6.1.4: Halogenation and Friedel-Crafts

Question 1

What is a Friedel-Crafts reaction?

Answer 1

An electrophilic substitution reaction where a hydrogen is exchanged for an alkyl or acyl chain.

Question 2

What happens when benzene is shaken with bromine water?

Answer 2

No reaction occurs.

Question 3

Why is there no reaction when benzene is shaken with bromine water?

Answer 3

• Benzene has a lower electron density between carbon atoms than an alkene.
• When non-polar molecules like bromine approach the benzene ring, there is not enough electron density between the carbon atoms to induce a dipole and start a reaction.

Question 4

Why does a halogen carrier allow alkylation to occur?

Answer 4

A stronger electrophile can be generated.

Question 5

How can you substitute a hydrogen for an alkyl chain in benzene?

Answer 5

• Haloalkanes such as chloromethane are mixed with a halogen carrier such as iron(III) chloide.
• The halogen carrier acts as a catalyst and is regenerated at the end of the reaction.
• A reactive carbocation is made which undergoes electrophilic substitution with the benzene ring.

Question 6

Reagents and conditions for a Friedel-Crafts reaction (alkyl)

Answer 6

• Haloalkane
• Halogen carrier
• 50 degrees Celsius

Question 7

Multiple substitutions are likely and a mixture of products is made, how can the products be separated?

Answer 7

Using fractional distillation or chromatography.

Question 8

How can the actual yield of a singly substituted ring be improved?

Answer 8

By adding excess benzene.

Question 9

What causes the mixture of products?

Answer 9

• Each successive substitution makes the delocalised pi-bonds more nucleophilic and therefore more susceptible to electrophilic attack.
• This increase in reactivity is caused by the alkyl chain donating electrons to the aromatic ring.

Question 10

Which functional group does acyl chloride have?

How reactive?

Answer 10

• RCOCL

- Very reactive

Question 11

What can an acyl chloride be used as in a Friedel-Crafts reaction?

Answer 11

-As a halogen carrier to substitute just one halogen atom.

Question 12

Why does only one substitution occur when using acyl chloride?

Answer 12

-Because as the carbonyl group withdraws electrons from the aromatic ring, a less reactive ketone is made.

Question 13

What are the conditions and reagents for acylation?

Answer 13

• The reaction mixture is held about 60 degrees Celsius for about 30 minutes under reflux for the reaction to occur.
• RCOCL
• AlCl3 catalyst
• Anhydrous conditions