Diels Alder Reaction - speed of reaction

themonk

1. The problem statement, all variables and given/known data
My professor asked the class to see if he is correct or if the book is correct over the speed of a Diels-Alder reaction (DA). It was whether 1,3-butadiene (calling it A) or if 2,3-dimethyl-1,3-butadiene (calling it B) reacts faster. The book says that the reaction of of B reacts 5 times faster than A because of the electron donating powers (EDG) of the methyl groups. My professor says that it is the conformation that makes it faster.

2. Relevant notes
Here it mentions that the reaction is 10 times faster because of the conformation:
http://www.askthenerd.com/NOW/CH22/22_6.pdf

3. The attempt at a solution
Now, they are both correct in my opinion because not just one thing will affect the reactivity or speed of a reaction. Which one has more of an influence is probably a better consideration. I think that the EDG of Methyl is not strong enough to give a 5 times faster reaction in DA. The energy required to change the conformation has more to do with it. The s-cis has more energy and is more likely to react, so one should consider this more. A might have higher energy in the s-cis formation and a dihedral angle of 0°, but B has more s-cis formation than s-trans, so there are more of them to react with. This makes B more likely to react and therefore increase the speed of the reaction.

Anything I am forgetting or anywhere where my logic does not make sense?

This was also provided: