Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: pantone159 on December 08, 2005, 02:14:56 PM
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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?
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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.