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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
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substitution or addition? –  permeakra Dec 26 '12 at 17:59
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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
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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
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3 Answers 3

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The "classical" way

enter image description here

Direct addition of $\ce{H-X}$ to alkenes proceeds via the addition of a proton to the $\ce{C=C}$ double bond in the initial step, i.e via a carbocation. This works nicely for strong mineral acids, but not for the weak acid $\ce{HCN}$ with the rather weak nucleophile $\ce{CN-}$.

Synchronous addition?

Q: Can't $\ce{H-CN}$ add synchronously add to $\ce{C=C}$?

A: NO! Such a reaction would involve the interaction of the HOMO (highest occupied molecular orbital) of one reaction partner with the LUMO (lowest unoccupied molecular orbital) of the other. The LUMO of an alkene has a nodal plane orthogonal to the $\ce{C-C}$ bond, while there is - obviously - no nodal plane but a $\sigma$ bond between hydrogen and carbon in $\ce{HCN}$.

The synchronous addition of $\ce{H-CN}$ to $\ce{C=C}$ is symmetry-forbidden!

Catalysis?

Yes! Since the first catalytic hydrocyanation using $\ce{Co2(CO)8}$ (see Philipp's excellent comment above), other catalysts have been developed, namely $\ce{Ni(0)}$ based species using a trypticene-derived chelating phosphine ligand (see image).

trypticene-based ligand

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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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Summarizing Phillip and Siddahrtha's comments.

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^{-} $

This is not energetically favorable under normal conditions. Under high pressure and with a catalyst the reaction would be possible.

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I'm not sure this is relevant, since this electrophilic substitution is an acidic reaction. Also, please check the balance of your equation. –  Eric Brown May 21 '13 at 6:17
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