Alkenes react with hydrogen halides (HCl, HBr, HI) at room temperature to form halogenoalkanes. The H adds to one carbon of the C=C and the halogen to the other:CH2=CH2 + HBr → CH3CH2Br (bromoethane) The halogen is more electronegative than hydrogen, so the H–X bond is polar, Hδ+–Xδ–, and the δ+ hydrogen is the electrophile. The reaction follows the two-step electrophilic addition mechanism: the C=C attacks the H, the H–X bond breaks heterolytically to give a carbocation and X–, then a lone pair on X– bonds to the positive carbon.
Ethene reacts with HCl in the same way to form chloroethane, CH3CH2Cl.Reactivity increases down the group: HI > HBr > HCl. The H–X bond must break in step 1, and bond enthalpy decreases as the halogen atom gets larger (H–Cl 432, H–Br 366, H–I 298 kJ mol–1). So HI, with the weakest bond, reacts fastest.Worked example: what forms when but-2-ene reacts with HBr? The C=C is between carbons 2 and 3 and the molecule is symmetrical, so whichever carbon the H joins, Br ends up on carbon 2. There is only one product: CH3CHBrCH2CH3, 2-bromobutane.Symmetrical alkenes such as ethene and but-2-ene give a single product. With an unsymmetrical alkene such as propene, two different products are possible; that is the subject of the next lesson.
Ethene reacts with HCl in the same way to form chloroethane, CH3CH2Cl.Reactivity increases down the group: HI > HBr > HCl. The H–X bond must break in step 1, and bond enthalpy decreases as the halogen atom gets larger (H–Cl 432, H–Br 366, H–I 298 kJ mol–1). So HI, with the weakest bond, reacts fastest.Worked example: what forms when but-2-ene reacts with HBr? The C=C is between carbons 2 and 3 and the molecule is symmetrical, so whichever carbon the H joins, Br ends up on carbon 2. There is only one product: CH3CHBrCH2CH3, 2-bromobutane.Symmetrical alkenes such as ethene and but-2-ene give a single product. With an unsymmetrical alkene such as propene, two different products are possible; that is the subject of the next lesson.