question_answer

Account for the following: (a) Chloromethane reacts with KCN to form ethanitrile as main product and with AgCN to form methyl isocyanide as chief product. (b) Use of DDT was banned in United States in 1973. (c) Benzylic halides show high reactivity towards \[{{S}_{N}}1\] reaction.

Answer:

                (a) Cyanide ion is an ambident ion. It has the following two resonating structures: Since KCN is predominantly ionic, the reaction can occur either through carbon or nitrogen. Because C-C bonds are stronger than C-N bonds, therefore, carbon end of \[C{{N}^{-}}\] ion acts as the nucleophile on chloromethane to give enthanenitrite as the main product. In contrast, Ag?CN: is predominantly covalent. Therefore, only electron pair on nitrogen is available for nucleophilic attack on chloromethane to form methyl isocyanide as the chief product (b) Due to chemical stability of DDT and its fat solubility, DDT is not completely metabolized very rapidly by the animals. Instead it gets deposited and stored in fatty tissues of animals which causes toxicity. As a result, use of DDT was banned in U.S. in 1973. (c) Benzyl halides reality undergo ionization to give benzyl cation which is stabilized by resonance (refer to page 10/31). Since carbocations are reactive chemical species, therefore, benzyl halides are very reactive in \[{{S}_{N}}1\] reactions.