13.4 Electrophilic addition in alkenes

define electrophile

an atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair

describe the steps of electrophilic addition between an alkene and hydrogen bromide

1) bromine is more electronegative than hydrogen so hydrogen bromide is polar

2) the electron pair in the pi-bond is attracted to the partially positive hydrogen atom causing the double bond to break

3) a bond forms between the hydrogen atom of the hydrogen bromide molecule and a carbon atom part of the c=c bond

4) The hydrogen bromide bond breaks by heterolytic fission. Bromine receives the electron pair

5) a Br^-and a carbocation are formed

6) the lone pair on Br^- transfers to the positive carbon on the carbocation to form the addition product

describe the steps of electrophilic addition between an alkene and a non-polar molecule bromine

1) bromine approaches the alkene and the pi-electrons interact with electrons in the Br-Br bond causing polarisation of the Br-Br bond. This is an induced dipole

2) the electron pair in the pi bond is attracted to the partially positive end of the molecule causing the double bond to break

3) a bond forms between one of the C atoms of the double bond and a bromine atom

4) Br-Br bond breaks by heterolytic fission

5) Br^- and carbocation are formed

6) Br^- reacts with the carbocation to form the addition product

Markownikoff’s rule

when a hydrogen halide reacts with an unsymmetrical alkene. the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms. a tertiary carbocation will be formed because it is more stable

define primary carbocation

carbocation where the positive charge is on a carbon atom at the end of a chain

define secondary carbocation

carbocation where the positive charge is on a carbon atom with two carbon chains attached

explain why carbocation stability increase with the increased number of alkyl groups

1) each alkyl group donates and pushes electrons towards the positive charge of the carbocation

2) the positive charge is spread over the alkyl groups

3) more alkyl groups attached to the positive carbon atom = the more the charge is spread out = the more stable the ion is