Question

In the case of halogenation reaction of alkanes, abstract of primary hydrogen yields a primary radical, and abstract of secondary hydrogen yields a secondary radical.
\(\mathrm{R-H} + \mathrm{Br}^- \longrightarrow \mathrm{R}^- + \mathrm{Br-H}\)
The above example is of:

• A. High reactivity and low selectivity
• B. Low reactivity and high selectivity
• C. High reactivity and high selectivity
• D. Low reactivity and low selectivity

Answer 1

The Correct Option is B

The question revolves around the concept of halogenation in alkanes. In the example provided:

\(\mathrm{R-H} + \mathrm{Br}^- \longrightarrow \mathrm{R}^- + \mathrm{Br-H}\)

This reaction is essentially a free radical substitution reaction. The halogenation process, specifically with bromine, exhibits low reactivity and high selectivity. Let's break down these terms:

1. Reactivity: Reactivity refers to how readily the reactants participate in the chemical reaction. In halogenation of alkanes, bromine (Br₂) is less reactive than chlorine (Cl₂). This low reactivity implies that the reaction proceeds slowly, giving more time for the reaction to occur selectively.
2. Selectivity: Selectivity is the preference for forming a specific radical intermediate and consequently a particular product. Bromine shows higher selectivity because it preferentially abstracts (removes) hydrogen from the more substituted carbon – for example, a secondary or tertiary hydrogen, leading to a more stable radical.

The option "Low reactivity and high selectivity" explains the nature of bromination (using Br₂) in alkanes. This results in low activation energy requirement and the formation of the most stable radical intermediate, thus giving highly selective product formation. Therefore, the correct answer is Low reactivity and high selectivity.

Ruling out other options:

• High reactivity and low selectivity: This describes chlorination, where Cl₂ is more reactive and less selective.
• High reactivity and high selectivity: This combination is not typical in free radical halogenation reactions.
• Low reactivity and low selectivity: This is incorrect for bromination as it reflects a non-selective process, contrary to bromine's selectivity.

Hence, the bromination of alkanes is characterized as having low reactivity and high selectivity.