HSAB theory#Kornblum.27s rule

The Kornblum 's rule is a concept of organic chemistry, which allows the estimation of the preferred product formed a nucleophilic substitution with ambident nucleophiles. The rule is based on the concept of hard and soft acids and bases ( HSAB concept). In an SN1 reaction preferentially attacks the harder ( more electronegative ) position of the nucleophile to, at a SN2 reaction against the softer ( more nucleophilic ) part.


The Kornblum rule does not apply for all reactions with ambident nucleophiles, it is only an estimate, which arises between main and by-product. In the following example, with the cyanide anion | N ≡ C- | -, however, the nucleophile is met:

This means that engages at an SN1 - type reaction of nitrogen and at a SN2-type of carbon.

This creates in the following SN1 reaction: CH2 = CH -CH2- Cl KCN preferably the isonitrile compound CH2 = CH- CH2 -NC, while also conceivable nitrile compound CH2 = CH- CH2 -CN is formed only as by-product.

It should be noted that the direction of reaction depends on the reaction conditions. Polar aprotic solvents favor SN2 reactions because here the scarcely solvated anions can fully demonstrate their nucleophilicity, while in protic solvents, the solvated anions are hindered in their reactivity and so SN1 reactions occur preferentially.

Even cations can affect the reaction direction: we replace in the above example KCN by AgCN, we obtain a higher yield of isonitrile because the silver preferred coordinates as a soft Lewis acid to the (softer ) carbon atom of the cyanide ion and this thereby preventing the attack. Thus, the reaction direction is moved to " SN1 - like". In the reaction of alkyl halides with silver nitrite is obtained - as compared with alkali metal - more alkyl nitrites, since the silver ion to the - coordinated nitrogen atom - in this case smoother. Furthermore, Ag ions support the replacement of chloride, bromide and iodide by coordination to the halogen, whereby the positive polarization of the adjacent carbon atom is amplified; If necessary, the complex dissociates to the silver halide and the carbenium ion, which then also rearrangements are observed.