Electrophilic addition to alkenes

Last time

Before we start: last time, in Functional groups, formulae and structural isomers, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

The double bond is an electron-rich target

Shake bromine water with ethene and the orange colour vanishes at once. Shake it with ethane and nothing happens. The difference is the C double bond C in ethene, rich in electrons, which attacks any electron-poor species that comes close. That is electrophilic addition, the characteristic reaction of alkenes.

Key terms

An addition reaction is a reaction in which two reactants combine to form a single product. An electrophile is an electron-pair acceptor. A nucleophile is an electron-pair donor. Heterolytic fission is the breaking of a covalent bond where one atom takes both electrons, forming a positive and a negative ion. A carbocation is a positive ion with a positively charged carbon atom.

The mechanism

The C double bond C pi electrons attack the delta plus hydrogen of HBr. A curly arrow goes from the double bond to the hydrogen. The H–Br bond breaks heterolytically. A second curly arrow goes from the bond to the bromine. A carbocation and a bromide ion form. The bromide ion uses a lone pair to attack the carbocation. A curly arrow goes from the lone pair to the C plus. Show dipoles, charges and lone pairs, and curly arrows starting from a bond or a lone pair, never from an atom.