[helper]

hi;

the question is in this link,

http://i.cubeupload.com/BqgPTf.png

thank you ahead!

[Borek]

I have attached the image to the post. Please read the forum rules.

[orgopete]

Good question. While I would argue this is consistent with carbon being an electron donor, hence more is better, there are others who argue for hyperconjugation. Let the hyperconjugation supporters explain why I am wrong (again).

[Dan]

Start by identifying the rate determining step of the reaction, then consider which substrate will require less energy to undergo this step. As orgopete has said, there is more than one explanation.

[helper]

First of all, thank you all.

So, if I understand correctly, there are two answers for the question:
on one side, the more bulky are the substituents the more effective the hyperconjugation effect and the rate of forming the carbocation is higer and the second unser will be right,  and on the other hand the bulky substitution may interfer the leaving group to leave - so th first unser will be right.

[orgopete]

Oh, oh, I didn't mean to make a confusing comment. I, for one, believe that as one progresses across the periodic table the atoms become progressively more electron withdrawing. This can be understood as a nuclear charge effect, the greater the charge, the more withdrawing. Lithium and beryllium are notably electron donors. The tetravalent form of boron is also an electron donor. Carbon is at the dividing line. You may find many textbooks will describe carbon as an electron donor in the chapter in which carbocations are discussed (hoping you won't comment that in chapter one, it was more electron withdrawing than hydrogen). This argument may satisfy the increased stability that tertiary carbocations are more stable than secondary carbocations. This could have been the end of the discussion and explained why the second answer was correct. Additional electron donating carbon atoms would further stabilize a carbocation.

On the other hand, there are many who are troubled by the electronegativity arguments that tell you that carbon is more electron withdrawing than hydrogen, so replacement of a hydrogen with a carbon should destabilize a carbocation. To overcome this apparent paradox, hyperconjugation has been introduced. If hyperconjugation were to take place, then a weakening of the beta C-H bond can use those electrons to stabilize a neighboring carbocation. Certainly, there is some validity to this argument.

The reason I commented on this problem (though not explaining why 2 should be the correct answer) was hyperconjugation would appear to favor 1 and carbon as electron donor, 2. I was angling for someone in the hyperconjugation camp to explain why the answer is 2 and because of hyperconjugation.

[helper]

Thank you very much