[kra1173beast]

Clearly a sp hybridised carbon atom is more electronegative than a sp2 hybridised one. Then comparing an alkyl and a vinyl carbocation, wouldn't the vinyl carbocation be more stable on account of its electron withdrawing effect thereby decreasing the positive charge on the central C-atom. However sources cite the alkyl carbocation yo be more stable... Could you elaborate on this

[orgopete]

If an atom attracts electrons and if you take electrons away from it, will that improve its stability?

[kra1173beast]

Take electrons away from it? I don't understand. Wouldn't partial neutralisation of positive charge stabilise the carbocation?

[kriggy]

If you have positive charge and take away more electrons then the positive charge increses and is not neutralized. Its neturalized when you have electron donating group(s)

[kra1173beast]

Ok, but how exactly are electrons getting taken away?

[PhDoc]

"Then comparing an alkyl and a vinyl carbocation..."

The presumption is that your carbocation, an unstable positively charged species, was formed from something neutral; hence, a nucleus or group with a pair of electrons must have left the neutral species to form it; hence the neutral species must have lost a pair of electrons.

A vinyl carbocation is one wherein there is a positive charge on an sp-hybridized carbon, whereas an alkyl carbocation is one wherein there is a positive charge on an sp2-hybridized carbon. As you indicated, sp-hybridized carbons (50% s-character) are more electronegative than sp2-hybridized carbons (67% s-character). The logical deduction is that an sp-hybridized carbocation is of higher potential energy than a sp2-hybridized carbocation. There are molecular orbital-based arguments (Fleming) not worth going into.

Please keep in mind that, for the sp-hybridized carbocation to form, an sp2-hybridized carbon (more electronegative) must lose an electron pair; for the sp2-hybridized carbocation to form, an sp3-hybridized carbon (less electronegative) must lose an electron pair.

Which is more apt to GIVE UP electrons via spontaneous departure of a leaving group, the sp2-hybridized carbon (more electronegative) or the sp3-hybridized carbon (less electronegative)? Conversely, which carbocation would want that negatively charged leaving group back most? Perhaps the most electronegative, i.e. the vinyl cation.

"Typical" vinyl cations are so unstable as to have been reported mostly from α-aryl vinyl halides (Grob and co-workers). More stable β-silyl vinyl cations (β-silicon hyperconjugative effect) have been reported (Miller and co-workers), with supportive x-ray crystal structures.

[kra1173beast]

Thanks. Sorry for the late reply. It makes more sense to me now. I still feel I haven't grasped the concept perfectly, but I think my rudimentary understanding of it will do for the moment. Again, thanks a lot.

[orgopete]

Ok, but how exactly are electrons getting taken away?

There are two possible ways to generate an sp-carbocation, solvolysis of an alkenylbromide or halide or addition of an electrophile to an acetylene, such as simple protonation. If you were to write a mechanism for the generation of a carbocation from the alkenylbromide, your mechanistic arrow would indicate the electrons are taken from the carbon and given to the bromide. Similarly, the curved arrow for protonation of an acetylene would show a pair of electrons are taken from one of the carbons. In each case that is how an sp-carbocation might be generated. Because an sp-carbocation is so electron withdrawing, a solvolysis of an alkenylbromide will not occur, but it may be easier to understand how a bromide could 'take electrons away' from a carbon.