Ethyl acetate has the structure CH3CO2CH2CH3. Which set of m/z peaks is most consistent with this
structure?
A) 88, 73, 43
B) 88, 76, 45
C) 88, 73, 45
D) 88, 76, 43
E) 88, 72, 70, 44
The Correct Answer and Explanation is :
The correct answer is C) 88, 73, 45.
Explanation:
Ethyl acetate (CH₃CO₂CH₂CH₃) is a simple ester that consists of an ethyl group (C₂H₅) attached to an acetate group (CH₃CO₂). When subjected to mass spectrometry (MS), the molecular ion (m/z 88) represents the molecular weight of ethyl acetate. The fragmentation of this molecule typically follows patterns where the molecule breaks into various ion fragments.
- Molecular Ion (m/z 88): The molecular ion corresponds to the intact molecule of ethyl acetate (CH₃CO₂CH₂CH₃) with a mass of 88 g/mol. This is the parent peak.
- Fragment at m/z 73: The ion at m/z 73 corresponds to the loss of a methyl group (CH₃), which leaves behind an acetyl ion (CH₃CO), with a mass of 73 g/mol. This is a common fragmentation pattern for esters, where the ester group can break down into an acylium ion (CH₃CO) and an ethyl radical (C₂H₅).
- Fragment at m/z 45: The fragment at m/z 45 corresponds to the loss of an ethyl group (C₂H₅). When the molecule breaks down, this ion is formed by the loss of the ethyl group from the ester structure, leaving behind an acylium ion (CH₃CO), which has a mass of 43. This is another common fragmentation pattern.
Why other options are incorrect:
- A) 88, 73, 43: The peak at m/z 43 is common in other esters and smaller molecules, but for ethyl acetate, the correct fragment after the loss of an ethyl group would be m/z 45, not 43.
- B) 88, 76, 45: The peak at m/z 76 does not fit the expected fragmentation pattern for ethyl acetate.
- D) 88, 76, 43: Similarly, m/z 76 is an unusual fragment in this case, and the correct fragment is m/z 45, not 43.
- E) 88, 72, 70, 44: This set includes higher m/z values (72 and 70) that do not correspond to typical fragmentation patterns for ethyl acetate.
Thus, the correct fragmentation pattern for ethyl acetate, which fits best with the expected peaks, is 88, 73, and 45, making option C the correct answer.