Question

Give one fragment in the mass spectrum and one peak in the IR spectrum that could be used to distinguish between these two isomers: (Designate which compound will show that diagnostic fragment or peak by preceding the value with the letter of the compound, i.e. "a47" or "b1730". Do not include any units.)

Answer 1

The question does not contain the structures of the isomers.  The complete question is below (the structures are also in attachment)

Question:

Give one fragment in the mass spectrum and one peak in the IR spectrum that could be used to distinguish between these two isomers: (Designate which compound will show that diagnostic fragment or peak by preceding the value with the letter of the compound, i.e. "a47" or "b1730". Do not include any units.)

a) CC(CC=O)(C)C

b) C(C)C(=O)CCC

Answer:

a1730

b43

Explanation:

These two isomers a and b contain different functional groups viz., aldehyde and ketone respectively. They show different signals in their mass and IR spectra, hence, can be distinguished.

In mass spectrometer analysis, both species will undergo α-cleavage, i.e., carbonyl group will lose alkyl group next to it. Only the mass spectra of compound b will show a peak at 43 due to the loss of propyl carbocation. This peak is not observable in the mass spectra of compound a, although a peak at 44 might appear due to the McLafferty rearrangement of compound a.

IR spectra of both species contain characteristics peaks of their functional group. Ketone shows its C=O stretching peak at 1715 cm⁻¹, while aldehyde shows its C=O stretching peak at 1730 cm⁻¹ along with a should peak near 2830 to 2695 cm⁻¹ due to O=C-H stretching.