In radical chlorination of alkanes, non-equivalent hydrogens react with chlorine

Question

In radical chlorination of alkanes, non-equivalent hydrogens react with chlorine atoms at different rates. At 35 °C, primary, secondary, and tertiary bonds react at relative rates of 1 3.9 5.2 respectively. These are conditions of kinetic control where product ratios are determined by relative rates of formation. For example, if A is formed twice as fast as B, the A:B product ratio will be 2. Consider chlorination of the alkane below at 35 oC. 1. Specify the most reactive C-H bond(s, a-c. Two non-equivalent C-H bonds of comparable reactivity should be separated by commas, 1.e. a,c. 2. Specify the site of chlorination in the major monochloro substitution product, a- Two products that form in comparable quantities should be separated by commas, 1.e. a.c

Explanation / Answer

Ans. The secondary hydrogen's are more reactive when compared to primary the b carbon has one secondary hydrogen which is more active because it stabilizes the secondary radical formed .

So b is the most reactive c-h bond

No.of primary hydrogen's = 12

No.of tertiary hydrogen's = 2

Ratio of primary to tertiary hydrogen's is 6

This means that for every 1 tertiary hydrogen reacts there would be 6 primary hydrogen's reacting

So a position has more chances of forming major product.

B.) Similar concept can be applied to case b,

Number of primary hydrogen atoms = 12

Number of secondary H2 = 4

Number of tertiary hydrogen atoms= 2

Most reactive c-H bonds are a,b,c

Major product are c,b,a

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