[Nescafe]

Why do we often refer to the carbons in the halide bonds being " activated carbon centers?".

My guess would be due to the resonance stabilization of the partial positive charge on the carbon?

Nescafé

[discodermolide]

That would seem to be about correct. As you are aware the bromine partially polarises the C-Br bond producing a slight +ve charge go the Benzylic C atom. Thus you can draw resonance structures involving the pi system to help stabilise this partial +ve charge.
The end effect is to make the benzylic carbon atom more susceptible towards nucleophiles.

[Nescafe]

That would seem to be about correct. As you are aware the bromine partially polarises the C-Br bond producing a slight +ve charge go the Benzylic C atom. Thus you can draw resonance structures involving the pi system to help stabilise this partial +ve charge.
The end effect is to make the benzylic carbon atom more susceptible towards nucleophiles.

I don't understand how stabilization of the positive charge makes it more reactive. Wouldn't you want a localized positive charge rather than delocalized?

[discodermolide]

The resonance structures are only a means of explanation. The partial positive charge certainly sits on the benzylic carbon atom due to the electron withdrawing nature of the bromine. And as I said the resonance structures invoking the pi system are only a means to try and explain this relative stability.

[Nescafe]

The resonance structures are only a means of explanation. The partial positive charge certainly sits on the benzylic carbon atom due to the electron withdrawing nature of the bromine. And as I said the resonance structures invoking the pi system are only a means to try and explain this relative stability.

Could it also be explained by the fact that sp2 centered carbons are more electro negative than sp3 carbons (speaking of a regular alkyl halide) so that centre is more electron deficient than an alkyl halide?

[discodermolide]

I'm not sure I follow you with that last comment.

[Nescafe]

I'm not sure I follow you with that last comment.

That's what happens when I try to type a question on my iphone. Sorry.

I was trying to compare Br-CH2CH3 to an activated carbon such as that in propargyl bromide Br-CH2-C≡CH. In the case of the alkyne functionality, the CH2 is attached to a SP carbon which is more electron withdrawing than the CH2 in the Ethyl bromide which is attached to a SP3 carbon. So I am asking whether one could say the same thing explaining why the benzyl bromide CH2 is an activated carbon. In the case of benzyl bromide we have the CH2 attached to SP2 carbon which is more electronegative than the Ethyl Bromide CH2 etc..

Nescafe.

[discodermolide]

I suppose you could say that but remember the sp2 carbon of an aromatic ring is not quite the same as one attached to an alkene.

[NotExactly]

That would seem to be about correct. As you are aware the bromine partially polarises the C-Br bond producing a slight +ve charge go the Benzylic C atom. Thus you can draw resonance structures involving the pi system to help stabilise this partial +ve charge.
The end effect is to make the benzylic carbon atom more susceptible towards nucleophiles.

I don't understand how stabilization of the positive charge makes it more reactive. Wouldn't you want a localized positive charge rather than delocalized?

Perhaps an interesting perspective: a localized positive charge would be more unstable and therefore more reactive compared to a delocalized charge.  However, without the stabilization of that positive charge there is no way (under normal conditions) that the positive charge could exist. 

[james_a]

From a molecular orbital perspective, part of the reason why benzylic and allylic halides are more reactive toward nucleophiles is that the Pi orbital can donate into the sigma star orbital of the C-halide bond. This stabilizes the transition state where carbon bears a partial positive charge, thereby increasing the rate.

[orgopete]

... Pi orbital can donate into the sigma star orbital of the C-halide bond. This stabilizes the transition state where carbon bears a partial positive charge, thereby increasing the rate.

I argue the extremes of nucleophilic attack can be described as initial formation of a completely dissociated carbocation in an SN1 reaction and an initial attack before bond cleavage in an SN2 reaction. Actual reactions will be somewhere between theses extremes.

With that model, electron donation would not improve nucleophilic attack. An electron withdrawing effect of an sp or sp2 carbon seems consistent with the increased reactivity of alpha-halo ketones and esters.

This thinking does leave the paradox of how can a double bond be both an electron withdrawing unit and an electron donating group. I cannot provide a simple answer for this, but iodine is also the best leaving group and best nucleophile.

[Nescafe]

... Pi orbital can donate into the sigma star orbital of the C-halide bond. This stabilizes the transition state where carbon bears a partial positive charge, thereby increasing the rate.

I argue the extremes of nucleophilic attack can be described as initial formation of a completely dissociated carbocation in an SN1 reaction and an initial attack before bond cleavage in an SN2 reaction. Actual reactions will be somewhere between theses extremes.

With that model, electron donation would not improve nucleophilic attack. An electron withdrawing effect of an sp or sp2 carbon seems consistent with the increased reactivity of alpha-halo ketones and esters.

This thinking does leave the paradox of how can a double bond be both an electron withdrawing unit and an electron donating group. I cannot provide a simple answer for this, but iodine is also the best leaving group and best nucleophile.

I really like your post. Is iodine that good of a nucleophile? I thought when it comes to nucleophilocity we look at the pkas and that of HI is -9 meaning that the conjugate base I- is very stable.

[orgopete]

... Is iodine that good of a nucleophile? I thought when it comes to nucleophilocity we look at the pkas and that of HI is -9 meaning that the conjugate base I- is very stable.

I have found this to be a difficult topic to find data that I can understand very well. As I recall, changing from protic to aprotic solvents increases the rate of reaction of flouride with less of a change in iodide. That seems satisfactory with hydrogen bonding slowing flourides reactivity. It doesn't explain why iodide should be a good nucleophile in the first place.

What seems the most satisfactory is that while HI is a strong acid, this is a thermodynamic result. In a nucleophilic substitution reaction, the reaction gives the kinetic product. That gets me half way there. Iodide still must be pulling its electrons in to explain its being a strong acid and a good leaving group. The polarizability arguments seem to make the most plausible explanation. It seem reasonable that electron pair repulsion from shell to shell could extend the reach of the valence electrons of larger atoms, if even only for a kinetic effect. I presume this may have been part of Peason's thinking in suggesting 'hard and soft' theory.