Aromatic Chemistry

Question 1

When was benzene first isolated?

Answer 1

1925

Question 2

Describe Kelules structure of benzene, and when was it proposed?

Answer 2

1965
Proposed that benzene had two lengths of bonds. The short bond was 0.135nm and was the double bond. The single bonds had lengths of 0.147nm

Question 3

What is the evidence against Kelules benzene model?

Answer 3

Benzene does not decolourise bromine water
All the carbon bond lengths are equal, and have intermediate lengths between single and double carbon bonds
Thermochemical data for the hydrogenation of benzene suggests its more stable than the model. The enthalpy change is expected to be -360 and is instead -208

Question 4

Describe the structure of benzene

Answer 4

Planar structure, each carbon atom has three covalent bonds so has a spare electron and a trigonal planar shape
Each bond angle is 120
Length of each bond is intermediate between C=C and C-C length
P orbitals overlap to form a Pi orbital of delocalised electrons. Electron dense and stable
Less endothermic

Question 5

When will a compound exhibit delocalised electrons due to double bonds?

Answer 5

When a compound has four or more adjacent p-orbitals that arose from doible bonds

Question 6

What is an electrophile?

Answer 6

An electrophile is an electron pair acceptor

Question 7

Why are electrohiles attracted to benzene?

Answer 7

Due to the high electron density above and below the Pi bond in benzene

Question 8

Why does benzene undergo substitution and not addition?

Answer 8

In order to preserve the benzene ring of delocalised electrons, so its still stable

Question 9

What is the mechanism, conditions and reagents of nitration of benzene?

Answer 9

Electrophilic substitution
Reagents- comc. HNO3 and coc. H2SO4
Conditions- reflux at 50*

Question 10

What is the mechanism, conditions and reagents of acylation of benzene?

Answer 10

Electrophilic substitution
Reagents- AlCl3, acyl chloride
Conditions- reflux at 50*

Question 11

What is the mechanism, conditions and reagents of acylation of benzene?

Answer 11

Electrophilic substitution
Reagents- AlCl3, acyl chloride
Conditions- reflux at 50*

Question 12

Describe the three stages for electrophilic substitution with benzene

Answer 12

1- generation of the electrophile using the catalyst
2- formation of the organic products via electrophilic
3- regeneration of the catalyst

Question 13

What is the mechanism and reagents of alkylation of benzene

Answer 13

Electrophilic substitution
RCl and AlCl3

Question 14

What is the mechanism and reagents of halogenation of benzene?

Answer 14

Electrophilic substitution
AlCl3 and Cl2

Question 15

Why is cyclohexene more efficient during electrophilic substitution than benzene?

Answer 15

Benzene contains a ring of delocalised electrons in Pi orbital
Therefore is less electron dense than the double carbon bonds on cyclohexene
So it doesnt attract electrons as efficiently

Question 16

How do methyl groups on benzene effect its reactivity?

Answer 16

Methyl groups release electrons due to the positive inductive effect. Therefore the benzene ring becomes more dense and attracts electrophiles more efficiently.
Therefore they increase reactivity

Question 17

What are the products of the nitration of benzene, and what can they be used for?

Answer 17

Aromatic nitro- benzene compounds that are used as explosives

Question 18

What are the reagents and products when nitro-benzene compounds are reduced after nitration?

Answer 18

Reagents- Tin and conc. HCl
Prodices water and amines

Question 19

How is the reduction of nitrobenzene compounds shown in a balanced equation?

Answer 19

[H]

Question 20

Why is benzene more stable than cyclohexene molecules?

Answer 20

Due to the delocalisation of its electrons, meaning more energy is required to overcome its bonds