Kornblum rule
The Kornblum rule is a concept of organic chemistry that allows estimation of the preferentially formed product of a nucleophilic substitution with ambident nucleophiles. The rule is based on the concept of hard and soft acids and bases (HSAB concept). In the case of an SN1 reaction, the harder (more electronegative) position of the nucleophile is preferred, whereas in the case of an SN2 reaction, the softer (more nucleophilic) part Edit sample source text
The Kornblum rule does not apply to all reactions with ambident nucleophiles; it is only an estimate of the main and the by-product. In the following example, using the cyanide anion | N≡C | as nucleophile, however, it is fulfilled:
This means that in a SN1-like reaction, the nitrogen attacks and in an SN2-like the carbon.
This results in the following SN1 reaction: CH2 = CH-CH2-Cl + KCN is preferentially the isonitrile compound CH2 = CH-CH2-NC, while the likewise conceivable nitrile compound CH2 = CH-CH2-CN only as By-product.
Please note that the reaction direction depends on the reaction conditions. Polar-aprotic solvents favor SN2 reactions, since here the hardly solvated anions can play their nucleophilicity fully, whereas in protic solvents the solvated anions are impaired in their reactivity and SN1 reactions preferentially proceed
Cations can also influence the direction of the reaction. If KCN is replaced by AgCN in the above example, a higher yield of isonitrile is obtained since the silver ion as a soft Lewis acid is preferentially coordinated to the (softer) C atom of the cyanide ion Attack. As a result, the reaction direction is shifted to "SN1-like". In the reaction of alkyl halides with silver nitrite, more alkyl nitrites are obtained compared to alkali metal cyanides, since the silver ion coordinates to the so-called softer N atom. Furthermore, Ag ions assist the detachment of chloride, bromide and iodide by coordination to the halogen, thereby enhancing the positive polarization of the adjacent C atom; the complex dissociates to the silver halide and the carbenium ion, where rearrangements can also be observed. Single-level Edit source text
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