Which of the following chlorides can be made in good yield...

Which of the following chlorides can be made in good yield by light-catalyzed monochlorination of the corresponding hydrocarbon? (a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Cl}$ (c) $\left(\mathrm{CH}_3\right)_3 \mathrm{CCH}_2 \mathrm{Cl}$ (b) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Cl}$ (d) $\left(\mathrm{CH}_3\right)_3 \mathrm{CCl}$

Which of the following chlorides can be made in good yield by light-catalyzed monochlorination of the corresponding hydrocarbon?
(a) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Cl}$
(c) $\left(\mathrm{CH}_3\right)_3 \mathrm{CCH}_2 \mathrm{Cl}$
(b) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Cl}$
(d) $\left(\mathrm{CH}_3\right)_3 \mathrm{CCl}$

Key Concepts

Free Radical Halogenation

This is a chain reaction mechanism where light activates a halogen molecule (in this case, chlorine) to form radicals, which then react with the hydrocarbon by abstracting a hydrogen atom, generating a carbon radical. This radical then reacts with another chlorine molecule to form the chlorinated product and a new chlorine radical that propagates the chain. The process is inherently less selective when using chlorine compared to bromine, which can lead to mixtures if multiple types of hydrogen atoms are present.

Radical Stability

The stability of the carbon radical intermediates is a critical factor affecting the outcome of free radical halogenation. More substituted radicals, like tertiary radicals, are generally more stable than secondary or primary radicals. Although chlorine's reactivity does not discriminate as much as bromine’s, differences in radical stability can still influence the regioselectivity and overall yield of the monochlorinated products.

Regioselectivity in Radical Substitution

Regioselectivity pertains to the preferential formation of one constitutional isomer when a hydrogen atom is abstracted during the halogenation reaction. In substrates where only one type of hydrogen or a sterically/electronically favored hydrogen is available, the monochlorination can yield predominantly a single product, leading to good yield. However, if the hydrocarbon possesses multiple different types of hydrogens, the low selectivity of chlorine can result in a mixture of products rather than a single major one.

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