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Chapter 14: Problem 8

Both \(\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}\) and \(\mathrm{C}_{7} \mathrm{H}_{14}\) have the same nominal mass, namely 98 . Show how these compounds can be distinguished by the \(m / z\) ratio of their molecular ions in highresolution mass spectrometry.

Short Answer

Expert verified
Answer: The molecular compounds C6H10O and C7H14 can be distinguished using their high-resolution mass spectrometry m/z ratios by comparing their exact masses. For C6H10O, the high-resolution m/z ratio is approximately 98.1434, while for C7H14, the high-resolution m/z ratio is approximately 98.1868. Since these two m/z ratios are different, the compounds can be distinguished.

Step by step solution

01

Calculate the exact mass of each compound

In high-resolution mass spectrometry, the molecular masses are measured with higher accuracy (more decimal places). So, to compute exact masses, we'll need to use accurate atomic masses for carbon (C), hydrogen (H), and oxygen (O). Let's calculate the exact mass for each compound: For C6H10O, we use 6 atoms of carbon, 10 atoms of hydrogen, and 1 atom of oxygen: Exact mass = (6 * 12.011) + (10 * 1.00784) + (15.999) Exact mass = 72.066 + 10.0784 + 15.999 Exact mass ≈ 98.1434 For C7H14, we use 7 atoms of carbon and 14 atoms of hydrogen: Exact mass = (7 * 12.011) + (14 * 1.00784) Exact mass = 84.077 + 14.10976 Exact mass ≈ 98.1868
02

Compare the mass-to-charge (m/z) ratios

Now that we have the exact mass for each compound, we can compare their m/z ratios in high-resolution mass spectrometry to distinguish between them. Since their charge will be the same and not affect the comparison, we just consider their masses: For C6H10O, the high-resolution m/z ratio is approximately 98.1434. For C7H14, the high-resolution m/z ratio is approximately 98.1868. These two m/z ratios are different, and thus, these compounds can be distinguished by their m/z ratios in high-resolution mass spectrometry.

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Most popular questions from this chapter

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.

For which compounds containing a heteroatom (an atom other than carbon or hydrogen) does the molecular ion have an even-numbered mass and for which does it have an odd-numbered mass? (a) A chloroalkane with the molecular formula \(\mathrm{C}_{n} \mathrm{H}_{2 n+1} \mathrm{Cl}\). (b) A bromoalkane with the molecular formula \(\mathrm{C}_{n} \mathrm{H}_{2 n+1} \mathrm{Br}\). (c) An alcohol with the molecular formula \(\mathrm{C}_{n} \mathrm{H}_{2 n+1} \mathrm{OH}\). (d) A primary amine with the molecular formula \(\mathrm{C}_{n} \mathrm{H}_{2 n-1} \mathrm{NH}_{2}\). (e) A thiol with the molecular formula \(\mathrm{C}_{n} \mathrm{H}_{2 n+1} \mathrm{SH}\).

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

Carboxylic acids often give a strong fragment ion at \(m / z(\mathrm{M}-17)\). What is the likely structure of this cation? Show by drawing contributing structures that it stabilized by resonance.

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?
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