# SN1 or SN2 reaction?

Here, this is a 1° allylic halide. If it goes by SN1, the intermediate will be resonance stabilised.

Also, since it is 1° Halide, it can also go via SN2. Which one will be preferred?

• Don't forget $S_{\mathrm{N}}2'$... – Zhe May 8 '17 at 21:29
• Sn2 might be preferable because {\ce{CN-}} is a strong nucleophile and they didn't mention of any polar solvent... – Suraj S May 9 '17 at 3:59

This reaction will happen by both SN1 and SN2 mechanism, apart from this SN1' and Sn2' (allylic mechanism ) will also occur. Thus from SN1 and Sn2 one will get primary cyanide and from SN1' and SN2' one will get secondary cyanide .( read organic chemistry by jerry march for the detailed explanation). I am attaching a screenshot of clayden which explains this beautifully.

• Please add the reference for the table and the highlighted block of text. – Klaus-Dieter Warzecha Jan 6 '17 at 9:28
• organic chemistry by clayden - oxford university press - 1st edition -2000 page 424 – aks0854 Jan 6 '17 at 9:33
• this reaction does not include any polar solvent so SN2 is prefered over SN1. – Het May 9 '17 at 2:57

Although I haven't found this exact example in the literature, I suspect this reaction is exclusively SN2.

The analysis with respect to the electrophile is good: the allylic leaving group is suitable for both SN1 and SN2. That means that the other conditions will determine which is preferred.

Cyanide is among the best nucleophiles for SN2 reaction. First, it is negatively charged. Second, because it only has two atoms, it is extremely unhindered.

As @CupC_56 suggests, solvent (unlisted) could play a role. However, typically for SN1 reactions, a polar protic solvent acts also as the nucleophile. If one was trying to obtain a nitrile product, it would be a poor choice to run the reaction in the presence of a polar protic solvent that would (1) reduce the nucleophilicity of cyanide and (2) compete with cyanide to react with the electrophile.

Well, Your Question seems a bit off as the type of solvent has not been mentioned. Is it:-

1. Non-polar
2. Polar (protic or aprotic)

Obviously, SN1 can occur only in Polar protic solvents, and in reactions where the alkyl halide is a conflicting one (reaction can occur both by SN1 and SN2), the mentioning of the said solvent is essential.