Arenes

Question 0

Evidence for delocalised structure of benzene in terms of bond length

Answer 0

X-Rays show that the carbon-carbon bond lengths are all the same in benzene (0.139 nm), somewhere between that of a C-C bond and a C=C bond.
The delocalised system means that the bonds are all the same type and therefore the same length.

Question 1

Compare the kekulé and delocalised structure of benzene

Answer 1

With the kekulé structure:
-3 double bonds are isolated
-would be very reactive
Delocalised structure:
-each carbon atom has four outer shell electrons (3 bonded to two other C atoms and 1 bonded to H)
-three bonds are sigma bonds and the 4th outer shell electron is in a 2p orbital above and below the plane of C atoms.
-this electron overlaps with electrons in the p-orbitals of the C atoms surrounding it
-Results in a ring of electron density above and below the plane of the carbon atoms.
-this overlap produces a system of pi bonds which spread over all 6 C atoms.
-the p electrons are now spread over the whole ring and are delocalised.

Question 2

Evidence for delocalised structure of benzene in terms of enthalpy

Answer 2

The enthalpy change of hydrogenation of benzene is -208kJmol^-1, when it is expected to be -360 so it is more stable than expected.
The delocalisation of the pi electrons would stabilise the structure and lower energy released when hydrogenated.

Question 3

Evidence for delocalised structure of benzene in terms of resistance to reaction

Answer 3

Benzene does not decolorise and therefore does not react with bromine water. Benzene needs a catalyst for this reaction.
The delocalised pi electron system has insufficient electron density to polarise the Br-Br bond and react.

Question 4

Explain the difference in reactivity of benzene and alkenes with bromine in terms of localised vs delocalised electron density.

Answer 4

• Benzene has delocalised pi electrons spread over the 6 carbon atoms. Alkenes have pi electrons localised above and below the two carbon atoms in the double bond.
• Benzene has a lower pi electron density than alkenes.
• when a non-polar molecule approaches benzene, there is insufficient pi electron density above and below any two C atoms to cause the necessary polarisation of the bromine.
• a halogen carrier is needed to generate more powerful electrophile, Br+ so the reaction can take place.

Question 5

What are the conditions and reagents for nitration of benzene?

Answer 5

Concentrated nitric acid, concentrated sulfuric acid (catalyst), 50C

Question 6

What is the electrophile in the nitration of benzene?

Answer 6

(NO2) +, nitronium ion

Question 7

Why is the intermediate of electrophilic substitution of benzene unstable?

Answer 7

Because the electrons in the intermediate are partially delocalised.

Question 8

Conditions for halogenation of benzene.

Answer 8

AlCl3 catalyst, room temperature, dark

Question 9

How is the electrophile for halogenation of benzene produced?

Answer 9

Heterolytic fission of a halogen to form a positive and negative halogen ion.

Question 10

When phenol reacts with sodium, what do we observe?

Answer 10

Effervescence because hydrogen gas is being produced.

Question 11

What is formed when phenol reacts with sodium?

Answer 11

Sodium phenoxide and hydrogen gas.

Question 12

What is formed when phenol reacts with aqueous sodium hydroxide?

Answer 12

Sodium phenoxide and water

Question 13

Why does bromine react with phenol more readily than benzene?

Answer 13

• Increased reactivity is due to the lone pair of electrons on an oxygen atom in phenol
• this creates a higher electron density in the ring structure
• increased electron density polarises bromine molecules.

Question 14

Uses of phenol

Answer 14

Alkyl phenols: found in detergents
Chlorophenols: found in antiseptics and disinfectants
Salicylic acid: used in production of aspirin
Bisphenol: used in production of paints