Haloalkane Reactivity and SN2 Reaction Mechanism

Reactivity of Ambident Nucleophiles with Haloalkanes

Haloalkanes react with potassium cyanide ($KCN$) to form alkyl cyanides as the main product, whereas reaction with silver cyanide ($AgCN$) yields isocyanides as the chief product. Potassium cyanide ($KCN$) is predominantly ionic in nature and dissociates to provide cyanide ions in solution. Although both carbon and nitrogen atoms in the cyanide ion are in a position to donate electron pairs, the nucleophilic attack takes place mainly through the carbon atom rather than through the nitrogen atom because the resulting $C-C$ bond is more stable than a $C-N$ bond. Conversely, silver cyanide ($AgCN$) is mainly covalent in nature. As a result, the nitrogen atom is free to donate its electron pair, resulting in nucleophilic attack through nitrogen and forming isocyanide as the main product.

Nucleophiles that possess two distinct points of attachment are classified as ambident nucleophiles. The cyanide ion is an example of an ambident nucleophile; attack through its carbon atom results in alkyl cyanides, whereas attack through its nitrogen atom leads to isocyanides. Similarly, the nitrite ion represents an ambident nucleophile with two different points of linkage, represented as $[-O-N=O]^{-}$. Linkage through an oxygen atom results in the formation of alkyl nitrites, whereas linkage through the nitrogen atom leads to nitroalkanes.

Nucleophilic Substitution Bimolecular (S_N2) Reaction Mechanism

Nucleophilic substitution reactions in haloalkanes proceed via distinct mechanisms, one of which is Substitution Nucleophilic Bimolecular ($S_N2$). The reaction between methyl chloride ($CH_3Cl$) and hydroxide ion ($OH^-$) to yield methanol ($CH_3OH$) and chloride ion ($Cl^-$) follows $S_N2$ pathways. This reaction follows second-order kinetics, meaning that the rate of the reaction depends directly upon the concentration of both reacting species.

In the year 1937, Edward Davies Hughes and Sir Christopher Ingold proposed the mechanism for an $S_N2$ reaction. The mechanism depicts a bimolecular nucleophilic substitution process where the incoming nucleophile interacts directly with the alkyl halide. This interaction causes the carbon-halide bond to break while a new bond is simultaneously formed between the carbon atom and the attacking nucleophile. In the reaction of methyl chloride with hydroxide ion, the formation of the new $C-O$ bond between carbon and the hydroxide group ($-OH$) and the cleavage of the carbon-chlorine bond occur simultaneously in a single concerted step.

Spatial representation of molecular structures follows standard three-dimensional conventions detailed in Section 12.3.2 of Class XI: a solid wedge represents a chemical bond coming out of the plane of the paper toward the viewer, a dashed line represents a bond going down away from the plane of the paper, and a straight line represents a bond lying directly in the plane of the paper. In diagrammatic representations of $S_N2$ reactions, such as Fig. 6.2, a red ball represents the incoming hydroxide ion ($OH^-$), and a green ball represents the outgoing halide ion ($Cl^-$).