[rleung]

Hi,

I have a quick question about why why (Z)-2-butene reacts faster than (E)-2-butene.  My book says that since (Z)-2-butene is less stable, it has a smaller free energy of activation and should react more rapidly, but in my book, it shows that the more stable carbocation has a smaller free energy of activation, while the less stable carbocation has a greater free energy of activation.  Which is right?  I am using Bruice's book, and the page where it shows less stable carbocations to possess the greater free energy of activation is on pg. 146.  Thanks.

Ryan

[Garneck]

I guess it has something to do with steric effects..  (Z)-2-butene is not energetically preferred, because the two methyl groups are "zusammen" (that's what Z stands for - "together"). It has a smaller energy of activation which means it reacts easier.

The other thing about the stability of carbocation is mixed up and, IMO, incorrect..

[rleung]

Hmm, so let me get this straight.  A less-stable carbocation (tertiary) has LESS free energy OF ACTIVATION, while a more-stable carbocation (secondary) has greater free energy of activation, right?  Meanwhile, a less-stable carbocation has a GREATER free energy CHANGE  (i.e., /_\G is more positive), while a more-stable carbocation has a LOWER free energy change (i.e., /_\G is less positive).  Am I correct?  Sorry for the caps, but I needed them for my own sake since these are confusing to me, LOL.  Thanks so much.

Ryan

[movies]

Hold on, both (Z)- and (E)-2-butene would form secondary carbocations right?  The carbocation intermediates would be identical and therefore must have the same energy.  The difference is the starting point.  As Garneck pointed out, the Z isomer has more steric repulsions and is therefore higher in energy.  So, the difference in energy between the Z isomer and the carbocation is smaller than the difference in energy than the E isomer and the carbocation (the carbocation is higher in energy than either of the alkene forms).

[rleung]

OOHHH!  I see.  I was confused because I thought both the (Z)-2-butene and (E)-2-butene would have the same starting point, but have different peaks since the cis is more unstable than the trans.  

[movies]

Heh, a common mistake to be sure!