Addition to Unsymmetrical Alkenes (HL) (DP IB Chemistry): Revision Note

Addition to unsymmetrical alkenes

• A carbocation is a positively charged carbon atom with only three covalent bonds instead of four

• There are three types of carbocations:

  • Primary

  • Secondary

  • Tertiary

Inductive effect

• The alkyl groups attached to the positively charged carbon are electron-donating groups

  • This is known as the inductive effect

• Arrowheads show how alkyl groups push electron density towards the positively charged carbon:

  • Reducing the positive charge on the carbocation

  • Spreading the positive charge over the molecule

• This makes the carbocation more stable

Primary, secondary and tertiary carbocations

• Tertiary carbocations are the most stable because they have three alkyl groups helping to stabilise the positive charge

• Due to their positive charge, carbocations act as electrophiles

Markovnikov’s rule

• Markovnikov’s rule predicts the outcome of electrophilic addition reactions and states that:

  • In electrophilic addition of a hydrogen halide (HX) to an alkene, the halogen bonds to the more substituted carbon atom

  • In addition reactions involving molecules with two different halogen atoms (e.g. ICl), the more electronegative halogen bonds to the more substituted carbon

• This rule applies to unsymmetrical alkenes such as propene and but-1-ene

• Electrophilic addition occurs when an electrophile reacts with the C=C double bond (as discussed previously)

• The mechanism is slightly different for unsymmetrical alkenes, e.g. propene + hydrogen bromide

Step 1: Formation of the carbocation intermediate

• The electrophile can add to either carbon in the double bond, forming one of two possible carbocation intermediates

• If it adds to the less substituted carbon:

  • A more stable carbocation forms

  • This will form the major product

• If it adds to the more substituted carbon:

  • A less stable carbocation forms

  • This will form the minor product

Step 2: Nucleophilic attack on the carbocation

• The nucleophile (e.g. Br⁻) attacks the positively charged carbon in the carbocation intermediate

• This leads to the formation of either the major or minor product, depending on the stability of the intermediate

Summary: Major and minor product formation

• The following diagram summarises the full electrophilic addition mechanism of HBr to propene

  • This includes both major and minor product pathways