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Chemistry · Practice book · Senior 6 · Unit 3 of 14

Benzene

Senior 6 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 3 Benzene

Key unit competence: Relate the chemistry and uses of benzene to its nature and structure.

Key ideas

  • Benzene, C6H6, is a planar hexagonal ring: all C–C bonds are equal (0.139 nm, between C–C 0.154 and C=C 0.134 nm), bond angles 120°, each carbon sp2 hybridised. The six p electrons form a delocalised π system above and below the ring (drawn as a circle in a hexagon).
  • Evidence against the Kekulé structure (alternating single and double bonds): equal bond lengths; only one 1,2-disubstituted isomer; benzene does not decolourise bromine water easily; its enthalpy of hydrogenation (−208 kJ mol−1) is about 152 kJ mol−1 less exothermic than expected for three C=C (3 × −120 = −360): this is the delocalisation (resonance) energy.
  • Properties: colourless liquid, b.p. 80 °C, m.p. 5.5 °C, insoluble in water, good solvent for non-polar substances, burns with a very smoky flame (high C : H ratio). Benzene is toxic and carcinogenic (leukaemia), so methylbenzene is often used instead.
  • Preparation: heating sodium benzoate with soda lime (decarboxylation): C6H5COONa + NaOH → C6H6 + Na2CO3; trimerisation of ethyne over a hot iron tube (3C2H2 → C6H6); reduction of phenol with zinc dust; catalytic reforming of hexane.
  • Because delocalisation is stable, benzene undergoes electrophilic substitution rather than addition: nitration (conc. HNO3 + conc. H2SO4, below 55 °C, electrophile NO2+); halogenation (Br2 with FeBr3 or AlCl3 carrier); Friedel–Crafts alkylation (CH3Cl/AlCl3) and acylation (CH3COCl/AlCl3); sulfonation (fuming H2SO4, SO3).
  • Addition happens only under harsh conditions: hydrogenation with Ni at about 150 °C gives cyclohexane; Cl2 in UV light gives 1,2,3,4,5,6-hexachlorocyclohexane. Disubstituted benzenes: 1,2 (ortho), 1,3 (meta), 1,4 (para) isomers.

Common misconceptions

  • "Benzene has three single and three double bonds that stay in place." No: All six C–C bonds are identical; the π electrons are delocalised round the ring.
  • "Benzene reacts like an alkene and readily decolourises bromine water." No: Benzene resists addition; it reacts with bromine only with a halogen carrier, by substitution.
  • "Nitration of benzene uses dilute nitric acid." No: It needs concentrated HNO3 and concentrated H2SO4 to form the NO2+ electrophile.

Worked examples

Example 1. Describe the mechanism of the bromination of benzene with FeBr3.

Solution

Formation of the electrophile: Br2 + FeBr3 → Br+ + FeBr4−.

The Br+ accepts a pair of π electrons from the ring, forming a positively charged intermediate (the ring's delocalisation is partly lost).

FeBr4− removes H+ from the carbon bearing Br; the delocalised ring is restored: C6H5Br + HBr.

FeBr3 is regenerated: it is a catalyst (halogen carrier).

Example 2. Give three pieces of evidence that benzene is not 'cyclohexa-1,3,5-triene'.

Solution

All C–C bonds have the same length (0.139 nm) instead of alternating 0.154 and 0.134 nm.

Enthalpy of hydrogenation is −208, not −360 kJ mol−1: benzene is 152 kJ mol−1 more stable.

Benzene does not decolourise bromine water and prefers substitution to addition.

Only one 1,2-dibromobenzene exists (Kekulé predicts two).

Practice questions

  1. State the conditions for the nitration of benzene.

    Show answer

    Concentrated HNO3 and concentrated H2SO4, temperature below 55 °C (reflux about 50 °C).

    Common mistake: Using dilute acids.

  2. Why is methylbenzene often used instead of benzene as a solvent?

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    Methylbenzene is far less toxic and not known to cause leukaemia.

    Common mistake: Saying methylbenzene is a stronger acid.

  3. Draw (describe) the structure of nitrobenzene.

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    A benzene ring with one H replaced by –NO2 attached through N.

    Common mistake: Attaching the group through O.

  4. Name the catalyst in Friedel–Crafts reactions.

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    Anhydrous aluminium chloride, AlCl3.

    Common mistake: Saying aqueous AlCl3.

  5. Write the equation for the complete combustion of benzene.

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    2C6H6 + 15O2 → 12CO2 + 6H2O

    Common mistake: Not balancing the oxygen.

  6. Name C6H4(CH3)2 with methyl groups at positions 1 and 3.

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    1,3-dimethylbenzene (meta-xylene).

    Common mistake: Calling it 1,5-dimethylbenzene.

  7. Give two uses of benzene.

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    Making styrene (polystyrene), phenol, nylon intermediates, detergents, dyes, solvents (any two).

    Common mistake: Listing 'cooking fuel'.

  8. Why is the AlCl3 used in Friedel–Crafts reactions kept dry?

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    Water hydrolyses AlCl3, destroying the catalyst.

    Common mistake: Saying water makes it more active.

  9. What is meant by delocalised electrons?

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    Electrons shared by more than two atoms, spread over the whole ring.

    Common mistake: Saying they are lone pairs.

  10. What colour change shows benzene is NOT an alkene when shaken with bromine water?

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    The orange colour remains (no decolourisation).

    Common mistake: Saying it turns colourless.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Hydrogenation of benzene with nickel at about 150 °C gives:a. non-polar
    2. The boiling point of benzene is about:b. C6H6
    3. Benzene is insoluble in water because it is:c. cyclohexane
    4. The molecular formula of benzene is:d. 80 °C
    Show answer

    1 → c (cyclohexane) · 2 → d (80 °C) · 3 → a (non-polar) · 4 → b (C6H6)

    • Three H2 add.
    • It is a volatile liquid.
    • No hydrogen bonding with water.
    • Six carbons, six hydrogens.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    equal in length  ·  FeBr3  ·  H2SO4  ·  nitrobenzene
    1. In benzene, all carbon–carbon bonds are .
    2. The product of nitration of benzene is .
    3. Bromination of benzene needs a catalyst such as .
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    1: equal in length · 2: nitrobenzene · 3: FeBr3

    • Delocalisation makes them identical.
    • C6H5NO2.
    • A halogen carrier polarises Br2.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. The main health hazard of benzene is that it is:a. para
    2. How many isomers of dimethylbenzene exist?b. delocalisation makes benzene more stable
    3. The 1,4-isomer of a disubstituted benzene is also called:c. carcinogenic
    4. The enthalpy of hydrogenation of benzene is less exothermic than expected because:d. 3
    Show answer

    1 → c (carcinogenic) · 2 → d (3) · 3 → a (para) · 4 → b (delocalisation makes benzene more stable)

    • It can cause leukaemia.
    • 1,2-, 1,3- and 1,4-dimethylbenzene.
    • Opposite positions.
    • Delocalisation energy about 152 kJ mol−1.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    sp2  ·  90°  ·  electrophilic substitution  ·  120°
    1. The hybridisation of carbon atoms in benzene is .
    2. The C–C–C bond angle in benzene is .
    3. Benzene typically undergoes .
    Show answer

    1: sp2 · 2: 120° · 3: electrophilic substitution

    • Three σ bonds per carbon in a plane.
    • Planar hexagon, sp2 carbons.
    • The delocalised ring is kept.

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