Quick answer: Benzene, C₆H₆, is a planar ring with six delocalised π electrons and six equal C–C bonds of 0.140 nm. This Chemistry 11-12 worksheet on benzene gives 20 problems from easy to hard: describing benzene, bond lengths and naming arenes. The worked answers are on a separate page, and the worksheet is a free printable PDF.
These 20 problems cover the structure of benzene, the evidence for delocalisation, and the mechanisms of nitration, halogenation and Friedel–Crafts reactions. Some require calculations of enthalpy, yield and atom economy, and others ask you to explain and describe.
The level is A Level chemistry, IB Chemistry and CBSE Class 11-12. Use Ar values from your data sheet and give each final answer in bold with a unit where needed.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.10
Problem 1 Describing benzene ★★★
Answer the questions about benzene.1. State its molecular formula.
2. Then state the bond angle between the carbon atoms in the ring.
3. Next, state the number of delocalised electrons in the ring.
4. Finally, state the shape of the molecule.
Problem 2 Bond lengths ★★★
The figure shows typical bond lengths.1. State which bond length benzene’s C–C bonds are closest to.
2. Then explain what the figure tells you about the structure of benzene.
3. Finally, explain why this result is not what the Kekulé model predicts.
Problem 3 Naming arenes ★★★
Name each compound.1. C₆H₅NO₂
2. C₆H₅Br
3. C₆H₅CH₃
4. C₆H₅NH₂
Problem 4 Reagents for substitution ★★★
State the reagents and any catalyst for each reaction of benzene.1. Nitration
2. Bromination
3. Reaction with CH₃COCl
Problem 5 Why electrophiles? ★★★
Benzene reacts with electrophiles, not nucleophiles.1. Define the term electrophile.
2. Then explain why the benzene ring attracts electrophiles.
Problem 6 Stabilisation energy ★★★
Use the enthalpy level diagram. ΔH(hydrogenation) of cyclohexene is −120 kJ/mol and of benzene is −208 kJ/mol.1. Calculate the predicted value for a Kekulé benzene.
2. Then calculate the stabilisation energy.
3. Finally, state what this energy shows.
Problem 7 Bromine water test ★★★
Cyclohexene decolourises bromine water immediately, but benzene does not.1. Name the type of reaction of cyclohexene.
2. Then explain the difference in behaviour using electron density.
3. Finally, state what conditions would make benzene react with bromine.
Problem 8 Making the nitronium ion ★★★
1. Write the equation for the formation of NO₂⁺ from HNO₃ and H₂SO₄.2. Then state the role of sulfuric acid.
3. Finally, write the equation for the nitration of benzene.
Problem 9 Describing the mechanism ★★★
The figure shows the intermediate formed when bromine attacks benzene.1. State what the dashed arc represents.
2. Then describe how the intermediate forms from benzene.
3. Next, describe how it forms bromobenzene.
4. Finally, explain why the catalyst FeBr₃ is needed.
Problem 10 Halogen carrier ★★★
1. Write the equation for the reaction of Br₂ with FeBr₃ that produces an electrophile.2. Then write the equation for the overall bromination of benzene.
3. Finally, explain why FeBr₃ is called a catalyst.
Problem 11 Yield of nitrobenzene ★★★
Bilal nitrates 15.6 g of benzene (M = 78.0 g/mol) and obtains 18.5 g of nitrobenzene (M = 123.0 g/mol).1. Calculate the moles of benzene.
2. Then calculate the theoretical mass of nitrobenzene.
3. Finally, calculate the percentage yield.
Problem 12 Friedel–Crafts acylation ★★★
Phenylethanone is made from benzene and ethanoyl chloride, CH₃COCl.1. Write the equation.
2. Then name the catalyst and explain why the reaction must be dry.
3. Finally, name the electrophile.
Problem 13 Kekulé and isomers ★★★
Kekulé suggested two forms of 1,2-dibromobenzene, one with a C=C bond between the Br-carbons and one with a C–C bond.1. Explain why this was a problem for the model.
2. Then state how the delocalised model solves it.
Problem 14 Reading a table of reactivity ★★★
Compare the observations in the table.Reagent: bromine water | acidified KMnO₄ | Br₂ with FeBr₃
Cyclohexene: decolourises | decolourises | decolourises
Benzene: no change | no change | decolourises, HBr fumes
1. State what the first two columns show about benzene.
2. Then state what the third column shows about the type of reaction benzene undergoes.
Problem 15 Two-step synthesis ★★★
The figure shows benzene converted to phenylamine.1. Name the conditions for step 1 and give the temperature limit.
2. Then write an equation for step 2 using [H] for the reducing agent.
3. Finally, explain why the temperature limit is needed in step 1.
Problem 16 Atom economy ★★★
Nitration: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.1. Calculate the atom economy to 3 significant figures (M: C₆H₆ 78.0, HNO₃ 63.0, C₆H₅NO₂ 123.0).
2. Then suggest one reason the actual yield is less than 100 %.
Problem 17 Dinitration ★★★
If the temperature rises above 55 °C, a second nitro group enters the ring.1. Write the equation for the formation of 1,3-dinitrobenzene (C₆H₄N₂O₄) from nitrobenzene.
2. Then calculate the mass of 1,3-dinitrobenzene (M = 168.0 g/mol) from 0.100 mol of nitrobenzene with 100 % yield.
3. Finally, explain why the temperature is kept low.
Problem 18 Why substitution wins ★★★
After an electrophile attacks benzene, the ring can either add a nucleophile or lose H⁺.1. Explain which path gives a more stable product.
2. Then use the stabilisation energy of 152 kJ/mol in your explanation.
3. Finally, state what the term aromatic means.
Problem 19 Hydrogenation data ★★★
Enthalpy changes of hydrogenation: cyclohexene −120 kJ/mol; cyclohexa-1,3-diene −232 kJ/mol; benzene −208 kJ/mol.1. Calculate the value predicted for cyclohexa-1,3-diene with two isolated double bonds.
2. Then compare the extra stability of the diene with that of benzene.
3. Finally, explain what the data suggest about benzene.
Problem 20 Is benzene unreactive? ★★★
Write a short paragraph evaluating the statement: benzene is unreactive. Use evidence from its stability, its mechanism and the need for strong reagents.
