Question

acetone shows a single i r peak while it's isomer ,acetaldehyde shows two i r.peak ​

Answer 1

The right question is given below -

Acetone shows a single IR peak while its isomer, acetaldehyde shows two IR peaks . Why?

1. Acetone is an unsymmetrical compound.

2. Acetaldehyde contains C-O and C-H stretchings

3. +I effect of methyl group increases C=O frequency

4. None of these

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The correct answer to this question is option 4. None of these.

Explanation:

• Acetone owns 6 carbon-hydrogen stretching vibrations in the area of $3000cm^{-1}$.

• 6 carbon-hydrogen deformations in the area of $1400cm^{-1}$ are also there.

• And four methyl-rocking vibrations at around $1000cm^{1}$.  

• There are many bands, predominantly in the gas-phase infrared spectrum are severely overlapped, and hence we get a single peak.

• For an aldehyde, the doublet of peaks that are seen in the range $2860-2700cm^{-1}$ is a result of Fermi resonance.  

• The second band emerges when the aldehyde C-H stretching vibration is paired with the first overtone of the intermediate -intensity aldehyde C-H bending vibration emerging in the range $1400-1350cm^{-1}$.