Chapter 25: Benzene and its compounds
Homework questions
1
Benzene is a very important chemical with its own branch of organic chemistry. It has the
molecular formula C6H6.
a
b
c
d
Explain why the preferred structure of benzene is
and not
.
Your answer should refer to the following pieces of evidence:
• thermochemical evidence
• X-ray evidence
• chemical reactions.
Draw a labelled diagram of benzene showing how the orbitals produce a
delocalised structure.
Explain why benzene undergoes electrophilic substitution reactions.
If benzene is represented by the Kekulé formula, the following reaction takes place when
1 mol of benzene undergoes complete combustion in oxygen:
i
Use the bond energies in the table below to calculate the enthalpy change for
this reaction.
Bond
Bond energy / kJ mol−1
ii
C–H
413
C=C
612
C–C
347
O=O
498
C=O
805
[9]
[3]
[2]
[4]
O–H
464
When the enthalpy change is determined experimentally, the value is −3267 kJ mol−1.
Comment on the two values and explain any discrepancy.
[2]
Total = 20
Cambridge International AS and A Level Chemistry © Cambridge University Press 2014
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2
Methylbenzene reacts with chlorine in two different ways. Reaction A produces one organic
product, and reaction B produces two organic products.
a
b
i Write the complete equation for reaction A.
[1]
ii What type of reaction is taking place here?
[1]
iii Use equations to describe the mechanism for the reaction. State the names for each step
in the reaction.
[7]
iv One of the by-products of the reaction has the formula C14H14. Identify this compound
and write the equation for its formation.
[2]
i Complete the equation for reaction B.
[2]
ii What type of reaction is taking place here?
[1]
iii Write the equation for the generation of the electrophile in reaction B.
[1]
iv Use curly arrows to draw the mechanism for the reaction.
[4]
v Show how the catalyst is regenerated.
[1]
Total = 20
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3
When the acidity of phenol is compared with that of water and ethanol, the following order of acid
strength is shown:
a
b
Explain this order of acidity.
4-nitrophenol has the structure shown below.
[5]
i
The –NO2 (nitro) group is an electron-withdrawing group. The pKa of phenol 9.95 whilst
the pKa of 4-nitrophenol is 7.15. Comment on the two pKa values and explain the
difference.
[4]
ii What is the pH of a 0.01 dm−3 mol solution of 4-nitrophenol?
[4]
iii Complete the equation below and label the acid, the base and the conjugate acids and
bases.
[4]
c
iv 4-nitrophenol is formed by the reaction of phenol with dilute nitric acid whilst the
nitration of benzene requires the formation of nitronium ions using concentrated nitric
acid and concentrated sulfuric acid.
Explain the much milder conditions used for phenol.
[3]
The compound shown below (methoxybenzene) is formed by the reaction of sodium
phenoxide with chloromethane (CH3Cl).
i
ii
Write the equation for this reaction.
[1]
Use curly arrows to draw the mechanism for the reaction between the phenoxide ion
and the chloromethane.
[3]
iii What type of reaction is this?
[1]
iv Explain why phenol is a solid at room temperature while methoxybenzene is a liquid,
even though methoxybenzene has a higher relative molecular mass (Mr = 108)
than phenol (Mr = 94).
[3]
Total = 28
Cambridge International AS and A Level Chemistry © Cambridge University Press 2014
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