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Chapter 14: Problem 17

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

Short Answer

Expert verified
Answer: The cation responsible for the m/z 29 peak in the mass spectrum of most aldehydes is HC(CH2)C+H2.

Step by step solution

01

Understand m/z 29 peak

The characteristic peak at m/z 29 means that there is a fragment of the aldehyde molecule with a mass of 29 atomic mass units (amu) that carries a positive charge.
02

List known structures of aldehydes

Aldehydes have a general formula of RCHO, where R is a hydrocarbon chain and CHO represents the carbonyl and hydrogen atoms. We need to identify the fragment with a mass of 29 amu and a positive charge.
03

Calculate the mass and configuration of atoms required to form m/z 29

The atomic masses of carbon, hydrogen, and oxygen are approximately 12, 1, and 16, respectively. Thus, the cation with 29 amu could be composed of 2 carbon atoms, 5 hydrogen atoms, and no oxygen atoms. The mass of this cation would be: \(2(12) + 5(1) = 24 + 5 = 29\).
04

Identify the fragment that can be formed with this composition

Based on the atomic configuration calculated in Step 3, we can form the following fragment: \(\mathrm{H}\mathrm{C}(\mathrm{H})_{2} \mathrm{C}^{+}\mathrm{H}_{2}\). This fragment, containing 2 carbons and 5 hydrogens, satisfies the mass of 29 amu and the positive charge condition.
05

Identify the cation

The cation responsible for the m/z 29 peak in the mass spectrum of most aldehydes is \(\mathrm{H}\mathrm{C}(\mathrm{H})_{2} \mathrm{C}^{+}\mathrm{H}_{2}\), which is structurally different from the ethyl cation, \(\mathrm{CH}_{3} \mathrm{CH}_{2}{ }^{+}\).

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Most popular questions from this chapter

The molecular ions of both \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{~S}\) and \(\mathrm{C}_{6} \mathrm{H}_{14} \mathrm{O}\) appear at \(m / z 102\) in low- resolution mass spectrometry. Show how determination of the correct molecular formula can be made from the appearance and relative intensity of the \(M+2\) peak of each compound.

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

The molecular ion for compounds containing only \(\mathrm{C}, \mathrm{H}\), and \(\mathrm{O}\) always has an even mass-to-charge value. Why is this so? What can you say about the mass-to-charge ratio of ions that arise from fragmentation of one bond in the molecular ion? From fragmentation of two bonds in the molecular ion?

The following is the mass spectrum of compound \(\mathrm{C}, \mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}\). Compound \(\mathrm{C}\) is infinitely soluble in water, undergoes reaction with sodium metal with the evolution of a gas, and undergoes reaction with thionyl chloride to give a water-insoluble chloroalkane.

Electrospray mass spectrometry is a recently developed technique for looking at large molecules with a mass spectrometer. In this technique, molecular ions, each associated with one or more \(\mathrm{H}^{+}\)ions, are prepared under mild conditions in the mass spectrometer. As an example, a protein (P) with a molecular mass of 11,812 gives clusters of the type \((\mathrm{P}+8 \mathrm{H})^{8+},(\mathrm{P}+7 \mathrm{H})^{7+}\), and \((\mathrm{P}+6 \mathrm{H})^{6+}\). At what mass-to-charge values do these three clusters appear in the mass spectrum?
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