Reactions of alkenes

Mechanism of electrophylic addition

Addition of a halogen

1. Non-polar halogen molecule

2. Induced dipole in halogen

3. Halogen acts as electrophile

4. Pi bond of C=C bond breaks

5. Forms dative covalent bond with positive end of Br molecule

6. Br-Br bond breaks - heterolytic fission

7. Carbocation intermediate formed

8. Lone pair of electrons on bromide ion forms dative covalent bond with the C of carbocation

Addition of a hydrogen halide (symmetrical alkene)

1. Permanent dipole in HBr

2. Pi bond of breaks

3. Forms dative covalent bond with H

4. HBr bond breaks - heterolytic fission

5. Carbocation intermediate

6. Lone pair of electrons on Br^- forms dative covalent bond with C of carbocation

Addition of a hydrogen halide (unsymmetrical alkene)

Same as symmetrical alkene except there are two possible carbocation intermediates

• This means a major and a minor product is formed

• Major product is formed from the most stable carbocation

How to determine stability of carbocation intermediate

The more alkyl groups the C is bonded to, the more stable

1. Primary carbocation = C⁺ bonded to 1 alkyl group

2. Secondary carbocation = C⁺ bonded to 2 alkyl groups

3. Tertiary carbocation = C⁺ bonded to 3 alkyl groups

What is the most stable type of carbocation?

Tertiary

Addition of steam to form alcohols

Conditions = 330*C, 200 atm, H₂O(g), phosphoric acid catalyst

1. H⁺ from H₃PO₄

2. Pi bond breaks

3. Forms dative covalent bond with H⁺

4. Carbocation intermediate

5. Lone pair of electrons on oxygen of H₂O forms dative covalent bond with C of carbocation

6. Heterolytic fission of O-H bond

7. H⁺ catalyst is regenerated