Reaction of 1-chloro-2-propanol with hydrogen halides

[maverick280857]

HI

This problem follows from a discussion in Morrison and Boyd (6th ed) Page 251.

"Our second example involves 1-chloro-2-propanol. Although technically a secondary alcohol, it reacts with hydrogen halides "abnormally" slowly, and at about the rate of a primary alcohol. This time we are dealing, not with a steric effect but with a polar effect. The rate of an S_{N1} reaction, we have seen, depends upon the stability of the carbocation being formed. Let us compare, then, the 1-chloro-2-propyl cation with a simple secondary cation, the isopropyl caton, say. Electronegative chlorine has an electron-withdrawing inductive effect. As we have seen this intensifies the positive charge on the electron deficient carbon and makes the carbocation less stable. This same electron withdrawl destabilizes the incipient cation in the transition state, raises E_{act}, and slows down the reaction." (Quoted from the book)

Now if you draw the 1-chloro-2-propyl cation, you will see that there's a +ve charge on a carbon adjacent to the carbon bonded to the chlorine group (which in this state is indeed exerting a -I effect). But if I now perform a hydride shift from the adjacent carbon (the one bonded to chlorine) to this electron deficient carbon, I will get the 1-chloro-1-propyl cation, which I claim to be more stable than the precussor because now the lone pair of the chlorine can donate its electrons to the electron deficient carbon atom to give a new resonance form which will be substantially more stable than either cation we started out with. So this way, the reaction with 1-chloro-2-propanol should be faster than that with say isopropyl chloride.

One might argue that the reactivity depends on the initially formed (non-rearranged) cation in which case my arguments will fail, but a hydride shift is still possible and to me, nothing seems to preclude it. (I am not against experimental evidence anyway, but my reasoning follows from my organic chemistry lessons which tends to suggest just this).

Cheers
Vivek

 

[movies]

I think that in the mechanism you propose (which I find very interesting, by the way) the limiting step is the hydride migration. Breaking a covalent C-H bond is quite difficult, and in the presence of excess water the reverse reaction of the initial SN1 step would be very fast.

Finally, the resonance stabilization by chlorine is a very apt observation. However, the resonance effect is probably not as great as you would expect because it involves a 3p orbital on chlorine and a 2p orbital on carbon. The difference in size has a big impact in the amount of overlap between those two orbitals. Chlorine is generally small enough to have some resonance as you have described, but certainly less than fluorine. Bromine and iodine are too large to give significant overlap with carbon.

I hope this helps.

 

[GCT]

hydride shifts occur to produce the more stable carbocation, and as movies explained the primary carbocation you mentioned is not known for its resonance stabilization, also the inductive effect is intensified with your carbocation. Thus the hydride shift will not be favorable in the first place.

 

[maverick280857]

Hi movies and GCM,

Thanks for your help. I would like to stretch this a bit further and ask you this question: how do I know if a hydride shift in such a situation is a viable option if I am not given numerical values for E_{act}? More generally, is it correct to base conclusions on the initially formed carbocation--which in this case is unstable and as you have pointed out is not known for its resonance stabilization?

Thanks and cheers
vivek

 

[GCT]

I'm not an expert on this subject but I imagine what you can do is to analyze this with common sense and from a free energy diagram perspective, the reaction being the hydride shift. You might want to note that to get to your proposition of a primary resonance stabilized cation (which is not stabilized in the first place), you are completely ignoring the initial reaction discussed by Morrison. That is in order for the hydride shift to occur, you'll have to surpass the situation of a high Eact transition state proposed by text.

Also, the product of the hydride shift is actually less stable I imagine. You've got a primary cationic charge right next to the chlorine.

 

[movies]

I think this is what GCT has already said, but I'll reiterate it anyway just to be sure:

To get to the hydride shift you have to get to the secondary carbocation anyway, I think we've all agreed on that. However, we've also agreed that this carbocation is higher energy than a "normal" secondary carbocation. So you are piling two high-energy processes on top of one another.

Furthermore, once you do the hydride shift you aren't really comparing the same reaction anymore.