CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.11 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.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top