Question

Propose structures for compounds that fit the following data: (a) \(\mathrm{A}\) ketone with \(\mathrm{M}^{+}=86\) and fragments at \(\mathrm{m} / \mathrm{z}=71\) and \(\mathrm{m} / \mathrm{z}=43\) (b) An alcohol with \(\mathrm{M}^{+}=88\) and fragments at \(\mathrm{m} / \mathrm{z}=73, \mathrm{~m} / \mathrm{z}=70\), and \(m / z=59\)

Short answer

Compound (a) is 2-pentanone; compound (b) is 2-butanol.

Step-by-step solution

Start with the Mass-to-Charge Ratio

Identify the molecular structures based on the given mass-to-charge (m/z) ratios for the given compounds. For compound (a), the molecular ion peak \(\mathrm{M}^+ = 86\) suggests a molecular weight of 86. For compound (b), the molecular ion peak \(\mathrm{M}^+ = 88\) suggests a molecular weight of 88.

Propose Structure for Compound (a)

For a ketone with molecular weight 86, consider common functionalized carbon chains. One possibility is 2-pentanone, where the molecular formula is \(\mathrm{C}_5\mathrm{H}_{10}\mathrm{O}\). This fits the molecular weight and can fragment to yield \(m/z = 43\) for a common acyl fragment \(\mathrm{C}_2\mathrm{H}_3\mathrm{O}^+\), as well as \(m/z = 71\), which can be a result of losing an ethyl group (mass 15 from \(\mathrm{C}_3\mathrm{H}_7\mathrm{O}^+\)).

Propose Structure for Compound (b)

For an alcohol with molecular weight 88, consider simple alcohol structures that fit the mass. One possibility is 2-butanol, having the molecular formula \(\mathrm{C}_4\mathrm{H}_{10}\mathrm{O}\). It can produce a fragment at \(m/z = 73\) by losing a methyl group (mass 15 from \(\mathrm{C}_3\mathrm{H}_7\mathrm{O}^+\)), at \(m/z = 70\) if two hydrogens are lost along with the methyl group, and \(m/z = 59\) through fragmentation of the carbon chain.

Key concepts

Ketone Structure Determination

Determining the structure of a ketone using mass spectrometry involves analyzing the mass-to-charge (m/z) ratio of its fragmented ions. Ketones are characterized by a carbonyl group (C=O) nestled between carbon atoms. In mass spectrometry, ketones often fragment to produce various ions that fall apart in predictable patterns.

For a ketone with a molecular ion peak at M⁺ = 86, such as the example given in the problem, one potential structure is 2-pentanone. This corresponds to the molecular formula C₅H₁₀O, matching the weight of 86. Key fragments that help in confirming this structure include:

• An m/z = 43 fragment, often seen with ketones, representing a common acyl cation ( C₂H₃O⁺).
• An m/z = 71 fragment, potentially formed by losing an ethyl group (C₂H₅), resulting in C₃H₇O⁺.

Identifying these specific fragments in the mass spectrum helps to confidently determine the ketone's structure. The ability to break down a molecule and interpret the resulting mass fragments is crucial in defining the carbon backbone and locating the functional groups.

Alcohol Structure Determination

In mass spectrometry, alcohols can be identified and differentiated by the analysis of the mass-to-charge ratio of their fragmented ions. Alcohols contain the hydroxyl group (-OH), which influences their fragmentation pattern in a mass spectrometer due to the rupture of bonds adjacent to this group.

For an alcohol with a molecular ion peak at M⁺ = 88, such as 2-butanol, the analysis begins by looking for major fragment ions consistent with typical alcohol fragmentation patterns. Consider C₄H₁₀O, weighing in at 88, as a potential candidate. Some characteristic fragments include:

• An m/z = 73 fragment produced by the loss of a methyl group (C₃H₇O⁺), a common occurrence in alcohols.
• An m/z = 70 fragment resulting from the loss of two hydrogens, in addition to the methyl group, affecting the hydroxyl-bearing carbon.
• An m/z = 59 fragment due to breaking the carbon chain, which is a typical fragmentation pattern for alcohols.

By examining these characteristic fragment peaks, the structure of the alcohol can be effectively proposed, showing how the hydroxyl group influences the breakdown pattern.

Mass-to-Charge Ratio Analysis

The mass-to-charge ratio (m/z) is a fundamental concept in mass spectrometry used to identify molecular structures by analyzing their charged fragments. It combines the mass of an ion with its charge, providing critical insight into the structure of an unknown compound. Understanding m/z is crucial for interpreting mass spectra and identifying the molecular ion, which indicates the entire unfragmented molecule in the spectrum.

Given the exercise's ketone and alcohol, interpreting their m/z values involves:

• Molecular Ion Peak: For both compounds, the M⁺ peak represents their complete molecular weight, which helps in determining possible molecular formulas.
• Fragment Ions: Analyzing smaller m/z peaks, produced by the cleavage of chemical bonds, aids in piecing together potential structures by matching them with known fragmentation patterns.
• Charge of Ions: Mass spectrometers typically produce singly charged ions, making the m/z value essentially equivalent to the ion's mass.

By mapping ion peaks in the spectra to known fragmentation patterns, chemists can deduce structural features and arrive at probable structures for the molecules under investigation. Understanding m/z analysis allows for logically piecing together fragmented information to rebuild the complete molecular story.