[rleung]

Hi,

I have a question about which one of these has the greater dipole moment.  The correct answer is the one on the left, but I am not sure why.  It would seem that the one on the right would have a greater dipole moment since the methyl group faces up, which would make the dipole moment face up more.  Thanks.

Ryan

[rleung]

Hey,

Just wanted to *Ignore me, I am impatient* this post up since I originally posted w/o a picture and had to modify it, so I am not sure if people noticed...

[Mitch]

I wouldn't of expected a methyl group to produce a dipole moment with an alkene.

[rleung]

Ahh!  Sorry, I realized just now that I wrote the wrong "right" answer in my original post, which I have just modified.  The actual right answer is the structure on the left, but I am not sure of why.  Thanks.

[movies]

I think that C-C sp²-sp³ bonds are slightly polarized towards the more electronegative sp² end.  Based on that, in the structure on the left, the two dipoles are aligned and reinforce one another, wheras in the structure on the right they are not aligned and so the net dipole is decreased somewhat.

[ksr985]

umm the bond moment vectors could be shown as below, and simple vector addition would result in greater dipole moment for the structure on the right, ie, the trans form.

[movies]

umm the bond moment vectors could be shown as below, and simple vector addition would result in greater dipole moment for the structure on the right, ie, the trans form.

You switched which one is left and right in your picture relative to the original picture.

Answer should be the same though: trans has a bigger dipole.