A highly selective photochemical chlorination of esters,...

A highly selective photochemical chlorination of esters, amides, and alcohols can be carried out in $70-90 \% \mathrm{H}_{2} \mathrm{SO}_{4}$ using $N$-chlorodialkylamines as the chlorinating agents. Mechanistic study indicates that the reaction occurs by the following chain sequence: A very interesting feature of the reaction is that the chlorine is introduced with high selectivity at the next-to-terminal position for molecules with $n=4$ to 6. In contrast, chlorination in nonpolar solvents does not show comparable selectivity. Rationalize these observations.

A highly selective photochemical chlorination of esters, amides, and alcohols can be carried out in $70-90 \% \mathrm{H}_{2} \mathrm{SO}_{4}$ using $N$-chlorodialkylamines as the chlorinating agents. Mechanistic study indicates that the reaction occurs by the following chain sequence:
A very interesting feature of the reaction is that the chlorine is introduced with high selectivity at the next-to-terminal position for molecules with $n=4$ to 6. In contrast, chlorination in nonpolar solvents does not show comparable selectivity. Rationalize these observations.

Key Concepts

Radical Chain Mechanism

This concept relates to reactions that proceed through a series of steps—initiation, propagation, and termination—in which radicals are formed and then react with substrates to sustain the chain. In photochemical chlorination, a reactive chlorine radical abstracts a hydrogen atom from the substrate, leading to the formation of a substrate-centered radical which then reacts further to maintain the chain reaction.

Solvent Polarity Effects

The behavior of reactive intermediates is significantly influenced by the polarity of the solvent. In highly concentrated sulfuric acid, the strongly polar environment can stabilize charged or polarizable radicals and transition states, thereby affecting the reactivity and selectivity of the chlorination process compared to nonpolar solvents.

Substrate Protonation and Polar Effects

Under strongly acidic conditions, functional groups such as esters, amides, and alcohols may undergo protonation, altering their electronic properties. This protonation can stabilize specific radical intermediates and direct the reaction pathway, resulting in a selective chlorination at the next-to-terminal position in molecules with certain chain lengths.

Reaction Site Selectivity in Radical Halogenation

Selectivity in radical halogenation often arises from the interplay between the stability of generated radicals and the influence of electronic and steric factors. The combination of polar effects and protonation in an acidic medium preferentially stabilizes radicals at specific positions, such as the next-to-terminal carbon, thereby controlling the site at which chlorination occurs.

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