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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