Final answer:
The cyclic halonium intermediate forms due to the stabilization provided to a carbocation by the inductive effect, resulting in a planar intermediate that allows nucleophilic attack and potential formation of enantiomers. Enzymes provide chiral specificity to this process. Some reactions, like electrophilic aromatic substitution, do not involve such cyclic intermediates to preserve aromatic stability.
Step-by-step explanation:
Formation of the Cyclic Halonium Intermediate
The cyclic halonium intermediate forms during certain chemical reactions because it represents a high-energy transition state that is an important part of the mechanism leading to the final product. A carbocation may be formed easily on tertiary carbons due to the stability provided by the inductive effect of neighboring carbons. This facilitates the heterolytic cleavage of the bond to the carbon atom by a more electronegative moiety. The resulting carbocation is planar, allowing attack from a nucleophile on either side without preference, leading to the potential formation of a mixture of enantiomers if the product is chiral.
In enzymatic reactions, such indiscriminate addition is not observed because the active sites are chiral, allowing for only one side to be reactive. This mechanism is typical for tertiary alkyl halides substitutions and allylic phosphates. On the other hand, electrophilic aromatic substitution represents a case where such cyclic intermediates do not form due to the energetic penalty of losing aromatic stability.
Another mechanism involving a cyclic intermediate is the E1cB elimination reaction, which is relevant in biological transformations, particularly due to the stable carbanionic intermediates formed from carbonyl compounds. The E1cB mechanism illustrates a scenario where a carbanion is formed first, followed by the elimination of an electrophilic group.