CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.2.8 Markovnikov's Rule

Alkenes: 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)

 

Hydrocarbons Inductive Effect, downloadable AS & A Level Chemistry revision notes

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

 

Hydrocarbons Step 1_Markovnikov(s) Rule, downloadable AS & A Level Chemistry revision notes

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

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

 

Hydrocarbons Step 2_Markovnikov(s) Rule, downloadable AS & A Level Chemistry revision notes

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

 

Hydrocarbons Step 3_Markovnikov(s) Rule, downloadable AS & A Level Chemistry revision notes

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 halogen halide (HX) to an alkene, the halogen ends up bonded to the most substituted carbon atom.

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