1. Electrophilic addition 2
Hydrocarbons
2 January, 2019

2. Different Products
Notice that with the addition of HBr, 2 products are possible
The important thing to remember is that carbocation is always formed on the most substituted carbon atom.

3. Electrophilic addition with unsymmetrical alkenes
Hydrogen bromide with propene – two possible products H
CH3 C Br H
CH3 C
+ HBr
1-bromopropane H
CH3 C Br
2-bromopropane

4. Electrophilic addition with unsymmetrical alkenes
Can you draw the two possible mechanisms for hydrogen bromide with propene?

5. So what product do we get?
The amount of each product formed depends on the stability of the carbocations formed.
Each time there is an electrophilic addition reaction with an alkene, 3 possible carbocations are possible.
An R represents an alkyl group e.g. CH3, C2H5 etc R H C + R C + R H C +
Primary (1˚) carbocation
Secondary (2˚) carbocation
Tertiary (3˚) carbocation

6. Stability of Cations R H C + R H C + R C +
Primary (1˚) carbocation
Secondary (2˚) carbocation
Tertiary (3˚) carbocation
The alkyl groups are ‘electron releasing’, pushing their electrons towards the positive charge so they themselves become +.
This makes carbocations with more alkyl groups more stable and thus energetically favourable for the ion to bond to.

7. Stability of Cations R H C + R H C + R C +
Primary (1˚) carbocation
Secondary (2˚) carbocation
Tertiary (3˚) carbocation
More product will be made via the stable carbocation.

8. Hydrogen bromide with propene
CH3 C
primary
carbocation H
CH3 C + - + Br H H
CH3 C Br Br -
Minor product formed via primary carbocation
1-bromopropane

9. Hydrogen bromide with propene
CH3 C
secondary
carbocation H
CH3 C + - + Br H H
CH3 C Br Br -
Major product – formed via secondary carbocation
2-bromopropane

10. Knowledge check
Draw the mechanisms to show the reaction between hydrogen bromide and 2-methylbut-2-ene.
Name each product and identify the major product formed and why.
CH3 H C
+ HBr

11. Answers - CH3 H C tertiary carbocation H3C CH3 H C + - + Br H H3C
Major product – formed via tertiary carbocation
2-bromo-2-methylbutane

12. Answers - CH3 H C secondary carbocation H3C CH3 H C + - + Br H H3C
Minor product – formed via secondary carbocation
2-bromo-3-methylbutane

13. Knowledge Check
Complete the worksheet as best you can.