3.4 Alkenes

Alkenes are unsaturated hydrocarbons Alkenes contain a carbon- carbon double bond somewhere in their structure

CnH2n

Numbers need to be added to the name when positional isomers can occur

• C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond • π bonds are exposed and have high electron density • They are therefore vulnerable to attack by species which ‘like’ electrons: these species are called electrophiles

an electron pair acceptor The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions

A reaction where two molecules react together to produce one

The intermediate formed during electrophilic addition reactions of alkenes, which has a positive charge on the carbon atom

the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate

tertiary > secondary >primary

Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms

• There is restricted rotation around the C=C double bond • There are two different groups/atoms attached both ends of the double bond

• The atom with the bigger atomic number is classed as the priority atom

• First determine the priority groups on both sides of the double bond

a reaction where the molecule is split by the addition of water

• Electrophile attracted to double bond • Electrophiles are positively charged so accept electron pair from double bond • A positive ion (carbocation) is formed • A negatively charged ion forms a bond with the carbocation

Most stable. On the secondary or tertiary carbocation. You need to put the electrophile on the carbon closest to the end

less stable, on primary carbocation