CD.4

Question 1

Because benzene molecules contain areas of high electron density, it reacts with which type of molecules?

Answer 1

Electrophiles

Question 2

What type of mechanism is it when alkenes react with electrophiles?

Answer 2

Electrophilic addition

Question 3

What type of molecule is the product when alkene undergo addition reaction with electrophile?

Answer 3

Saturated molecule

Question 4

Why are reaction mechanisms different for alkene and benzene with electrophiles?

Answer 4

Because benzene contains a system of delocalised electrons making it very stable.

Question 5

Why won’t benzene undergo addition reaction with electrophiles?

Answer 5

Because a ring with two double bonds is higher in energy than just the benzene ring, therefore it is energetically unfavourable to disrupt the delocalisation in an addition reaction.

Question 6

What type of mechanism is it when benzene reacts with electrophiles?

Answer 6

Electrophilic substitution.

Question 7

Why would benzene undergo substitution with electrophiles?

Answer 7

Because at the end of the substitution, the stable delocalisation of benzene is kept, only a hydrogen is replaced by the electrophile group.

Question 8

What is the first step of the mechanism of electrophilic substitution of benzene?

Answer 8

The delocalised electrons in benzene attacks the electrophile, one pair of double bond opens up to allow the electrophile to join. As a result, the ring contains two double bonds, one carbon is attached to the electrophile and a hydrogen, and the other is a carbocation. The delocalisation is disrupted.

Question 9

Is the first step of the mechanism fast or slow?

Answer 9

Slow, because the stable benzene doesn’t like the delocalisation to be disrupted, setting off this step requires energy, therefore the presence of a catalyst and heat is often required.

Question 10

What is the second step of the mechanism of electrophilic substitution of benzene?

Answer 10

The hydrogen donates its bonding electrons to reform the delocalised system and leaves as a hydrogen ion.

Question 11

Is the second step of the mechanism fast or slow?

Answer 11

Fast, because the benzene would preferred the delocalisation to be reformed therefore this step happens fairly spontaneously.

Question 12

What is the reaction known as when benzene reacts with liquid bromine in the presence of a catalyst?

Answer 12

Bromination

Question 13

What catalyst is often used in bromination?

Answer 13

Iron filling or iron(III) bromide, FeBr3.

Question 14

What condition is required for bromination?

Answer 14

Heat under reflux.

Question 15

What product is produced other than bromobenzene in bromination?

Answer 15

Hydrogen bromide, which is a gas.

Question 16

What acts as the electrophile in bromination?

Answer 16

Br+ ions.

Question 17

How is the electrophile Br+ generated in bromination from bromine molecules?

Answer 17

The catalyst, iron(III) bromide, FeBr3, removes a Br- ion from a bromine molecule to for FeBr4-, leaving a Br+ ion, which is better electrophile than Br2.

Question 18

How is the FeBr4- regenerated back to iron(III) bromide, the original catalyst?

Answer 18

The H+ ion substituted of the benzene ring reacts with FeBr4- to produce HBr and iron(III) bromide.

Question 19

What is the reaction known as when benzene reacts with chlorine in the presence of a catalyst?

Answer 19

Chlorination

Question 20

What catalyst is often used in chlorination?

Answer 20

Aluminium chloride, AlCl3.

Question 21

What condition is required for chlorination and why?

Answer 21

Anhydrous conditions, because the catalyst aluminium chloride reacts violently with water

Question 22

Why is heat not required for chlorination?

Answer 22

Because the chlorine is very reactive and AlCl3 is a powerful catalyst therefore no heat is required to provide energy for the first step of the mechanism.

Question 23

What product is produced other than chlorobenzene in chlorination?

Answer 23

Hydrogen chloride, which is a gas.

Question 24

What acts as the electrophile in chlorination?

Answer 24

Cl+

Question 25

Molecules like FeBr3 and AlCl3 which are able to catalyse electrophilic reaction are known as what?

Answer 25

Halogen carriers.

Question 26

What is the reaction known as when an alkyl group is introduced onto a benzene ring?

Answer 26

Alkylation

Question 27

What is reacted with benzene in alkylation?

Answer 27

alkyl chloride, or other alkyl halide molecules.

Question 28

What catalyst is used in alkylation?

Answer 28

Anhydrous aluminium chloride, or similar halogen carriers to suit the alkyl halide.

Question 29

What condition is required for alkylation?

Answer 29

Heat under reflux.

Question 30

What products are formed in alkylation?

Answer 30

Alkyl benzene, and a hydrogen halide, depending on what alkyl halide was used.

Question 31

What is the reaction known as when an acyl group is introduced onto a benzene ring?

Answer 31

Acylation

Question 32

What is reacted with benzene in acylation?

Answer 32

Acyl chloride or acid anhydride.

Question 33

What catalyst is used in acylation?

Answer 33

Anhydrous aluminium chloride.

Question 34

What condition is required for acylation?

Answer 34

Heat under reflux.

Question 35

What products are formed in acylation

Answer 35

Acyl benzene, and hydrogen chloride.

Question 36

What are Friedel-Crafts reactions (alkylation and acylation) important for synthesis?

Answer 36

They provide a way of forming new C-C bonds and to add side groups to benzene rings.