Free Benzene and Aromatic Compounds MCQs with Answers

1. The Kekule structure of benzene failed to explain

• A. the molecular formula C6H6
• B. the ring shape of the molecule
• C. why benzene resists addition reactions and has all its carbon to carbon bonds of equal length
• D. why benzene contains only carbon and hydrogen

Explanation: Alternating single and double bonds would predict two different bond lengths and the ready addition reactions of an alkene, but experiment shows a single intermediate bond length of 0.139 nm and a strong preference for substitution. The modern picture replaces the alternating bonds with a delocalised ring of six pi electrons above and below the plane. That delocalisation is what makes benzene unusually stable.

2. Benzene undergoes electrophilic substitution rather than addition because substitution

• A. preserves the stable delocalised pi system
• B. produces a smaller molecule
• C. requires no catalyst
• D. is always exothermic

Explanation: Addition would destroy the aromatic sextet and lose the large resonance stabilisation, whereas replacing a hydrogen leaves the delocalised ring intact. This is why benzene reacts with bromine only in the presence of a halogen carrier and gives bromobenzene rather than a dibromo addition product. Nitration, sulphonation and Friedel Crafts reactions follow the same pattern.

3. The nitration of benzene is carried out using

• A. dilute nitric acid alone at room temperature
• B. a mixture of concentrated nitric and concentrated sulphuric acids at about 50 to 60 degrees Celsius
• C. sodium nitrate solution
• D. nitrogen dioxide gas in sunlight

Explanation: Sulphuric acid protonates nitric acid so that it loses water and generates the nitronium ion, which is the actual electrophile attacking the ring. The temperature is kept near 55 degrees because higher temperatures give unwanted dinitrobenzene. Nitrobenzene is important industrially as the starting point for aniline and hence for dyes.

4. Benzene is described as aromatic because it

• A. has a pleasant smell
• B. contains only carbon and hydrogen
• C. reacts readily by addition
• D. is planar, cyclic and has a delocalised system of six pi electrons obeying Huckel's rule

Explanation: Aromaticity in the modern sense requires a planar cyclic conjugated system containing 4n plus 2 pi electrons, which for benzene means six, and this confers exceptional stability. The word originally referred to the smell of such compounds, which is why the first option is the historical but chemically wrong answer. Aromatic compounds characteristically undergo substitution rather than addition.

5. Which one of the following is not a characteristic of benzene?

• A. Aromaticity
• B. Stability
• C. To follow Huckel rule
• D. High reactivity

Explanation: The delocalised ring of six pi electrons makes benzene unusually stable, so it resists addition and reacts only under forcing conditions, preferring substitution which preserves the aromatic system. Aromaticity, stability and obedience to Huckel's 4n plus 2 rule are all genuine features. Expecting benzene to behave like a reactive alkene is precisely the misconception this question targets.

6. Chlorination of benzene in the presence of iron (III) chloride involves mechanism of:

• A. Electrophilic addition
• B. Electrophilic substitution
• C. Free radical substitution
• D. Free radical halogenation

Explanation: Iron three chloride acts as a halogen carrier, polarising the chlorine molecule to generate an electrophile that attacks the electron rich ring, and a hydrogen is then lost so the aromatic system is preserved. Substitution rather than addition is what keeps the delocalised sextet intact. In ultraviolet light and without the catalyst, benzene undergoes free radical addition instead, which is why the conditions matter.