Electrophilic addition
HBr adds across ethylene (Markovnikov)
How the HBr adds across ethylene (Markovnikov) 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: HBr (gas or aq.). Conditions: CH₂Cl₂ or AcOH, 0–25 °C. Bonds formed: C–H σ; C–Br σ; bonds broken: C=C π; H–Br σ. Stereochemistry: Planar cation: Br⁻ attacks either face (anti and syn). Rate and selectivity: Rate = k[alkene][HBr]; more substituted alkene reacts faster; Markovnikov.
At a glance
Step 1: π bond attacks H–Br
The π electrons of the alkene's double bond are electron-rich and act as the nucleophile — they attack the electrophilic, slightly positive hydrogen of H–Br.
Step 2: Carbocation + bromide
As the π bond forms a new C–H bond, the H–Br pair leaves entirely onto bromine, giving a carbocation on the other carbon and a free bromide ion.
Step 3: Bromide captures the cation
Bromide's lone pair then attacks the carbocation, forming bromoethane. The net result: H and Br add across the double bond.
Abbreviations and terms
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- AcOH
- Acetic acid. Weak acid and polar protic solvent.
- HBr
- Hydrobromic acid. Strong acid; bromide is a good nucleophile.
- 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
| HBr adds across ethylene (Markovnikov) | Acid-catalyzed hydration of an alkene | Br₂ addition to an alkene (vicinal dihalide) | |
|---|---|---|---|
| Reagents | HBr (gas or aq.) | Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O | Br₂ |
| Conditions | CH₂Cl₂ or AcOH, 0–25 °C | 25–100 °C; heat and excess water favour the alcohol | CH₂Cl₂ or CCl₄, 25 °C, dark |
| Stereochemistry | Planar cation: Br⁻ attacks either face (anti and syn) | Planar carbocation: racemic; Markovnikov regiochemistry | Anti addition via bromonium; trans product (racemic) |
| Rate / selectivity | Rate = k[alkene][HBr]; more substituted alkene reacts faster; Markovnikov | Protonation of the alkene is rate-determining | Bromonium formation rate-limiting; fast |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the HBr adds across ethylene (Markovnikov)?
HBr (gas or aq.). Conditions: CH₂Cl₂ or AcOH, 0–25 °C.
Which bonds are formed and which are broken?
Formed: C–H σ; C–Br σ. Broken: C=C π; H–Br σ.
What is the stereochemical outcome?
Planar cation: Br⁻ attacks either face (anti and syn)
What controls the rate or the selectivity?
Rate = k[alkene][HBr]; more substituted alkene reacts faster; Markovnikov
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