Isomerism in Alkenes (AQA A Level Chemistry)

Isomerism in Alkenes

Unsaturated compounds

• In unsaturated compounds, the groups attached to the C=C carbons remain fixed in their position

• This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a π bond

The presence of a π bond in unsaturated compounds restricts rotation about the C=C bond forcing the groups to remain fixed in their position and giving rise to the formation of geometrical isomers

Stability of Cations & Markovnikov's Rule

• Carbocations are positively charged carbon atoms with only three covalent bonds instead of four

• There are three types of carbocations: primary, secondary and tertiary

Inductive effect

• The alkyl groups attached to the positively charged carbon atoms are ‘electron donating groups’

• This is also known as the inductive effect of alkyl groups

• The inductive effect is illustrated by the use of arrowheads on the bonds

• The alkyl groups push electrons away from themselves towards the positively charged carbon

• This causes the carbocation to become less positively charged

• As a result of this, the charge is spread around the carbocation which makes it energetically more stable

• This means that tertiary carbocations are the most stable as they have three electron-donating alkyl groups which energetically stabilise the carbocation

• Due to the positive charge on the carbon atom, carbocations are electron-loving species (electrophiles)

Alkyl groups push electron density towards the carbocation making it energetically more stable; the more alkyl groups the carbocation is bonded to, the more stabilised it is

Markovnikov’s rule

• In addition reactions, an electrophile reacts with the double bond of alkenes

The electrophile reacts with the electron-rich C-C double bond

• The electrophile will add to the carbon to give the most stable carbocation

The most stable carbocation is the major product of the nucleophilic attack on the C-C double bond

• Therefore, the nucleophile will bond to the C-C carbon atom with the highest number of alkyl groups bonded to it

The nucleophile ends up to the most substituted C-C carbon atom

• This is also known as the Markovnikov’s rule which predicts the outcome of addition reactions and states that:

  • In an addition reaction of a hydrogen halide (HX) to an alkene, the halogen ends up bonded to the most substituted carbon atom.