Module 1

Question 1

What are the problems with the Kekule structure of benzene?

Answer 1

It doesn’t explain benzene’s low reactivity: we would expect it to decolourise bromine water but it doesn’t.
The carbon carbon bond length is equal for all bonds in benzene but the kekule structure would indicate 3 shorter double bonds and three longer single bonds.
The enthalpy change for the hydrogenation of benzene is much lower than expected (ie lower than 3 times that of cyclohexene)

Question 2

What is the bond angle in bezene?

Answer 2

120

Question 3

Fill in the gaps

Benzene struggles to take part in …….. reactions but will more readily take part in ……… reactions.

Answer 3

addition

substitution

Question 4

Define addition reaction

Answer 4

A reaction in which a reactant is added to an unsaturated molecule to make it a saturated molecule.

Question 5

Define substitution reaction

Answer 5

A reaction in which an atom or group of atoms is replaced by a different atom or group of atoms.

Question 6

Give the conditions for the preparation of nitro benzene.

Answer 6

Concentrated nitric acid (HNO3) + benzene

with an H2SO4 catalyst at a temperature of 50 degrees.

Question 7

Why must the temperature in the preparation of nitrobenzene be kept below 50 degrees?

Answer 7

Because at temperatures above 50 degrees more than one nitro group may be substituted.

Question 8

Give the equation for the nitration of benzene.

Answer 8

C6H6 + HNO3 –> C6H5NO2 + H2O

Question 9

Give some examples of halogen carriers

Answer 9

FeCl3 FeBr3 AlCl3 AlBr3

Iron metal can also be used as it will react with the halogen to form an Iron(iii) halide eg FeCl3.

Question 10

Give an equation for the formation of chloro benzene.

Answer 10

C6H6 + Cl2 —> C6H5Cl + HCL

Question 11

Give the equation for the formation of a nitronium ion.

Answer 11

HNO3 + H2SO4 —> NO2+ + HSO4- + H2O

Question 12

What is the equation for the formation of the bromonium ion?

Answer 12

FeBr3 + Br2 —> FeBr4- + Br+

Question 13

Give the equation for the reformation of the catalyst in the bromination of benzene.

Answer 13

FeBr4- + H+ —> FeBr3 + HBr

Question 14

Why will bromine water decolourise in the presence of cyclohexene and phenol but not in the presence of benzene?

Answer 14

In benzene pi electrons are delocalised over the hole six atoms of the ring structure. The electron density is too low to polarise the bromine and therefore it won’t react. In phenol the electrons form the oxygen delocalise into the ring and therefore increase the electron density of the ring so it can polarise the bromine.
In cyclohexene electrons in the pi bond are localised between two carbons and therefore have a high enough electron density to polarise the bromine.

Question 15

Is phenol an acid or a base?

Answer 15

An acid

Question 16

What would you observe if you added phenol to bromine water?

Answer 16

A white precipitate of 2,4,6-tribromophenol would be formed.

Question 17

Give some uses of phenols.

Answer 17

Chlorophenols are found in antiseptics
salicylic acid is used to make aspirin
bisphenol are used to make epoxy resins for paints.
Alkyl phenols are found in surfactants and detergents.

Question 18

Aldehydes are named starting from which carbon?

Answer 18

The carbon which is attached to the aldehyde.

Question 19

Give the formula for potassium dichromate.

Answer 19

K2Cr2O7

Question 20

What are the conditions needed to turn a primary alcohol into a carboxylic acid?

Answer 20

Heated under reflux with a concentrated H2SO4 catalyst.