12.2 Alkenes (Reaction with Bromine and Bromine (aq)

Question 1

Reaction with Bromine (1. POLARISATION)

Answer 1

• When the Bromine molecule comes close to ethene, it becomes polarised by the negative charge of the double bond.
• The electrons in the diatomic bromine molecule are repelled by the alkene and are pushed back along the molecule. It becomes polarised.
• The positively charged bromine atom acts as an electrophile, reacting with the double carbon bond.

Question 2

Reaction with Bromine (2. CARBOCATION)

Answer 2

◦A pair of electrons from the carbon double bond move onto the positive bromine atom.
◦The electron pair between the two bromine atoms then move towards the other bromine atom, creating a bromine atom with a negative charge.
◦One of the carbon atoms is now positively charged (it shares only six outer electrons).
◦An ion like this is called a carbocation. Carbocations are very unstable.
◦The carbocation goes on to react with the negative bromide ion.

Question 3

Reaction with Bromine (3. Nucleophile)

Answer 3

-The carbocation goes on to react with the negative bromide ion.

• The negatively charged bromide ion is acting as a nucleophile.
• The reaction is an addition reaction; it is an electrophilic addition reaction since the initial attack is by an electrophile.

Question 4

Reaction with Bromine (aq)

Answer 4

◦In Bromine water, there may sometimes be more water molecules than bromide ions present, and so the above reaction would be more likely to occur.
◦However, the bromine water would still be decolourised.

Question 5

Experimental Evidence for mechanism for electrophilic addition

Answer 5

If chloride ions are present when the ethane reacts with bromine. Two products are formed: 1,2-dibromohexane and 1-bromo-2-chlorohexane