benzene

Question 1

Chemists often use two different structures to represent a molecule of benzene.
Describe, with the aid of suitable diagrams showing orbital overlap, the difference in bonding between structure A and structure B.

Answer 1

M1
p-orbitals overlap (to form pi/π-bonds) 
M2
π-bond(s) are delocalised in structure B 
M3
π-bonds are localised/between two carbons in structure A

M4
AND
Diagrams show correct position of delocalised and
localised π-bonds/π-electrons
OR correct position of p-orbital overlap 

Question 2

Explain why bromine reacts more readily with salicylic acid than with benzene.

Answer 2

(In salicylic acid)
lone pair/pair of electrons on O(H)/phenol is ~
(partially) delocalised into the ring  
electron density increases/is high ORA  
Br2/electrophile is (more) polarised ORA 

Question 3

Explain the relative resistance to bromination of benzene compared with alkenes.

Answer 3

In benzene, electrons OR π-bond(s) are delocalised 
QWC requires delocalised/delocalized spelled correctly
and used in correct context
In alkenes, π-electrons are OR π-bond is
AND
localised OR between two carbons 
benzene has a lower electron density
OR alkene/C=C has a higher electron density 
Comparison essential
benzene polarises bromine / Br2 LESS
OR benzene attracts bromine / Br2 LESS
OR benzene induces a weaker dipole in bromine / Br2

Question 4

Chlorobenzene can be nitrated to form a mixture of products.

Suggest why the reaction forms a mixture of products.

Answer 4

substitution/nitration/NO2 at different positions (on the ring)
OR
forms different isomers
OR
multiple substitution/nitration 

Question 5

Explain why phenol reacts more readily with chlorine than benzene reacts with chlorine.

Answer 5

In phenol,
(lone) pair of electrons on O is (partially) delocalised into
the ring 
QWC: delocalised/delocalized/delocalise, etc must be spelt
correctly in the correct context for benzene OR phenol at
least once
electron density increases/is high  ORA
Cl2/electrophile is (more) polarised  ORA

Question 6

Explain the evidence that led scientists to doubt the model proposed by Kekulé.

Answer 6

Bond length intermediate between/different from (short) C=C
and (long) C–C


H hydrogenation less exothermic than expected (when
compared to

H hydrogenation for cyclohexene)

Only reacts with Br2 at high temp or in pres

Question 7

Explain the relative resistance to bromination of benzene compared to phenol and compared to cyclohexene.

Answer 7

MUST spell delocalised/delocalized or localised/localized
correctly once in the answer to obtain all 5 marks
benzene electrons or π-bonds are delocalised 
phenol a lone or non-bonded pair of electrons on the
oxygen or the OH group is (partially) delocalised into the
ring 
cyclohexene electrons are localised OR delocalised
between two carbons 
benzene has a lower electron density OR phenol has a
higher electron density OR cyclohexene has a higher
electron density 
benzene cannot polarise or induce a dipole in Br2 OR
phenol can polarise the Br2 OR cyclohexene can polarise
Br2 or the Br–Br bond 