Question

Following is the mass spectrum of 1-bromobutane. (a) Account for the appearance of the \(\mathbf{M}+2\) peak of approximately \(95 \%\) of the intensity of the molecular ion peak. (b) Propose structural formulas for the cations of \(\mathrm{m} / z 57,41\), and 29 .

Short answer

Question: Explain the appearance of the M+2 peak in the mass spectrum of 1-bromobutane and propose structural formulas for the cations with m/z 57, 41, and 29. Answer: The M+2 peak in the mass spectrum of 1-bromobutane is due to the presence of different isotopes of bromine, specifically \(^{79}Br\) and \(^{81}Br\). These isotopes have a natural abundance ratio of approximately 1:1, resulting in the M+2 peak having a similar intensity to the molecular ion peak. The cations with m/z 57, 41, and 29 correspond to the structural formulas of a butyl cation (\(\ce{CH3CH2CH2CH2+}\)), an ethyl cation (\(\ce{CH3CH2+}\)), and a methylene cation (\(\ce{CH2+}\)), respectively.

Step-by-step solution

Understanding the M+2 peak

The M+2 peak is an isotopic peak observed in mass spectrometry, and it means that the mass is 2 units higher than the molecular ion peak (M peak). The presence of an M+2 peak in the mass spectrum of 1-bromobutane can be attributed to the presence of different isotopes of bromine: \(^{79}Br\) and \(^{81}Br\). These two isotopes have a natural abundance ratio of approximately 1:1, which means that about 50% of the bromine atoms encountered in nature will be \(^{79}Br\), while the other 50% will be \(^{81}Br\). As a result, the M+2 peak for 1-bromobutane will be nearly equal to the intensity of the molecular ion peak.

Identifying the molecular ion peak

The molecular ion peak (M peak) represents the molecule's mass-to-charge ratio (m/z) when it has lost an electron but remains intact otherwise. The molecular formula for 1-bromobutane is C\(_4\)H\(_9\)Br, so its molar mass should be around 137 g/mol (\(4\times12.01 + 4\times1.01 + 79\times1\)). We would find the M peak around m/z = 137 and M+2 peak around m/z = 139.

Proposing structural formulas for cations of m/z 57, 41, and 29

We are given the m/z values for the fragments of the structure, and we need to propose their structural formula. - For m/z 57: The cation with m/z 57 is most likely formed by the cleavage of the carbon-bromine bond in 1-bromobutane,\(\ce{CH3CH2CH2CH2Br+} \rightarrow \ce{CH3CH2CH2CH2+} + \ce{Br}\). The structure that corresponds to this m/z value is a butyl cation: \(\ce{CH3CH2CH2CH2+}\) - For m/z 41: The cation with m/z 41 is formed by the cleavage of a carbon-carbon bond in the butyl cation generated in the previous step,\(\ce{CH3CH2CH2CH2+} \rightarrow \ce{CH3CH2+} + \ce{CH2CH2}\). The structure corresponding to this m/z value is an ethyl cation: \(\ce{CH3CH2+}\) - For m/z 29: The cation with m/z 29 is formed by the loss of a methyl group from the ethyl cation generated in the previous step,\(\ce{CH3CH2+} \rightarrow \ce{CH2+} + \ce{CH3}\). The structure corresponding to this m/z value is a methylene cation: \(\ce{CH2+}\) In summary, we have explained the presence of the M+2 peak in the mass spectrum of 1-bromobutane and proposed the structural formulas for the cations formed for m/z 57, 41, and 29.

Key concepts

M+2 Peak

The M+2 peak is a notable feature in mass spectrometry that signifies the presence of isotopes with an atomic mass two units greater than the most abundant isotope. This peak arises due to the contributions of heavier isotopes like ¹³C, ²H (deuterium), or in the case of bromine, ⁸¹Br in the molecule. These isotopes have a slightly higher mass, yet they're part of the same chemical element.

For instance, in the mass spectrum of 1-bromobutane, the near-equal intensity of the M+2 peak compared to the molecular ion peak is caused by the near-equal natural abundance of ⁷⁹Br and ⁸¹Br. In molecules containing chlorine or bromine, M+2 peaks have high intensities due to the significant natural abundance of their heavier isotopes, leading to an informative pattern that aids in molecule identification.

Isotopic Peaks

Isotopic peaks are a crucial aspect of mass spectrometry, providing insight into the elemental composition of the molecule. Different isotopes of an element have slightly different masses due to the difference in the number of neutrons. The presence of these isotopes leads to multiple peaks for a single element, which are mirrored in the mass spectrum.

The intensity of these peaks is directly proportional to the isotopes' natural abundance. For instance, carbon has two stable isotopes — ¹²C and ¹³C, with ¹³C present at about 1.1%, reflecting a much smaller isotopic peak at one mass unit higher than the main carbon peak. With isotopic peaks, chemists can make educated guesses about the types of atoms present in a molecule.

Molecular Ion Peak

The molecular ion peak, often referred to as the parent peak, is typically the highest mass peak in a mass spectrum and represents the entire molecule as a cation (positively charged species). It's identified by its mass-to-charge ratio (m/z), with a charge typically of +1. This peak occurs when the molecule loses an electron without fragmentation, reflecting the molecular weight of the compound in question.

For example, in 1-bromobutane, the molecular ion peak will be found at the m/z corresponding to the sum of the atomic masses of its atoms. However, because bromine exists as predominantly two isotopes, ⁷⁹Br and ⁸¹Br, the molecular ion peak will also include contributions from molecules containing either ⁷⁹Br or ⁸¹Br, with the latter resulting in the M+2 peak.

Structural Formulas for Cations

In mass spectrometry, understanding the structural formulas for cations is vital in deciphering the molecule's structure. When a compound is ionized, it often fragments into smaller ions or cations, and the mass spectrometer records the mass-to-charge ratio of these species. The structural formulas of these cations can indicate the pathways of fragmentation and give clues about the structure of the original molecule.

For instance, the cations corresponding to m/z values of 57, 41, and 29 in the mass spectrum of 1-bromobutane suggest specific fragmentations. These cations represent a butyl, ethyl, and methylene cation, respectively, indicative of the stepwise breaking of bonds as the original molecule dissociates under high energy.

Mass-to-Charge Ratio

The mass-to-charge ratio (m/z) is the foundation of mass spectrometric analysis. Mass spectrometry measures the m/z of ionized atoms or molecules, with the charge typically being +1 for most organic compounds. The result is a spectrum displaying a range of peaks that correspond to the various fragments of the molecule.

The molecular ion peak, isotopic peaks, and any fragment cations are all distinguished based on their m/z values. This allows for the reconstruction of the molecular weight and potential structure of the sample compound. Understanding the mass-to-charge ratio facilitates the identification of unknown substances and the elucidation of their structures in the realm of organic chemistry.