# Why cannot HCN react with ethene?

I expected ethene ($\ce{C2H4}$) to undergo electrophilic substitution due to $\ce{HCN}$ but it doesn't happen, as I am told by my textbook.

I couldn't figure out the reason, as I found out that $\ce{HCN}$ does react with $\ce{CH3CH2Br}$ by nucleophilic substitution. Upon ionisation, the proton of $\ce{HCN}$ is an electrophile,while the cyanide group is a nucleophile. Correct?

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Yes H+ is a electrophile while CN- is a nucleophile. –  Aditya Sriram Dec 26 '12 at 3:11
then is it a problem with acidity/basicity of ions? –  scienceauror Dec 26 '12 at 15:35
substitution or addition? –  permeakra Dec 26 '12 at 17:59
Ethene can't undergo electrophilic substitution; only addition is possible The nucleophilic Substitution in $\ce{CH3CH2Br}$ is a different reaction. If you can be a more specific, we will be able to help in a better way. –  Siddhartha Sinha Dec 27 '12 at 15:29
The reaction between Ethene and $\ce{HCN}$ can happen catalytically and under high pressure (pubs.acs.org/doi/abs/10.1021/ja01650a034). Since the Cyanide ion is not as nucleophilic as the Bromide ion the double bond of Ethene needs to be activated by a transition metal catalyst. –  Philipp Dec 27 '12 at 22:58

Ethene does not normally undergo nucleophile substitution reaction. Halogenation is an exception as the mechanism through which it goes forms a cyclic intermediate of Halonium ion and hence it substitutes electrophilically to $\pi$-bond.

But in $\ce{CN}$, no such case arises. (probably due to it being less nucleophilic than halogens)

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Under normal conditions ethene normally undergoes addition since the double bond is nucleophilic. In order to have a substitution reaction on ethene H $^{-}$, would need to be the leaving group.
$C_2H_4 + HBr \rightarrow CHCHBr + H^{-}$