Ochem Destroyer Question!

[urbanthread]

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127. Which gives the weakest IR signal for its functional group?

Trans isomer.

The solution says that it's because Trans has symmetry = no change of dipole so it's IR inactive...
Could someone explain this??
How is a trans isomer symmetric?

 

[Triflate]

so this might not be the best explanation but i'll give it a shot... first, you can tell that the trans isomer is symmetrical by cutting it in half. Second, the trans isomer (trans-2-butene) has two dipoles pointing towards the sp2 carbons. These dipoles cancel out, making its net dipole moment zero. IR takes advantage of vibrational modes in molecules. There are, however, vibrational modes that are IR inactive. The ones that are inactive would be the stretching vibrations of molecules that are completely symmetrical or the ones that have no net dipole moment. If you wanted to observe vibrational modes of something like trans-2-butene, you would have to use Raman spectroscopy.

 

[urbanthread]

You're the best 🙂

 

[Dotoday]

Yes. Looks like the answer is correct. When you have a trans isomer, dipoles cancel out. IR spef likes if a molecule has a dipole. Since trans isomer does not, IR spectra will be weak.

 

[BobcatDMD]

so this might not be the best explanation but i'll give it a shot... first, you can tell that the trans isomer is symmetrical by cutting it in half. Second, the trans isomer (trans-2-butene) has two dipoles pointing towards the sp2 carbons. These dipoles cancel out, making its net dipole moment zero. IR takes advantage of vibrational modes in molecules. There are, however, vibrational modes that are IR inactive. The ones that are inactive would be the stretching vibrations of molecules that are completely symmetrical or the ones that have no net dipole moment. If you wanted to observe vibrational modes of something like trans-2-butene, you would have to use Raman spectroscopy.

Thanks for the explanation, but this is a little over my head... Does the real DAT ask about Raman spectroscopy? I don't remember seeing it in chads.

 

[Triflate]

No it won't! Unless you were studying for the MCAT haha

 

[Jay20155555]

in the question there is also an answer choice of the cis form of 2-butene, why wouldnt this also not have a dipole?