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#1 2010-12-05 08:27:47

orgopete
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Why does cyanide only react in SN2 reactions?

Yahoo!Answers in chemistry wrote:

Why does cyanide only react in SN2 reactions?
Why can CN- not react as an SN1? I haven't found an answer yet. Or is it just one of those "because" things in Organic Chemistry? What is it about the triple-bonded structure with a negative charge on the carbon that requires that cyanide undergo SN2 reactions, given the SN2 reactions undergo inversion and are generally primary, and that SN1 reactions generally racemize and are generally tertiary or secondary?

It is not one of those "because" things. It is due to the greater inherent basicity of cyanide than most other SN1 nucleophiles. If the nucleophile is too basic in an SN1 reaction, the mechanism will change to an E2 elimination. If you have a tertiary halide and treat it with sodium ethoxide, the reaction is likely to be E2 because of the basicity of ethoxide. The lower basicity form of ethoxide is ethanol and it can be used in an SN1 reaction. The lower basicity form of cyanide is hydrogen cyanide and it reacts with carbocations on the nitrogen.

Secondly, another difference between SN1 and SN2 reactions is the acidity. In an SN2 reaction of an RX compound with a M(+)Nu(-) nucleophile, the by-products of this reaction will be M(+) X(-). In a reaction of RBr with NaCN, the products are RCN and NaBr.

In a similar SN1 reaction, an RX compound with H(+)Nu(-), the by-products are HX. Obviously, using HCN would change the reactivity. Therefore, SN1 reactions have very weak bases as nucleophiles compatible with the generation of HX from the RX substrate. These are things like Cl(-), Br(-), I(-), or ROH.

A reason SN1 reactions in general may not be found as frequently is because there is little control over whether a product will be substitution or elimination. Therefore, for the purposes of preparing a product, an SN1 strategy may not be the most productive. If the reaction is conducted in a way that an alkene starting material or intermediate is formed, that reaction can be used. For example, the converion of t-butanol to t-butyl bromide with HBr would be an effective reaction because any isobutylene generated would also be converted to the tertiary bromide or isobutylene would be used as the starting material.

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