# Alkylation of alkenes by nucleophilic substitution

A double bond/alkene reacts as a nucleophile. I have never seen a reaction where it takes part in an $S_N1$ or $S_N2$ reaction with a halogenoalkane for example. This would presumably be a very useful reaction yet I have never seen it. I assume this is because it doesn't work. Is this the case? If so why?

• A double bond/alkene reacts as a nucleophile. I have never seen a reaction where it takes part in an $S_N1$ or $S_N2$ reaction : If you've never seen a reaction where a double bond participates in a nucleophilic reaction, how can you say that it reacts as a nucleophile? – Binary Geek Apr 15 '15 at 15:20
• ...with a halogenoalkane (as it says in the question) – RobChem Apr 15 '15 at 15:54

The pi electrons in an alkene or alkyne can act as a nucleophile. This class of reactions is often taught a bit later than the standard $\ce{S_{N}1}$ and $\ce{S_{N}2}$ reactions and comes up when neighboring group participation or anchimeric assistance is covered.
The pi electrons in a double or triple bond are not strongly nucleophilic but when you arrange things in their favor, pi electrons can act as a nucleophile. The reactions involving double bonds that I am familiar with involve an $\ce{S_{N}1}$ mechanism. Here are a few examples.
In example (1) notice how the compound with the double bond solvolyzes $10^{11}$ times faster than the saturated model compound! Note too that only the product with acetate group anti to the double bond is formed.