂ or AcOH, 0–25 °C |
| Stereochemistry | Planar carbocation: racemic; Markovnikov regiochemistry | Anti addition via bromonium; trans product (racemic) | Planar cation: Br⁻ attacks either face (anti and syn) |
| Rate / selectivity | Protonation of the alkene is rate-determining | Bromonium formation rate-limiting; fast | Rate = k[alkene][HBr]; more substituted alkene reacts faster; Markovnikov |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Acid-catalyzed hydration of an alkene?
Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O. Conditions: 25–100 °C; heat and excess water favour the alcohol.
Which bonds are formed and which are broken?
Formed: C–H; C–O; O–H. Broken: C=C π.
What is the stereochemical outcome?
Planar carbocation: racemic; Markovnikov regiochemistry
What controls the rate or the selectivity?
Protonation of the alkene is rate-determining
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go