2A4 Alkenes

Question 1

Describe double bond effect on p orbital

Answer 1

Due to bond formed 1 electron is left out on each C

Which are above and below p orbital

So p orbital now overlap to make pi orbital

This is weaker than sigma bond

So high electron density, so reactive

Question 2

What are 4 types of addition reaction?

Answer 2

Hydrogenation: addition of hydrogen
Electrophilic addition: of bromide and bromine
Hydration: Addition of water
Polymerisation: Addition of itself

Question 3

Hydrogenation?

Answer 3

Adds H to alkenes

Alkenes with H under high temperature (150 c) and nickel catalyst

Allows margarine to be made from oil
Saturated fats usually considered unhealthy

Question 4

Draw mechanism for electrophilic addition of alkene with HBr?

Answer 4

HBr approaches ethene with the H first. This is positive (δ+) and is attracted to the
C=C

The Br behind is negative (δ─) and feels repulsion

The H─Br bond is strained and snaps

H takes the π electrons and attaches to the nearest C

Br gains the bonding electron pair and becomes the negative bromide ion which flies off into the surroundings

The other C loses the electron pair so becomes positively charged with three bonds
This is called a carbocation and is very unstable

It must rapidly meet another Br─ ion and uses a lone pair on the Br─ to make a link with it forming