The π electrons of an alkene attract electron-poor species. Two curly arrows make a carbocation, a third finishes the job, and the stability of that carbocation decides which product wins.
In electrophilic addition the π electrons of the C=C attack an electrophile. With Br2, the approaching π cloud induces a dipole in Br–Br; the π pair forms a bond to the Brδ+ atom while Br–Br breaks heterolytically, giving a carbocation and Br−, which then attacks the positive carbon. With HBr, Hδ+ is the electrophile. Alkyl groups push electron density towards a positive carbon (the positive inductive effect), so carbocation stability is tertiary > secondary > primary. With an unsymmetrical alkene the major product comes from the more stable carbocation: propene + HBr gives mainly 2-bromopropane.
Step through the mechanism in the simulator. Watch where each curly arrow starts: the first one leaves the π bond, not a bromine atom.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the reaction of ethene with bromine, the bromine molecule acts as:
Where does the first curly arrow in electrophilic addition start?
What is the intermediate in electrophilic addition?
Which carbocation is the most stable?
With an unsymmetrical alkene the electrophile can add to either carbon. The route through the more stable carbocation is faster, so its product dominates.
| Carbocation | Alkyl groups on C+ | Stability |
|---|---|---|
| primary, RCH2+ | 1 | least stable |
| secondary, R2CH+ | 2 | more stable |
| tertiary, R3C+ | 3 | most stable |
The first curly arrow must start from the C=C double bond (the π electrons) and point to the electrophile, not start at the bromine. Draw the carbocation with a full + charge on carbon, and the bromide ion with a lone pair and a − charge. The major product comes from the more substituted carbocation.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
What is the major product when propene reacts with hydrogen bromide?
Why is a secondary carbocation more stable than a primary one?
Ethene is bubbled into bromine water containing NaCl. Which product would NOT be expected?
What is the major product of 2-methylpropene with HBr?
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