[pantone159]

Hi - I'm working (on my own) through Solomons, and I have run into a problem I can't figure out.  (I did try google.)

The stated problem is as follows:
> Alkenes are more reactive than alkynes to electrophilic addition.  (E.g. adding Br2).  Yet, when alkynes are > treated with one molar equivalent of these reagents, it is easy to stop the addition at the "alkene stage".  > This appears to be a paradox and yet it is not.  Explain.

I understand the paradox part.  I figure that, (approximately!) the first half of the Br2 would add to the alkynes, giving you 0.5 dibromoalkene + 0.5 unreacted alkyne.  The second half of the 'molar equivalent' would then add to the more reactive dibromoalkene, and you'd end up with 0.5 tetrabromoalkane + 0.5 still unreacted alkyne.

Does the 'alkenes are more reactive than alkynes' not apply to the dibromoalkenes?  Perhaps the Br is so big that it gets in the way?



 

[movies]

Br is also going to inductively withdraw electron density from that alkene relative to a "normal" alkene.  Since the electrophilic addition is dependent on the electron denisty, it will be slower.