Which of the following organic compounds can undergo both addition and substitution reactions?

Step 1: Analyze the question We need a compound that can undergo both addition reactions (typical of unsaturated compounds) and substitution reactions (typical of saturated compounds and aromatic rings).

Step 2: Explain the concept

• Alkanes (pentane, propane, hexane) are saturated, with only single C-C and C-H bonds. They undergo substitution reactions, such as the halogenation of methane with chlorine in UV light. They do not undergo addition reactions because they have no double or triple bonds.
• Alkenes and alkynes undergo addition reactions because of their pi bonds.
• Benzene is an aromatic compound with a ring of delocalised electrons. It mainly undergoes substitution reactions, such as nitration, halogenation and alkylation. Under special conditions, it can also undergo addition reactions. For example, hydrogenation with a nickel catalyst at high temperature and pressure gives cyclohexane. Chlorine in strong sunlight or UV light adds to benzene to give hexachlorocyclohexane (benzene hexachloride).

Step 3: Apply to the options

• A. Pentane: alkane, substitution only
• B. Benzene: substitution and addition
• C. Propane: alkane, substitution only
• D. Hexane: alkane, substitution only

Step 4: Answer The correct option is B. Benzene.

Exam tip: Remember "alkanes substitute, alkenes add, benzene does both." If a question asks for a compound that does both, look for benzene among the options.