[boodyx1]

benzene is non polar but give IR  spectra? please explain to me why that?

[bromidewind]

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.

[boodyx1]

thank u so much

[Darwin]

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...

[bromidewind]

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.