Are alkynes more electrophilic than alkenes?

Question

I think that since alkynes have a greater electron density, they would be less electrophilic than alkenes.

Which functional group is more prone to nucleophilic attacks and why? An example of a known reaction would be greatly appreciated.

Answer 1

Here are what my notes from Bruice's Organic Chemistry textbook says:

"The clouds of electrons surrounding the sigma bond makes an alkyne an electron-rich molecule. They are therefore nucleophiles that react with electrophiles.

Thus alkynes, like alkenes, undergo electrophilic addition reactions because of their weak pi bonds. The same electrophilic reagents that add to alkenes also add to alkynes.

Despite being less stable than an alkene, an alkyne is less reactive than an alkene, since reactivity depends on ΔG ‡ , which depends not only on the stability of the reactant but also on the stability of the transition state.

When a proton adds to an alkene, an alkyl cation is formed. When a proton adds to an alkyne, a vinylic cation is formed. A vinylic cation has a positive charge on a vinylic carbon. It is less stable than a similarly substituted alkyl cation because the positive charge is on an sp carbon, which is more electronegative than an sp 2 carbon of an alkyl cation. This makes it less able to bear a positive charge."