Question

3. The following IR spectra are of two different compounds both with the molecular formula of CHO Identify the two compounds and interpret the significant signals in the IR spectra. w m

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Answer 1

Given the molecular formula C₄H₁₀O. with this molecular formula it can be either Alcohol or Ether.

but an Alcohol must contain a broad and strong peak (Hydrogen bonded -OH) at 3500-3300 cm^-1 which is not present in both the spectra.

And with Ether a characteristic stretching vibration of 1150-1060 cm^-1 (moderate to strong peak) must be present which is not present in spectra.

In first spectrum  characteristic strong stretching vibration peaks at 1715 cm^-1 which indicates a Ketone functional group and moderate stretching vibration peak at 2991 cm^-1 indicates -CH stretching (SP³ Carbon) and peaks containing C-C(O)-C bending vibrations present in between 1300-1100cm^-1​​​​​​​ . so with this information and with the given molecular formula in the question the compound is 2-Butanone C₄H₈O, not C₄H₁₀O.

CH₃-CH₂-CO-CH₃

In the second spectra at 1731 cm^-1 a sharp stretching vibration peak indicates an Aldehyde functional group and moderate stretching vibrations at 2976 cm^-1​​​​​​​ indicates -CH stretching vibrations (SP³ Carbon) and at 2827 cm^-1​​​​​​​ whic corresponds to aldehyde-CH stretching vibration, so with this information and with the given molecular formula in the question the compound is Butyraldehyde or Butanal with molecular formula C₄H₈O, not C₄H₁₀O.

CH₃-CH₂-CH₂-CHO.