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Organic Chemistry

Topic 9 of 34

An alkene donates its pi electrons to an electrophile

For bromine in a non-aqueous solvent, a bromonium intermediate is opened by bromide.

A-Level 9476 (2026-2027)

An alkene donates its pi electrons to an electrophile

For bromine in a non-aqueous solvent, a bromonium intermediate is opened by bromide.

The ethene π cloud polarises an approaching Br2 molecule. Electron density is donated towards its δ+ bromine as the Br-Br bond breaks heterolytically. The chemically accurate intermediate is a bridged bromonium ion: one Br is bonded to both original alkene carbons and carries positive formal charge. The other bromine leaves as Br-.

Form the bromonium ion: follow three electron pairs

An arrow goes from the ethene pi bond to the nearer bromine. Another goes from the bromine-bromine bond to the farther bromine. A third goes from a lone pair on the nearer bromine to the other alkene carbon, closing the three-membered bridge.

Reactive electron pairs are shown, not every lone pair. Full arrowheads represent pairs. In CCl4, bromide is the nucleophile for the next step.

Bromide opens the bridge

The intermediate is a triangle with two CH2 carbons and a Br+ apex. A lone pair from Br- attacks one carbon from the opposite side, while that carbon-bromine bond transfers its pair back to the bridging Br. The carbon-carbon bond remains a single bond.

The product is BrCH2CH2Br, 1,2-dibromoethane. The two new C-Br bonds arise from opposite-face attack; for ethene no chiral product results.

Addition of HX follows a different intermediate: the π bond accepts H+ from H-X as the H-X bond pair moves to X, forming a carbocation; X- then donates a pair to the positive carbon. For propene + HBr, protonation that gives the secondary carbocation is favoured over the primary alternative, so 2-bromopropane is the major product.

Apply Markovnikov's rule by comparing intermediates
  1. Draw both protonation choices

    For CH3CH=CH2, one gives CH3-C+(H)-CH3, the other CH3CH2CH2+.

  2. Compare carbocation stability

    Alkyl groups donate electron density and stabilise the positive centre; secondary is generally more stable than primary.

  3. Add the halide to the preferred positive centre

    The major product is CH3CHBrCH3. H has added to the carbon that initially bore more H atoms.

Markovnikov's rule predicts the usual major product of HX addition to an unsymmetrical alkene under the stated polar mechanism. It does not mean that a minor product is impossible, nor should a free carbocation automatically be substituted for the bromonium intermediate in Br2 addition. Radical peroxide pathways are outside the named mechanism here.