The following compounds undergo McLafferty rearrangement in the mass spectrometer. Predict the masses of the resulting charged fragments.
(b) 3-methylhexan-2-one

Verified step by step guidance

1. Identify the functional group in the compound. In 3-methylhexan-2-one, the functional group is a ketone (C=O) located at the second carbon.
2. Understand the McLafferty rearrangement mechanism. This involves a six-membered cyclic transition state where a hydrogen atom from the γ-carbon (the third carbon away from the carbonyl carbon) is transferred to the oxygen of the carbonyl group, leading to cleavage of the bond between the α-carbon (adjacent to the carbonyl) and the β-carbon.
3. Locate the γ-hydrogen in the molecule. In 3-methylhexan-2-one, the γ-carbon is the fourth carbon in the chain, and it has hydrogens available for the rearrangement.
4. Determine the fragments formed. The McLafferty rearrangement results in two fragments: (1) a neutral alkene formed from the γ-carbon and beyond, and (2) a charged fragment containing the carbonyl group and the α-carbon.
5. Calculate the mass of the charged fragment. Add the atomic masses of the atoms in the charged fragment (including the transferred hydrogen) to predict its mass. Use the molecular formula of the fragment to ensure accuracy.

Key Concepts

McLafferty Rearrangement

The McLafferty rearrangement is a fragmentation process observed in mass spectrometry, where a molecule undergoes a specific rearrangement to form charged fragments. This typically involves the transfer of a hydrogen atom and the cleavage of a bond, resulting in the formation of a double bond and a new charged species. It is particularly common in compounds with a carbonyl group, such as ketones and aldehydes.

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. In this process, molecules are ionized, and the resulting ions are separated based on their mass. The data obtained allows for the identification of molecular structures and the prediction of fragmentation patterns, such as those seen in the McLafferty rearrangement.

Fragmentation Patterns

Fragmentation patterns refer to the specific ways in which a molecule breaks apart during mass spectrometry. These patterns are influenced by the molecular structure and functional groups present. Understanding these patterns is crucial for predicting the masses of the resulting fragments, as they provide insights into the stability and reactivity of the ions formed during the ionization process.