Question 1

Why is benzene more stable than expected?

Accepted Answer

Benzene is more stable than the hypothetical cyclohexa-1,3,5-triene because its six p-electrons are delocalised over the entire ring, forming a continuous π-system. This delocalisation lowers the energy of the molecule by about 152 kJ/mol, a value known as the resonance energy. This extra stability means benzene does not undergo addition reactions like alkenes, as addition would disrupt the delocalisation and result in a less stable product.

Question 2

What is the difference between Kekulé and delocalised models of benzene?

Accepted Answer

The Kekulé model proposes alternating single and double bonds in a six-membered ring, with the double bonds rapidly oscillating. However, this model predicts two distinct bond lengths and that benzene should undergo addition reactions, which it does not. The delocalised model, supported by X-ray diffraction data, shows all carbon-carbon bonds are identical (0.139 nm) and the six p-electrons are spread evenly over the ring. This model explains benzene's stability and its preference for substitution over addition.

Question 3

How do you nitrate benzene?

Accepted Answer

Nitration of benzene is carried out using a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at around 50°C. The sulfuric acid acts as a catalyst, protonating nitric acid to form the nitronium ion (NO₂⁺), which is the electrophile. The nitronium ion attacks the benzene ring, forming an arenium ion intermediate, which then loses a proton to give nitrobenzene. The overall reaction is: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.

Question 4

What are activating and deactivating groups in aromatic chemistry?

Accepted Answer

Activating groups are substituents that increase the electron density of the benzene ring, making it more reactive towards electrophilic substitution. They are typically ortho/para directors. Examples include -OH, -NH₂, -OCH₃, and alkyl groups. Deactivating groups decrease electron density, making the ring less reactive, and are meta directors. Examples include -NO₂, -CN, -COOH, and -SO₃H. Halogens are unique: they are deactivating due to their strong inductive electron withdrawal but ortho/para directing because of their lone pairs that can be donated by resonance.

Question 5

Why does phenol react with bromine water but benzene does not?

Accepted Answer

Phenol has a hydroxyl group (-OH) attached to the benzene ring. The oxygen atom donates electron density into the ring via resonance, making the ring much more electron-rich than benzene. This increased electron density means phenol can react with electrophiles like bromine without a catalyst. In bromine water, phenol undergoes rapid electrophilic substitution at the ortho and para positions, forming 2,4,6-tribromophenol (a white precipitate). Benzene, lacking such activation, requires a halogen carrier (e.g., FeBr₃) and heat to react with bromine.

Question 6

What is Friedel-Crafts acylation and why is it useful?

Accepted Answer

Friedel-Crafts acylation is an electrophilic substitution reaction where an acyl group (RCO-) is attached to a benzene ring. It uses an acyl chloride (RCOCl) and a Lewis acid catalyst like AlCl₃. The catalyst helps generate the acylium ion (RCO⁺), which acts as the electrophile. This reaction is useful because it introduces a ketone group onto the benzene ring, which can then be reduced to an alkyl group or used in further synthesis. Unlike Friedel-Crafts alkylation, acylation does not suffer from polyalkylation because the product is less reactive than benzene.

Aromatic chemistry (A-level only)

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Related Topics in AQA A-Level Chemistry

Acids and bases (A-level only)

Alcohols

Aldehydes and ketones (A-level only)

Alkanes