Free Chemistry of Hydrocarbons MCQs with Answers

21. Terminal alkynes show acidic character due to:

• A. Terminal carbon atoms are sp hybridized
• B. Terminal carbon atoms are sp2 hybridized
• C. Terminal carbon atoms are sp3 hybridized
• D. Terminal carbon atoms show hydrogen bonding

Explanation: An sp hybrid orbital has 50 per cent s character, so it holds the bonding electrons closer to the carbon nucleus and the attached hydrogen is released more easily as a proton. This is why ethyne gives a precipitate with ammoniacal silver nitrate while ethene and ethane do not. Acidity therefore rises with s character in the order sp3 to sp2 to sp.

22. What is general requirement for free radical substitution reaction of ethane and halogen?

• A. Low pressure
• B. Low temperature
• C. High pressure
• D. Heat and UV light

Explanation: Initiation requires the halogen molecule to split homolytically into two radicals, and only ultraviolet light or a high temperature supplies enough energy to break that bond. In the dark and in the cold, an alkane and a halogen simply do not react, which is a standard demonstration of how unreactive alkanes are. Once started, the chain propagates on its own until two radicals combine.

23. 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.

24. 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.