All subjects · Chemistry · WCH11 (Unit 1)

Addition reactions of alkenes

Alkenes undergo addition across the C=C. Many of these reactions go by electrophilic addition, and the stability of the carbocation decides the major product.

Reactions and conditions

Hydrogen: nickel catalyst, about 150 °C: C2H4 + H2 -> C2H6 (hydrogenation)
Halogens: room temperature: C2H4 + Br2 -> CH2BrCH2Br
Hydrogen halides: room temperature: C2H4 + HBr -> CH3CH2Br
Steam: phosphoric acid catalyst, about 300 °C and 60 atm: C2H4 + H2O -> C2H5OH
Acidified potassium manganate(VII): oxidation to a diol; purple to colourless: ethene gives ethane-1,2-diol

Test for unsaturation

Shake with bromine water: the orange colour is decolourised by an alkene. Alkanes do not react (in the dark).

Electrophilic addition mechanism

With Br2: the π electrons repel the electrons in Br2, inducing a dipole (Brδ+ - Brδ-). A curly arrow goes from the C=C to the δ+ Br, and the Br-Br bond breaks heterolytically. A carbocation forms, and the Br- ion then donates a lone pair to the positive carbon.

With HBr the same pattern applies, with H as the δ+ end. Curly arrows always start from a bond or a lone pair and point to where the electrons go.

Which product?

Carbocation stability: tertiary > secondary > primary, because alkyl groups push electron density towards the positive carbon. So propene + HBr gives mainly 2-bromopropane (via the secondary carbocation CH3C+HCH3), with a little 1-bromopropane.

Worked example

Explain why propene and HBr give mainly 2-bromopropane.

H+ can add to either carbon of C=C.
Adding to the end carbon gives a secondary carbocation, CH3C+HCH3; adding to the middle gives a primary one.
The secondary carbocation is more stable because two alkyl groups release electrons to it.

Answer: The more stable secondary carbocation forms more often, so Br- adds to C2.

Key idea: Alkenes: H2/Ni, X2, HX, steam/H3PO4, KMnO4 to a diol. Bromine water turns colourless. Electrophilic addition goes via the most stable carbocation.

Check yourself

What is seen when bromine water is shaken with cyclohexene?
- Colourless to orange
- Orange to colourless
- Purple to colourless
- Orange to green
Show answer
Orange to colourless. Br2 adds across the C=C and is used up.
Which catalyst is used to hydrogenate an alkene?
- Phosphoric acid
- Iron
- Zeolite
- Nickel
Show answer
Nickel. Hydrogenation uses a nickel catalyst at about 150 °C.
What is the major product of propene with HBr?
- 1-bromopropane
- 2-bromopropane
- 1,2-dibromopropane
- Propan-2-ol
Show answer
2-bromopropane. It goes via the more stable secondary carbocation.
What type of species attacks the C=C in electrophilic addition?
- An electron-pair donor
- A free radical
- An electron-pair acceptor
- A hydroxide ion
Show answer
An electron-pair acceptor. An electrophile accepts a pair of electrons from the π bond.
Which carbocation is the most stable?
- (CH3)3C+
- CH3CH2+
- CH3+
- CH3CH2CH2+
Show answer
(CH3)3C+. The tertiary carbocation has three electron-releasing alkyl groups.
Ethene reacts with acidified potassium manganate(VII). What is the organic product?
- Ethanol
- Ethane
- Ethane-1,2-diol
- Ethanoic acid
Show answer
Ethane-1,2-diol. Two OH groups add across the double bond; purple turns colourless.
Which conditions are used to make ethanol from ethene and steam?
- Phosphoric acid catalyst, about 300 °C, about 60 atm
- Nickel catalyst, 150 °C
- UV light, room temperature
- Yeast, 35 °C
Show answer
Phosphoric acid catalyst, about 300 °C, about 60 atm. Hydration uses H3PO4 at high temperature and pressure.

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Last updated 7 November 2026. Spotted a mistake? Tell us.