Mr Cole Chemistry

Structure of Benzene Questions

Structure of Benzene

Aromatic Chemistry Worksheet

Thermodynamic Evidence

Thermodynamic data can be used to deduce the structure of benzene.
a) Cyclohexene can react with hydrogen to form cyclohexane. Write the equation for the reaction and state the conditions required for the reaction to occur.
Answer

C6H10 + H2 → C6H12

Condition: Nickel catalyst, 150°C.

b) Calculate the enthalpy change of the reaction given the following data.
Substance Enthalpy of Combustion (kJ mol-1)
Cyclohexene -3762
Hydrogen -285.8
Cyclohexane -3928
Answer

Enthalpy of Reaction = ΣΔHc(Reactants) – ΣΔHc(Products)

(-3762) + (-285.8) – (-3928) = -119.8 kJ mol-1

c) Draw a molecule of cyclo-1,3,5-triene.
Answer Cyclo-1,3,5-triene Structure
d) Predict the enthalpy change of hydrogenation of cyclo-1,3,5-triene based on the information above.
Answer

Based on the value calculated in (b) for one double bond:

-119.8 × 3 = -359.4 kJ mol-1

e) Benzene has an enthalpy of hydrogenation of -208.1 kJ mol-1. Explain why it is different to the value calculated for cyclo-1,3,5-triene.
Answer

Benzene is more stable than the theoretical cyclo-1,3,5-triene. This is because benzene has a delocalised ring of π-electrons rather than three isolated double bonds. The electron density is spread out over the whole ring, increasing stability and lowering the energy of the molecule (making the hydrogenation less exothermic).

Reactivity Evidence

The reactions of benzene can be used to deduce the structure of benzene.
a) Write the equation for the reaction between cyclohexene and bromine.
Answer

C6H10 + Br2 → C6H10Br2

b) Write the equation for the reaction between benzene and bromine.
Answer

C6H6 + Br2 → C6H5Br + HBr

(Note: This reaction requires a halogen carrier catalyst such as AlBr3 or FeBr3).

c) Explain how this proves that benzene is not an alkene.
Answer

Alkenes undergo electrophilic addition reactions (adding across the double bond), whereas benzene undergoes electrophilic substitution reactions (preserving the stable delocalised ring).

Crystallographic Evidence

Crystallography can be used to deduce the structure of benzene. Describe the structure of benzene and compare it to the proposed structure of cyclo-1,3,5-triene. Include bond angles and comparative bond lengths in your answer.
Answer

Benzene: A perfect planar hexagon. All carbon-carbon bond lengths are identical (intermediate between single and double bonds) and all bond angles are 120°.

Cyclo-1,3,5-triene (theoretical): Would have alternating single and double bonds. It would therefore have three short C=C bonds and three long C-C bonds. The molecule would likely not be planar, but puckered.

Effects of Delocalisation

Effect of delocalisation on functional group properties.
a) The enthalpy of hydrogenation of cyclohexa-1,4-diene is -240 kJ mol-1 and cyclohexa-1,3-diene is -232 kJ mol-1. Explain the difference.
Answer

The double bonds in cyclohexa-1,3-diene are separated by one single bond (conjugated). This allows for partial delocalisation of the π-electrons, which makes the molecule more stable (lower energy) compared to the isolated double bonds in cyclohexa-1,4-diene. Because it is more stable, less energy is released upon hydrogenation (it is less exothermic).

b) Both cyclohexanol and phenol can act as very weak acids. State which one is more acidic and explain why that is the case.
Answer

Phenol is a stronger acid than cyclohexanol.

This is because the phenoxide ion (C6H5O–) formed when phenol dissociates is stabilised by delocalisation. The lone pair on the oxygen atom overlaps with the delocalised ring system, spreading the negative charge over a larger area. This increased stability of the conjugate base encourages dissociation. Cyclohexanol forms an alkoxide ion with no such stabilisation.

c) Both cyclohexanamine and phenylamine can act as bases. State which one is more basic and explain why this is the case.
Answer

Phenylamine is a weaker base.

This is because the lone pair of electrons on the nitrogen atom delocalises into the benzene ring. This decreases the electron density on the nitrogen atom, making the lone pair less available to accept a proton (H+ ion).

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