Question

A characteristic peak in the mass spectrum of most aldehydes occurs at \(m / z 29\). Wha cation does this peak represent? (No, it is not an ethyl cation, \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{+}\).)

Short answer

Answer: The cation responsible for the characteristic peak at m/z 29 in the mass spectrum of most aldehydes is the CHO+ fragment. This fragment is part of the aldehyde functional group and is distinct from the ethyl cation (CH3CH2+), which has a different structure.

Step-by-step solution

Understand the mass spectrum of an aldehyde

To start, we need to recall the general structure of an aldehyde: R-CHO, where "R" is an alkyl group and "CHO" represents the aldehyde functional group (a carbonyl group bonded to a hydrogen atom).

Identify the atomic masses and possible fragments

Determine the atomic masses of carbon (C), hydrogen (H), and oxygen (O) from the periodic table: - Carbon (C): 12 - Hydrogen (H): 1 - Oxygen (O): 16 To find the fragment responsible for the peak at m/z 29, we can combine carbon, hydrogen, and oxygen atoms in different ways, ensuring the resulting fragment is related to an aldehyde structure and distinct from the ethyl cation (CH3CH2+).

Calculate the mass for the target fragment

Start by calculating the mass of a fragment that includes the CHO group (CHO+), which is the characteristic group in aldehydes. Mass of CHO+: 12 (C) + 1 (H) + 16 (O) = 29 The CHO+ fragment has a mass of 29, which corresponds to the given mass-to-charge ratio (m/z) value in the question.

Verify the fragment is indeed an aldehyde

Ensure that the derived fragment (CHO+) is related to an aldehyde and distinct from the ethyl cation (CH3CH2+). The CHO+ fragment is part of an aldehyde, so it passes this test. Moreover, the ethyl cation has a different structure (CH3CH2+), ruling it out as the sought fragment.

Write the conclusion

Based on the steps outlined above, the cation responsible for the characteristic peak at m/z 29 in the mass spectrum of most aldehydes is the CHO+ fragment, which is part of the aldehyde functional group and distinct from the ethyl cation (CH3CH2+).

Key concepts

Mass Spectrometry

Mass spectrometry (MS) is an analytical technique used to measure the mass-to-charge ratio of ions. It is a powerful tool for the identification and quantification of chemical compounds within a sample. A mass spectrometer works by ionizing the chemical substances to produce charged particles or ions. These ions are then sorted and detected according to their mass-to-charge ratio (m/z).

The process begins with the vaporization of the sample, followed by ionization, usually via methods like electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI). These ions are then accelerated into an analyzer, where they are separated based on their m/z using magnetic or electric fields. The output is a spectrum that displays the intensity of detected ions against their mass-to-charge ratios. This spectrum allows for the identification of compounds since each has a unique mass spectrum signature.

Fragmentation Patterns

In mass spectrometry, fragmentation patterns are crucial for understanding the structure of a molecule. When a molecule is ionized, it can break apart into smaller fragments, a process known as fragmentation. These fragments result in a pattern of peaks in the mass spectrum that serves as a characteristic 'fingerprint' for that molecule.

The types of fragments and their relative abundance can provide insight into the molecular structure and the presence of specific functional groups. Aiding in this analysis, characteristic fragmentation rules, such as the McLafferty rearrangement and the alpha-cleavage, help predict and explain the common pathways through which molecules break apart in the mass spectrometer. Understanding these patterns enables scientists to deduce the structure of unknown compounds and confirm the identity of known substances.

Aldehyde Functional Group

The aldehyde functional group is of particular importance when interpreting mass spectrometry data. It consists of a carbonyl group (a carbon atom double-bonded to an oxygen atom) with a hydrogen atom attached, represented by the chemical notation CHO. The aldehyde functional group is typically found at the end of a carbon chain, making it terminal.

The distinctive chemical behavior of aldehydes, including their propensity for oxidation and participation in nucleophilic addition reactions, is often reflected in their fragmentation patterns in mass spectrometry. For instance, a common fragmentation for aldehydes involves the loss of the hydrogen atom as a radical, leaving a positively charged acylium ion (RCO+), which can further break down into smaller fragments.

m/z (Mass-to-Charge Ratio)

The m/z ratio stands for mass-to-charge ratio, which is a dimensionless quantity measured in mass spectrometry. It represents the mass of an ion divided by its charge number. Most often in organic mass spectrometry, the ions are singly charged (charge = +1), which simplifies the term such that m/z essentially reflects the ion's mass.

In practice, the m/z value helps differentiate between ions in a mass spectrometer. For example, the characteristic peak at m/z 29 mentioned in the textbook solution corresponds to the mass of the CHO+ ion from an aldehyde, indicating the presence of the aldehyde functional group in a molecule. Analyzing the m/z values simplifies the identification of the ions produced, and hence the structure of the original compound.