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Topic: benzen is nonpolar but give IR spectra  (Read 7691 times)

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Offline boodyx1

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benzen is nonpolar but give IR spectra
« on: January 02, 2010, 03:53:39 PM »
benzene is non polar but give IR  spectra? please explain to me why that?

Offline bromidewind

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Re: benzen is nonpolar but give IR spectra
« Reply #1 on: January 03, 2010, 04:23:38 AM »
Just because it's nonpolar doesn't mean that it won't give an IR spectrum. Nonpolar substances like ethanol and acetone give IR spectra. As for a detailed explanation of why benzene gives an IR spectrum in the area of 1600 cm-1, here goes.

When two double bonds are one bond apart, they are said to be conjugated. Conjugated double bonds are found between 1620 and 1640 cm-1. Benzene has three conjugated double bonds in a six membered ring. Because of resonance stabilization, each of the bonds is more like 1 1/2 bonds. The reduced pi bonding causes the bonds to vibrate at a slightly lower frequency of 1600 cm-1.

Hope this helps a little bit.

Offline boodyx1

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Re: benzen is nonpolar but give IR spectra
« Reply #2 on: January 03, 2010, 10:30:32 AM »
thank u so much

Offline Darwin

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Re: benzen is nonpolar but give IR spectra
« Reply #3 on: January 05, 2010, 03:30:22 PM »
benzene is non polar but give IR  spectra? please explain to me why that?

Most non-polar molecules are in fact IR-active. For example CO2 has D∞h symmetry. As a consequence of its symmetry, it is non-polar. The number of degrees of vibrational freedom for CO2 is 3n - 5 = 3*3 - 5 = 4 (where n is the number of atoms). Therefore, there are four modes of vibration. The symmetric stretch does not give rise to a change in dipole moment, however, both the asymmetric stretch and the bend do. Therefore, there are two fundamental absorptions which are observed in the IR spectrum of CO2.

Just because it's nonpolar doesn't mean that it won't give an IR spectrum. Nonpolar substances like ethanol and acetone give IR spectra. As for a detailed explanation of why benzene gives an IR spectrum in the area of 1600 cm-1, here goes.

Ethanol & acetone = non-polar? Hmm...

Offline bromidewind

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Re: benzen is nonpolar but give IR spectra
« Reply #4 on: January 06, 2010, 11:55:50 AM »
Just because it's nonpolar doesn't mean that it won't give an IR spectrum. Nonpolar substances like ethanol and acetone give IR spectra. As for a detailed explanation of why benzene gives an IR spectrum in the area of 1600 cm-1, here goes.

Ethanol & acetone = non-polar? Hmm...

Wow, did I really write that? I must have been really tired... I apologize for my stupidity. Thanks for flagging it :) and your explanation of why nonpolar molecules are IR active is great too. It gives a more general explanation of why.

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