Question

In the mass spectrum of the following compounds, which is the tallest—the peak at m/z = 57 or the peak at m/z = 71? a. 3-methylpentane b. 2-methylpentane

Accepted Answer

Step 1: Understand the problem. The question is asking about the relative intensities of peaks at m/z = 57 and m/z = 71 in the mass spectra of two isomers: 3-methylpentane and 2-methylpentane. The tallest peak corresponds to the most stable fragment ion formed during fragmentation in the mass spectrometer. Step 2: Recall the concept of fragmentation in mass spectrometry. When a molecule is ionized, it can break into smaller fragments. The stability of these fragments determines the intensity of the corresponding peaks in the mass spectrum. More stable fragments produce taller peaks. Step 3: Analyze the fragmentation of 3-methylpentane. For m/z = 57, the fragment corresponds to a tert-butyl cation ($$C_{4}H$$_{9}^{+}), which is highly stable due to hyperconjugation and inductive effects. For m/z = 71, the fragment corresponds to a primary carbocation ($$C_{5}H$$_{11}^{+}), which is less stable. Step 4: Analyze the fragmentation of 2-methylpentane. For m/z = 57, the fragment corresponds to a secondary carbocation ($$C_{4}H$$_{9}^{+}), which is moderately stable. For m/z = 71, the fragment corresponds to a secondary carbocation ($$C_{5}H$$_{11}^{+}), which is also moderately stable. Step 5: Compare the stability of the fragments. In 3-methylpentane, the peak at m/z = 57 is taller because the tert-butyl cation is more stable than the primary carbocation at m/z = 71. In 2-methylpentane, the peaks at m/z = 57 and m/z = 71 are closer in intensity because both correspond to secondary carbocations with similar stability. Thus, the tallest peak depends on the compound being analyzed.

In the mass spectrum of the following compounds, which is the tallest—the peak at m/z = 57 or the peak at m/z = 71?
a. 3-methylpentane
b. 2-methylpentane

비슷한 문제에 대한 검증된 영상 답변:

주요 개념

Mass Spectrometry

Fragmentation Patterns

Isomeric Compounds