To understand the relative reactivity of secondary versus primary hydrogens in the free radical bromination of n-butane, we need to analyze the ratio of products formed. The ratio of 1-bromo to 2-bromobutane is given as 7:39.

1. Understanding the Reaction:
The free radical bromination of n-butane involves the substitution of a hydrogen atom by a bromine atom. This reaction is initiated by the abstraction of a hydrogen atom from n-butane by a bromine radical (Br•). This leads to the formation of a butyl radical (•C4H9), which reacts further with molecular bromine (Br2) to form brominated products.

2. Formation of 1-bromobutane:
In the free radical bromination of n-butane, the abstraction of a hydrogen atom from a primary carbon results in the formation of 1-bromobutane. As the ratio of 1-bromo to 2-bromobutane formed is 7:39, we can conclude that the abstraction of a hydrogen atom from a primary carbon (primary hydrogen) is less favorable compared to the abstraction from a secondary carbon (secondary hydrogen).

3. Formation of 2-bromobutane:
In the free radical bromination of n-butane, the abstraction of a hydrogen atom from a secondary carbon results in the formation of 2-bromobutane. As the ratio of 1-bromo to 2-bromobutane formed is 7:39, we can conclude that the abstraction of a hydrogen atom from a secondary carbon (secondary hydrogen) is more favorable compared to the abstraction from a primary carbon (primary hydrogen).

4. Relative Reactivity:
From the given ratio of products, we can infer that the secondary hydrogens in n-butane are more reactive compared to the primary hydrogens. This means that the abstraction of a hydrogen atom from a secondary carbon is more likely to occur during the free radical bromination of n-butane.

5. Answer:
The correct answer is option 'B' which states that the relative reactivity of secondary versus primary hydrogens in the free radical bromination of n-butane is 20:1. This ratio is derived from the given ratio of 1-bromo to 2-bromobutane formed (7:39).

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