Question

What are the masses of the charged fragments produced in the following cleavage pathways? (a) Alpha cleavage of pentan-2-one \(\left(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\right)\) (b) Dehydration of cyclohexanol (c) McLafferty rearrangement of 4 -methylpentan-2-one \(\left[\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\right]\) (d) Alpha cleavage of triethylamine \(\left[\left(\mathrm{CH}_{3} \mathrm{CH}_{2}\right)_{3} \mathrm{~N}\right]\)

Short answer

(a) 31, 43; (b) 18; (c) 30, 42; (d) 29.

Step-by-step solution

Identify the Cleavage and Fragment for Alpha Cleavage of Pentan-2-one

Alpha cleavage in pentan-2-one involves the breaking of the carbon-carbon bond adjacent to the carbonyl group. This yields two main fragments: acylium ion and a radical. For pentan-2-one, this results in the following fragments: 1. \[\text{Acylium Ion: } \mathrm{CH}_{3}\mathrm{C}^{+}=\mathrm{O} \\text{Radical: } \mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\]The mass of the acylium ion is calculated from: 1 Carbon = 12, 3 Hydrogens = 3, 1 Oxygen = 16 Total = 12 + 3 + 16 = 31 The mass of the radical is calculated from: 3 Carbons = 36, 7 Hydrogens = 7 Total = 36 + 7 = 43 Thus, the masses of the fragments are 31 and 43.

Determine the Mass Change in Dehydration of Cyclohexanol

Dehydration of cyclohexanol involves the loss of water \(\mathrm{H}_{2}\mathrm{O}\) and the formation of cyclohexene. The change in mass is due to the loss of water, which contains 2 Hydrogens (atomic mass 1 each) and 1 Oxygen (atomic mass 16): Mass of water = 1 + 1 + 16 = 18 Thus, the mass of the resulting charged cyclohexene is the mass of cyclohexanol minus the mass of the lost water.

Analyze McLafferty Rearrangement of 4-methylpentan-2-one

McLafferty rearrangement involves the shift of a hydrogen and breakage of a bond in a carbonyl compound, resulting in the formation of a neutral alkene and a charged enol. For 4-methylpentan-2-one, the fragments produced are:1. \[\text{Charged Enolate Ion: } \mathrm{CH}_{2}\mathrm{C(OH)=O} \\text{Neutral Alkene: } \mathrm{CH}_3\mathrm{CH}=\]The charged enolate ion involves the group from the oxygen carbon. Calculate its mass:1 Carbon (12), 2 Hydrogens (2), 1 Oxygen (16), and the charge.Total = 30The neutral alkene involves carbon 3 to the end:3 Carbons = 36, 6 Hydrogens = 6 Total = 42Thus, the masses are 30 and 42.

Calculate Alpha Cleavage Masses of Triethylamine

Alpha cleavage in triethylamine occurs next to the nitrogen atom, breaking the carbon-nitrogen bond. The resulting charged fragments are:1. \[\text{Ethyl Carbocation: } \mathrm{C}_{2}\mathrm{H}_{5}^{+} \\text{Fragmented Radicals: N(ET)_2 }\]Mass of the ethyl cation is:2 Carbons = 24, 5 Hydrogens = 5Total = 29Thus, the masses of the main fragments are 29 (cation) and whatever remains based on the lost cation.

Key concepts

Alpha Cleavage

Alpha cleavage is an essential reaction in organic chemistry that plays a major role in breaking down molecules into simpler charged fragments. Specifically, it involves the breaking of a bond adjacent to a functional group, often resulting in an ion and a radical. In the case of alpha cleavage in carbonyl compounds like pentan-2-one (\(\text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3\)), the process involves the cleavage of the carbon-carbon bond next to the carbonyl carbon. This leads to the formation of an acylium ion (\(\text{CH}_3\text{C}^{+}=\text{O}\)) and a radical (\(\text{CH}_2\text{CH}_2\text{CH}_3\)).
Here are some key points:

• The carbonyl's neighboring carbon is the focus of cleavage.
• The acylium ion is a common charged fragment in this pathway.
• Alpha cleavage is utilized widely in mass spectrometry to fragment molecules for analysis.

Dehydration Reaction

A dehydration reaction is an important chemical process where a molecule loses water. This is a common transformation in converting alcohols to alkenes. For example, cyclohexanol can undergo dehydration to produce cyclohexene. During this reaction:

• Water molecules, comprised of two hydrogens and one oxygen \((\text{H}_2\text{O})\), are removed from the starting molecule.
• The loss of water signifies a mass change equal to that of the water molecule, which is 18 amu (atomic mass units).
• This mass change leads to the formation of a more stable compound like cyclohexene.

Understanding dehydration reactions is crucial because they explain the observed differences in mass during such chemical transformations.

McLafferty Rearrangement

The McLafferty rearrangement is a fascinating reaction in mass spectrometry involving compounds with carbonyl groups. It facilitates the redistribution of hydrogen and the cleavage of bonds, resulting in new charged fragments. In 4-methylpentan-2-one (\(\text{CH}_3\text{COCH}_2\text{CH}(\text{CH}_3)_2\)), this rearrangement encompasses:

• The migration of a hydrogen to the carbonyl oxygen.
• The production of a charged enolate ion \(\text{CH}_2\text{C(OH)=O}\), and a neutral alkene \(\text{CH}_3\text{CH}=\text{CH}_2\).
• The distinctive feature is the hydrogen transfer, which is characteristic of McLafferty rearrangements, facilitating the fragmentation necessary for analysis.

Such rearrangements are crucial to identifying compound structures and elucidating molecular configurations.

Mass Spectrometry

Mass spectrometry is a powerful analytical technique used to determine the masses of particles and the composition of a sample by generating charged fragments. It plays a fundamental role in molecular analysis:

• It analyzes ionized fragments to determine molecular mass and structure.
• Different cleavage pathways, like alpha cleavage and McLafferty rearrangements, result in unique charged fragments that can be identified.
• Mass spectrometers generate a spectrum that helps in identifying unknown compounds and studying molecular breakdown.

With applications in chemistry, biology, and medicine, mass spectrometry is invaluable in detailed structural analysis.

Charged Fragments

Charged fragments are pivotal in the analysis of molecular structure via mass spectrometry. These fragments provide insights into the structure of organic compounds:

• They are produced when chemical bonds are broken during ionization processes like mass spectrometry.
• Charged fragments like acylium ions or carbocations result from specific cleavage pathways.
• Their masses and charges are key data points for structural identification and compound analysis.

Understanding the formation and significance of these charged pieces is fundamental in deducing the molecular framework.

Carbonyl Compounds

Carbonyl compounds are a central class in organic chemistry featuring a carbon-oxygen double bond structure. They are prevalent in various reactions, particularly in mass spectrometry:

• They consist of an oxygen atom double-bonded to a carbon atom, forming reactive centers for cleavage and rearrangement.
• Carbonyl-containing molecules like ketones and aldehydes often undergo reactions such as alpha cleavage and McLafferty rearrangement.
• The presence of the carbonyl group makes these compounds susceptible to specific chemical transformations, influencing their structural behavior during spectrometry analysis.

Recognizing carbonyl compounds' properties is crucial in interpreting their reactions and roles in molecular spectra.