F324 - Arenes

Question 0

What are arenes sometimes called?

Answer 0

Aromatic compounds

Question 1

What is the formula for benzene?

Answer 1

C6H6

Question 2

Describe the structure of benzene.

Answer 2

Cyclic molecule; contains 6 C atoms and 6 H atoms bonded together to form a hexagonal ring; each C is bonded to 2 other C and one H by sigma bonds; each C has one 4th electron left over occupying a p-orbital; overlap of p-orbitals above and below the plane to give a pi bond; pi electrons are delocalised around the ring

Question 3

What is the bond angle in benzene?

Answer 3

120°

Question 4

What is the delta H of hydrogenation for cyclohexene, cyclohexatriene, and benzene?

Answer 4

Cyclohexene: -120kJmol-1
Cyclohexatriene: -360kJmol-1
Benzene: -208kJmol-1

Question 5

What is the bond lengths for C-C, C=C, and C-C in benzene?

Answer 5

C-C 0.154nm
C=C 0.134nm
C-C in benzene 0.139nm

Question 6

What type of reaction does benzene tend to undergo and why?

Answer 6

Electrophilic substitution because the delocalised system is already stable and arenes react in such a way as to retain their stability

Question 7

Give the reagents and conditions of the nitration of benzene.

Answer 7

Reflux at 60°C
Concentrated nitric acid
Concentrated sulphuric acid
However if the reaction rises above 95°C a mixture of nitrobenzene and 1,3-dinitrobenzene is produced

Question 8

Give the overall equation of the nitration of benzene and the 2 step equations involved.

Answer 8

HNO3 + 2H2SO4 –> NO2+ + H3O+ + 2HSO4-

1) HNO3 + H2SO4 –> H2NO3+ + HSO4-
2) H2NO3+ + H2SO4 –> NO2+ + H3O+ + HSO4 -

Question 9

Give the two different halogen carriers during the bromination of benzene (halogenation).

Answer 9

FeBr3 or AlBr3

Question 10

Give three uses of benzene compounds.

Answer 10

Drugs/pharmaceuticals
Dyes
Explosives
Insecticides

Question 11

What reaction does cyclohexene undergo and why?

Answer 11

Electrophilic addition
Pi electrons are localised over 2 C compared to benzene; alkenes are more electron dense then benzene; cyclohexene is able to polarise halogens without halogen carriers

Question 12

Give the equation when phenol reacts with sodium hydroxide.

Answer 12

C6H5OH + NaOH –> C6H5O-Na+ +H2O

Question 13

Give the equation when phenol reacts with sodium.

Answer 13

C6H5OH + Na –> C6H5O-Na+ + 1/2H2

Question 14

Why are phenols not very water soluble?

Answer 14

There is a large hydrocarbon portion compared to the OH group

Question 15

Why does phenol undergo electrophilic substitution more readily than benzene?

Answer 15

Lone pair on O of OH; lone pair can be delocalised into the benzene ring; electron density increases; more attractive towards electrophiles; causes polarisation of halogen upon approach without the use of a halogen carrier

Question 16

Give 2 uses of phenols.

Answer 16

Antiseptics
Disinfectants
Flavouring in food
Resin for paints