Questions on benzene: its structure and the evidence for delocalisation

Question 1

Discuss the evidence that the Kekulé structure of benzene is wrong, and explain how the delocalised model accounts for each piece of evidence.

Accepted Answer

Indicative content

X-ray diffraction shows all six carbon–carbon bonds are 140 pm, whereas Kekulé predicts alternating bonds of 154 pm and 134 pm
in the delocalised model every pair of neighbouring carbons shares the same electron density, so every bond has one length between single and double
benzene does not decolourise bromine water at room temperature, whereas a compound with C=C bonds would add bromine at once
the delocalised electron density is spread over six carbons, too low to polarise bromine, and addition would lose the stability of the ring
the enthalpy of hydrogenation of benzene is −208 kJ mol⁻¹, far less exothermic than the −360 kJ mol⁻¹ expected for three C=C bonds
delocalising six electrons over the ring lowers the energy of the molecule, so benzene starts about 152 kJ mol⁻¹ lower than the Kekulé structure would

Levels

Level 3 (5–6 marks) all three pieces of evidence are given with the values, bond length, lack of addition with bromine and the enthalpy of hydrogenation, and each is matched to what the delocalised model predicts and the Kekulé structure does not
Level 2 (3–4 marks) two pieces of evidence are given with values and explained by delocalisation, or all three are named and one is left unconnected to the model
Level 1 (1–2 marks) one piece of evidence is named, often with benzene said to have stronger double bonds than an alkene, which is offered as the reason it resists addition

Question 2

Describe the shape of a benzene molecule and the lengths of its carbon–carbon bonds, as X-ray diffraction shows them, and state how each differs from what the Kekulé structure predicts.

Accepted Answer

B1 all twelve atoms lie in one plane, the carbons forming a regular hexagon with every bond angle 120° B1 all six carbon–carbon bonds are the same length, 140 pm; accept any value from 139 to 140 pm B1 the Kekulé structure predicts two lengths in turn, C–C 154 pm and C=C 134 pm, so an uneven hexagon B1 140 pm lies between the single and double bond lengths, so every bond is the same and none is a double bond

Question 3

Describe the bonding in a benzene molecule using the delocalised model.

Accepted Answer

B1 each carbon forms σ bonds to two neighbouring carbons and one hydrogen, so the ring is planar with bond angles of 120° B1 each carbon has one electron in a p orbital at right angles to the plane of the ring B1 each p orbital overlaps sideways with the p orbitals on both neighbouring carbons, giving rings of electron density above and below the plane B1 six electrons are delocalised around the whole ring rather than held in three separate double bonds; ignore 'resonance' unless the delocalisation is described

Question 4

Name the compounds C₆H₅CH₃, C₆H₅COOH and C₆H₅NH₂, and a benzene ring with chlorine atoms on two opposite carbons.

Accepted Answer

B1 C₆H₅CH₃ is named methylbenzene, a methyl group on the ring B1 C₆H₅COOH keeps its own name, benzoic acid B1 C₆H₅NH₂ is named phenylamine, the ring as a phenyl group; allow aminobenzene B1 the ring with two chlorines on opposite carbons is 1,4-dichlorobenzene; reject para-dichlorobenzene alone

Question 5

Compare the reactions of cyclohexene and benzene with bromine, and with hydrogen over a nickel catalyst, explaining the differences.

Accepted Answer

B1 cyclohexene adds bromine at room temperature, decolourising it, whereas benzene does not react with bromine water B1 benzene reacts with bromine only with a halogen carrier, and then by substitution, giving bromobenzene and HBr B1 both add hydrogen to give cyclohexane, but benzene releases 208 kJ mol⁻¹, far less than three times cyclohexene's 120 kJ mol⁻¹ B1 the differences arise because benzene's delocalised ring makes it more stable, and addition would destroy that ring

Question 6

Explain, in terms of the anion each forms, why the pKa values of ethanol, phenol and ethanoic acid are about 16, 10.0 and 4.8.

Accepted Answer

B1 each acid loses a proton from an O–H group, and the more stable the anion formed, the stronger the acid B1 in the phenoxide ion a lone pair on oxygen overlaps with the ring, so the negative charge is delocalised over the ring and the ion is stabilised B1 in the ethoxide ion the charge stays on one oxygen and the ethyl group releases electron density onto it, so the ion is least stable and ethanol is the weakest acid B1 in the ethanoate ion the charge is shared between two electronegative oxygen atoms, which stabilise it more than the ring does, so ethanoic acid is the strongest

Question 7

Compare the reactions of benzene and of phenol with bromine, giving the conditions, the organic product and the reason for the difference.

Accepted Answer

B1 benzene needs pure bromine and a halogen carrier such as FeBr₃, whereas phenol reacts with bromine water at room temperature B1 benzene gives bromobenzene, one bromine atom substituted, whereas phenol gives a white precipitate of 2,4,6-tribromophenol B1 a lone pair on phenol's oxygen is partly delocalised into the ring, raising its electron density above benzene's B1 phenol's ring can therefore polarise a bromine molecule itself, whereas benzene needs the carrier to make Br⁺

Question 8

Explain why cyclohexene decolourises bromine water at room temperature but benzene does not.

Accepted Answer

B1 the C=C bond in cyclohexene is a region of high electron density that polarises Br₂, which adds across the double bond B1 in benzene the π electron density is delocalised over six carbons, so it is too low to polarise a bromine molecule B1 addition to benzene would destroy the delocalised ring and lose its stability, so no addition takes place; reject 'benzene's double bonds are stronger'

Question 9

Calculate the delocalisation energy of benzene from these data-book values for the enthalpy of hydrogenation: cyclohexene −120 kJ mol⁻¹, benzene −208 kJ mol⁻¹.

Accepted Answer

M1 the Kekulé structure is expected to give 3 × (−120) = −360 kJ mol⁻¹ M1 difference = −208 − (−360), the measured value minus the expected value A1 delocalisation energy = 152 kJ mol⁻¹, benzene being 152 kJ mol⁻¹ more stable; allow −152 kJ mol⁻¹ if the stabilisation is stated

Question 10

Suggest what the following data show about the bonding and the reactions of cyclooctatetraene, C₈H₈, a ring of eight carbons: it decolourises bromine water rapidly, and its carbon–carbon bonds are 133 pm and 146 pm long, in turn.

Accepted Answer

B1 the two bond lengths show it has localised single and double bonds, so its electrons are not delocalised round the ring B1 each C=C bond is a region of high electron density that can polarise bromine, so it reacts like an alkene B1 it reacts by addition, because it has no delocalisation stability to lose when a double bond is used up

Question 11

Suggest how the number of isomers of dichlorobenzene, C₆H₄Cl₂, that can be made supports the delocalised model over the Kekulé structure. Only three isomers exist.

Accepted Answer

B1 the Kekulé structure predicts two different 1,2-isomers, one with the chlorines across a C=C bond and one across a C–C bond, so four isomers in all B1 in the delocalised model every bond is the same, so there is only one 1,2-isomer B1 the delocalised model predicts three isomers, 1,2, 1,3 and 1,4, which matches the three found

Question 12

Explain, in terms of energy, why benzene is more stable than the Kekulé structure would be.

Accepted Answer

B1 the p electrons in benzene are delocalised over six atoms instead of being held in three separate π bonds B1 delocalisation lowers the energy of the electrons and so of the molecule B1 benzene's enthalpy of hydrogenation is 152 kJ mol⁻¹ less exothermic than three times cyclohexene's, showing it starts 152 kJ mol⁻¹ lower in energy

Question 13

Evaluate the claim that benzene has three double bonds which swap places too quickly for the two forms to be separated.

Accepted Answer

B1 the claim explains why only one 1,2-disubstituted isomer is found, which is what it was proposed for B1 it cannot explain the 152 kJ mol⁻¹ of extra stability, because a molecule with three C=C bonds at any moment would be hydrogenated as three alkenes B1 X-ray diffraction shows six equal bonds at all times, so benzene has one delocalised structure and the claim is wrong

Question 14

Explain why chlorobenzene is not hydrolysed when boiled with aqueous sodium hydroxide, whereas 1-chlorobutane is hydrolysed on heating under reflux.

Accepted Answer

B1 a lone pair on the chlorine of chlorobenzene overlaps with the delocalised π system of the ring B1 this gives the C–Cl bond partial double-bond character, so it is shorter and stronger than the C–Cl bond in 1-chlorobutane B1 the delocalised electrons of the ring also repel the hydroxide ion as it approaches the carbon, so nucleophilic substitution does not happen

Question 15

Suggest a chemical test that distinguishes phenol from benzoic acid, giving the observation for each compound.

Accepted Answer

B1 add sodium carbonate solution, or solid sodium hydrogencarbonate, to each compound B1 benzoic acid fizzes, giving carbon dioxide, because it is strong enough an acid to react with carbonate B1 phenol gives no effervescence, because it is too weak an acid to release carbon dioxide; ignore whether it dissolves

Question 16

Calculate the pH of a 0.0250 mol dm⁻³ solution of phenol, whose pKa is 10.0.

Accepted Answer

M1 Ka = 10⁻¹⁰·⁰ = 1.0 × 10⁻¹⁰ mol dm⁻³ M1 [H⁺] = √(1.0 × 10⁻¹⁰ × 0.0250) = 1.58 × 10⁻⁶ mol dm⁻³ A1 pH = −log(1.58 × 10⁻⁶) = 5.80, to two decimal places; error carried forward from an incorrect [H⁺]

Question 17

Give the systematic name of the structure Kekulé proposed for benzene, and give the number of delocalised electrons in one benzene molecule.

Accepted Answer

B1 cyclohexa-1,3,5-triene, a six-membered ring of alternating single and double bonds; allow cyclohexatriene B1 six delocalised electrons, one from the p orbital of each carbon atom

Question 18

Explain why all of the carbon–carbon bonds in benzene are the same length.

Accepted Answer

B1 the six p electrons are delocalised evenly around the ring, so every pair of neighbouring carbons shares the same electron density B1 there are no separate single and double bonds, so every bond has the same length, between single and double