Infrared Spectroscopy Video Lessons

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Problem: Select the vibrations that should be infrared active.a) (CH3CH2)2C=O (C=O stretch) b) (CH3CH2)3C-Cl (C-Cl stretch) c) CH3CH2CH2C ≡ CCH2CH2CH3 ( C≡C stretch)d) trans-4-octene (C=C stretch) e) CH3CH2CH2CH2C≡CH (C≡C stretch) 

FREE Expert Solution

For this problem, we have to determine which among the compounds are IR active

  • Recall that in order for a vibrational node to absorb IR light, it must result in a periodic change in dipole moment of the molecule
  • In general, the more polar the bond, the stronger its IR absorption. 
  • Some vibrations are considered as infrared inactive where the stretching vibrations are completely symmetrical double and triple bonds do not result in a change in dipole moment, hence not resulting in any absorption of IR
  • Therefore, we need to find the alkenes and alkynes that are asymmetrical since these are IR active

Analyzing each molecule:

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Problem Details

Select the vibrations that should be infrared active.

a) (CH3CH2)2C=O (C=O stretch) 

b) (CH3CH2)3C-Cl (C-Cl stretch) 

c) CH3CH2CH2C ≡ CCH2CH2CH( C≡C stretch)

d) trans-4-octene (C=C stretch) 

e) CH3CH2CH2CH2C≡CH (C≡C stretch) 

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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Infrared Spectroscopy concept. You can view video lessons to learn Infrared Spectroscopy. Or if you need more Infrared Spectroscopy practice, you can also practice Infrared Spectroscopy practice problems.

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Based on our data, we think this problem is relevant for Professor Bolliger's class at OKSTATE.