[masteroc]

Just took a organic test covering (one part of the test) spectroscopy. One question that I can not stop thinking about was stated something like this: An unknown organic compound showed a peak on the IR spec in the 2200 cm-1 region. Which of the following could be a structure for the compound?

The two answers I narrowed it down to seemed to both be plausible to me. One was a benzene ring with only a nitrile group (c-triplebond-N). and the second had two benzene rings joined by a c-c triple bond. Im wondering why both of these could not be correct since on the handout we were given in class, both the c-n and c-c triple bonds fall into the 2200 region.

Thanks!

[pacifyer]

Just took a organic test covering (one part of the test) spectroscopy. One question that I can not stop thinking about was stated something like this: An unknown organic compound showed a peak on the IR spec in the 2200 cm-1 region. Which of the following could be a structure for the compound?

The two answers I narrowed it down to seemed to both be plausible to me. One was a benzene ring with only a nitrile group (c-triplebond-N). and the second had two benzene rings joined by a c-c triple bond. Im wondering why both of these could not be correct since on the handout we were given in class, both the c-n and c-c triple bonds fall into the 2200 region.

Thanks!

Diphenylacetylene is non-polar, so stretching of its CC triple bond would make little, if any effect, so its vibration will not show in the spectrum, or either it would be VERY small. The nitrile, instead, as it is a polar molecule, will show its CN stretching very nicely.

Please, check at the SDBS japanese database, so you could see by yourself what I state:

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng


Just look for the spectra of both compounds...

[masteroc]

Thanks for the info! Im still trying to figure out how we were suppose to know that or make that choice since we never really discussed polarity having anything to do with the vibrations picked up by the IR Spec.

[pacifyer]

Thanks for the info! Im still trying to figure out how we were suppose to know that or make that choice since we never really discussed polarity having anything to do with the vibrations picked up by the IR Spec.

For a molecule to absorb IR, the vibrations within a molecule must cause a net change in the dipole moment of the molecule.

[DearAntarctica]

Also, wouldn't the diphenyl you're describing be shifted quite a bit lower than a typical alkyne?

[Pradeep]

IR is vibrational spectroscopy. Vibration is an acceleration and deceleration. (Like simple harmonic motion) Accelerating charges emit EM radiation. Likewise EM radiations can accelerate (vibrate) charges. So.. To absorb IR radiation there should be a dipole-moment in the molecule. So .. Cyanobenzene has net dipole-moment and diphenyl acetiline does not have. So cayanobenzene ir IR active and diphenyl acetiline gives very week IR signal.

Generally we can see the IR signal of terminal Alayne groups, but not internal Alayne groups.

[pacifyer]

IR is vibrational spectroscopy. Vibration is an acceleration and deceleration. (Like simple harmonic motion) Accelerating charges emit EM radiation. Likewise EM radiations can accelerate (vibrate) charges. So.. To absorb IR radiation there should be a dipole-moment in the molecule. So .. Cyanobenzene has net dipole-moment and diphenyl acetiline does not have. So cayanobenzene ir IR active and diphenyl acetiline gives very week IR signal.

Generally we can see the IR signal of terminal Alayne groups, but not internal Alayne groups.

In my experience, the CC stretching of internal alkynes show in the IR as well as the terminal ones, the difference of course is that terminal alkynes do have a C(sp)-H stretching, and internal alkynes don't.
As you correctly state, IR is a vibrational spectroscopy, but it is rather an interaction of the molecule dipole with the electromagnetic radiation what is observed, and there is no such thing as accelerations or decelerations...
And also, please consider that CO2 has a net dipole of zero, however a clear absorption is seen in the IR spectra at about 2.7, 4.3 and 15 µm:
http://science.widener.edu/svb/ftir/ir_co2.html