[Big-Daddy]

Markovnikov's rule states that the nucleophile will add onto the "more substituted" carbon of the two in (what was) the double bond, and the electrophile onto the "less substituted" carbon of the two.

Firstly, does this mean alkyl substituents only, or any atom besides H? I somehow doubt it would be the latter.

Secondly, if both C atoms are substituted with the same number of substituents (of each type, if the starting atom of the substituent makes a difference), do we expect a mixture of products, regardless of whether one alkyl substituent is heavier than the other or not?

Example: - is there a single product we can predict, or will it be a mixture?

Yes I know this is a matter of carbocation stability and to restate this question in the terms of carbocation stability is trivial. I can do so if you really wish. But I can't find this material anywhere through searching (Wikipedia does not appear to have it).

[Rutherford]

Here is a text about carbocation stability: http://www.chemguide.co.uk/mechanisms/eladd/carbonium.html#top
I hope that it will make you some things more clear.

[Big-Daddy]

Here is a text about carbocation stability: http://www.chemguide.co.uk/mechanisms/eladd/carbonium.html#top
I hope that it will make you some things more clear.

The explanation given there is IMHO horribly lacking. "Alkyl groups do precisely the opposite and, rather than draw electrons towards themselves, tend to "push" electrons away" says the website, but if it were all about letting the C take more electrons (through the "inductive effect") you'd expect a compound with lots of H substituents - as H is less electronegative than C so the C would attract the electrons more - to be more stable than one with few H substituents. Whereas apparently the opposite is true: we want few H substituents (more C substituents, despite their higher electronegativity) to have a more stable carbocation.

Not only that but the website does not attempt to address my 2 problems.

[Rutherford]

You are getting it wrong. Use CH₃-CH₂-CH₂⁺ for example.
In CH₃-CH₂-CH₂⁺ the positively charged carbon atom pulls the electrons from the C-H bonds and from the ethyl group. The ethyl group has a bigger tendency to give away electrons and therefore the pulling of electrons from C-H bond will be reduced. Why is it so? Because of the inductive effect. The atom in the CH₃- group pulls electrons from 3 C-H bonds making it partially negative, but it shouldn't be, so it pushes away electrons towards the -CH₂- group. The carbon in this group gets a partially negative charge this way and by pulling electron from hydrogen atoms, so it has to push away the electrons towards the positive carbon atom which will be satisfied with this. The positive charge will decrease slightly and so the pulling from the C-H bond. If another alkyl group was attached to it instead of a hydrogen atom the pulling of electrons from the alkyl group wouldn't reduce like in the case of a C-H bond because the inductive effect happens in it the alkyl group like in the ethyl group. All in all, inductive effect is stronger than the pulling of electrons from the C-H bond towards the carbon atom.

[Big-Daddy]

You are getting it wrong. Use CH₃-CH₂-CH₂⁺ for example.
In CH₃-CH₂-CH₂⁺ the positively charged carbon atom pulls the electrons from the C-H bonds and from the ethyl group. The ethyl group has a bigger tendency to give away electrons and therefore the pulling of electrons from C-H bond will be reduced. Why is it so? Because of the inductive effect. The atom in the CH₃- group pulls electrons from 3 C-H bonds making it partially negative, but it shouldn't be, so it pushes away electrons towards the -CH₂- group. The carbon in this group gets a partially negative charge this way and by pulling electron from hydrogen atoms, so it has to push away the electrons towards the positive carbon atom which will be satisfied with this. The positive charge will decrease slightly and so the pulling from the C-H bond. If another alkyl group was attached to it instead of a hydrogen atom the pulling of electrons from the alkyl group wouldn't reduce like in the case of a C-H bond because the inductive effect happens in it the alkyl group like in the ethyl group. All in all, inductive effect is stronger than the pulling of electrons from the C-H bond towards the carbon atom.

Ah now I'm understanding. ChemGuide did not go into this depth that you just did.

By that logic if I understand correctly it should follow that only C will pass on the pushed electrons to the positive C, therefore carbocation stability is aided only by C substituents. Thus the first priority is the number of alkyl substituents. Second priority is: the less electronegative the atoms attached to those alkyl substituents are (in your example they are only H, but if one of them were Si instead the carbocation would be even more stable) the more partially negative the C atoms in each substituent will be and therefore the more they will push electrons away and the more the inductive effect will operate to stabilize the positive C.

The question that remains then is my first one. If you look at my structure that I posted in the OP, which C atoms would you expect E-Nuc to bond onto (i.e. which would take E, which would take Nuc), or would you expect a mixture without a clear major product?

[Rutherford]

The longer the alkyl group, the stronger the inductive effect will be.

[Rutherford]

One thing I skipped here. Hyperconjugation affects the stability of carbocations, too: http://www.adichemistry.com/organic/basics/hyperconjugation/hyperconjugation-1.html. Here you can see that alkyl groups again contribute to the stability of a carbocation.

Resonance is another thing to keep in mind.

[Big-Daddy]

One thing I skipped here. Hyperconjugation affects the stability of carbocations, too: http://www.adichemistry.com/organic/basics/hyperconjugation/hyperconjugation-1.html. Here you can see that alkyl groups again contribute to the stability of a carbocation.

Resonance is another thing to keep in mind.

Thank you, I think I am clear on this issue now.