Question

The base peak in the mass spectrum of propanone (acetone) occurs at \(m / z 43\). What cation does this peak represent?

Short answer

Answer: The cation corresponding to the base peak at m/z = 43 in the mass spectrum of propanone (acetone) is CH3CO^+.

Step-by-step solution

Determine the molecular formula of propanone (acetone)

The molecular formula of propanone (also known as acetone) is CH3-CO-CH3. Its molecular weight is 12(3) + 1(6) + 16(1) = 58.

Understand the mass spectrum concept

In mass spectrometry, a molecule is ionized and then fragmented into various smaller cations and a neutral molecule. These cations are separated based on their mass-to-charge (m/z) ratio and detected. The base peak is the highest peak in the mass spectrum, indicating the most abundant and stable cation produced during fragmentation.

Calculate the m/z values of possible cations

Since there isn't any clear rule for predicting the mass spectral fragments, one must consider all possible fragments and see whether any of the resulting cations match the given m/z value. For propanone, there are two major fragments which correspond to a m/z of 43: 1. Loss of the methyl (CH3) group: CH3—CO—CH3 → CH3CO^+ + CH3^• (12(2) + 1(5) + 16(1)) = 43 2. Loss of the carbonyl, and two methyl radical (CH3) groups: CH3—CO—CH3 → CH3—CO—CH2^+ + 2CH3^• (12(3) + 1(8) - 12 - 3*(1)) = 43

Identify the cation corresponding to the base peak

Based on the possible fragmentation pathways, one or both fragments could generate cations with m/z values of 43. However, remembering that the base peak represents the most abundant and stable cation formed during fragmentation, the loss of a single methyl radical group is more likely to occur. So, the cation corresponding to the base peak at m/z = 43 is: CH3CO^+ (acetone ion missing one CH3 group)