# Rate of SN2 reaction depending on following different conditions in case of primary halides

Most important points to remember when thinking of $\mathrm{S_N2}$ reactions are that they are nucleophilic reactions, a pentavalent transition state is formed, depends on steric hindrance, nucleophilic and substrate used i.e., what kind of leaving group is attached to it. Strength of the bonds bonds, etc. I know these, in the following conditions the rate of $\mathrm{S_N2}$ increases, but I don't seem to understand the reason behind it.

1) the presence of unsaturation on beta-$\ce{C}$ in primary halide increases $\mathrm{S_N2}$ reaction rate.

I really don't seem to understand this one, I know for the alpha $\ce{C}$ it would be difficult because the halide and the carbon will acquire partial double bond character which would make it very difficult to break.

2) the presence of carbonyl group at alpha carbon is most suitable for $\mathrm{S_N2}$ reaction.

my reason: it might be because of tautomerism, i.e., due to its conversion to enol form it will increase the reactivity.

3) presence of hetero $\ce{O,N,S}$ atom at alpha carbon in primary halides increase $\mathrm{S_N2}$ reaction.

This too, I fail the understand.

• 1) Please don’t use MathJax in titles due to searching issues. 2) Please use \ce{...} always and exclusively for chemical notations i.e. atoms and compounds. Note that $\mathrm{S_N2}$ reactions are better typeset using $\mathrm{S_N2}$ because the 2 should not be an index. 3) Please note that all punctuation marks except for opening brackets, opening quotation marks or mid-word apostrophes are followed by a space. (Groups of quotation marks — like at the end of this phrase — have a space at the end of the group.) That makes your post easier to read. – Jan Oct 27 '15 at 12:29