Ch. 14 - Structural Identification 1: Infrared Spectroscopy and Mass Spectrometry

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 14 - Structural Identification 1: Infrared Spectroscopy and Mass Spectrometry

Problema 62d

Capitolo 13, Problema 62d

For each of the molecules shown, predict the structure of at least one major fragment in the mass spectrum.
(d)

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Identify the molecular ion peak in the mass spectrum, which corresponds to the molecular weight of the compound. This peak is often the highest m/z value in the spectrum.

Consider the possible fragmentation patterns based on the structure of the molecule. Common fragmentation patterns include cleavage at the weakest bonds, such as C-C or C-H bonds, and the formation of stable cations.

Analyze the structure of the molecule to identify any functional groups or structural features that might lead to specific fragmentation patterns. For example, alcohols often lose a water molecule, while carbonyl compounds can undergo McLafferty rearrangement.

Predict the formation of a major fragment by considering the stability of the resulting cation. More stable cations, such as tertiary carbocations or resonance-stabilized ions, are more likely to appear as major fragments.

Use the predicted fragmentation pattern to identify the m/z value of the major fragment. This involves calculating the mass of the fragment and considering any isotopic patterns that might be present.

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Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It helps identify the composition of a sample by generating a mass spectrum, which displays the masses of fragments. Understanding how molecules break apart and form ions is crucial for predicting the major fragments in a mass spectrum.

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How to Read a Mass Spectrum

Fragmentation Patterns

Fragmentation patterns refer to the specific ways in which a molecule breaks apart into smaller ions during mass spectrometry. These patterns are influenced by the molecule's structure, such as the presence of functional groups and bond strengths. Recognizing common fragmentation pathways, like alpha cleavage or McLafferty rearrangement, aids in predicting major fragments.

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Common Splitting Patterns

Stability of Carbocations

The stability of carbocations plays a significant role in determining the major fragments in mass spectrometry. More stable carbocations are more likely to form, influencing the fragmentation pattern. Factors such as hyperconjugation, resonance, and inductive effects contribute to carbocation stability, guiding predictions of which fragments will be most prominent.

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Determining Carbocation Stability

Pratica correlata

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The first step of a reaction called electrophilic aromatic substitution is as follows:

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Rank the following acetophenone derivatives on the basis of the carbonyl stretching frequency (1 = highest ; 4 = lowest ).

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