Ambident nucleophile
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Ambident nucleophile
Jul 25, 2022, 16:45 IST
What is Ambident nucleophile
Some nucleophiles have lone pair of electrons on more than one atom and can attack through more than one site. Such nucleophiles are called ambident nucleophiles. In such cases, different products are possible due to attack through different sites. Attack by a specific site can be promoted under special conditions. Two well-known examples are discussed in detail.
Attack By CN- Ambident nucleophile (-:C o N:)
In CN-, carbon (negatively charged) will be a soft base as compared to nitrogen. So, if the reaction proceeds via SN1 mechanism, which produces a free carbocation (a hard acid), then attack through nitrogen (hard base) will take place. But if the reaction proceeds via SN2 mechanism (small positively charged carbon is soft acid) then attack through carbon (soft base) will take place. So, if we want to increase relative yield of nitriles, we can use NaCN or KCN etc in a less polar solvent, which facilitates SN2 substitution. Similarly, if we want to increase the yield of isonitriles, we can use AgCN. Ag+ has very strong affinity for X-, so it favours the formation of R+ and the reaction proceeds via SN1 mechanism. This will result in attack by hard base giving R-NC. Further if we compare primary, secondary and tertiary alkyl halides, formation of R-NC should be favoured due to more favourable SN1 substitution in tertiary alkyl halide. But the exception is that tertiary alkyl halides undergo elimination and the yield decreases. This is because CN- is a strong base, which can also cause elimination reaction.
Attack By NO2- Ambident nucleophile(-O-N=O)
alkane nitrite nitro alkane In NO 2- , oxygen (negatively charged) will be a hard base as compared to nitrogen. So, if the reaction proceeds via SN1 mechanism then attack through oxygen (hard base) will take place to produce alkane nitrite. But if the reaction takes place via SN2 mechanism then attack through nitrogen (soft base) takes place to give nitro alkane. If we want to increase the yield of nitro alkane, the reaction should proceed via SN2 mechanism. i.e. we can use NaNO2, KNO2 etc. Moreover, the yield will be best if we use primary alkyl halide and less polar solvent. Formation of nitrite will dominate, if we use tertiary alkyl halide, more polar solvent and AgNO2 because Ag+ has strong affinity for X- and can form a carbocation to force the reaction to proceed via SN1 mechanism. Primary alkyl halide with AgNO2 chiefly gives nitro alkane but if secondary and tertiary alkyl halides are used then AgNO2 will yield nitrite as the major product.read class 12 Notes for more detail .
Find below reactions of Ambident nucleophile
