14.6b Fragmentation Patterns of Alkyl Halides, Alcohols, and Amines | Organic Chemistry

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20 Sept 201804:56
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TLDRThe script delves into the fragmentation processes of alkyl halides, alcohols, and amines during mass spectrometry analysis, focusing on the mechanisms of cleavage and alpha cleavage. It starts by explaining how alkyl halides lose halogen atoms, resulting in carbocations, illustrated by the M minus 79 peak. It then explores alcohol fragmentation, particularly the loss of water leading to the M minus 18 peak, and alpha cleavage, which forms stable carbocations. Lastly, it discusses amines, noting similar fragmentation patterns through alpha cleavage that lead to resonance-stabilized cations. These common fragmentation patterns enhance the understanding of molecular structures in mass spectrometry.

Takeaways
  • 🧩 Fragmentation of alkyl halides occurs in two primary ways: cleavage of the halogen itself and alpha cleavage.
  • 🌟 Cleavage of the halogen involves the loss of a nonbonding electron, leading to the formation of a carbocation and a halogen radical.
  • 📉 The loss of bromine results in a mass peak known as the M minus 79 peak, which is a common detection in mass spectrometry for alkyl halides.
  • ✂️ Alpha cleavage involves breaking the bond next to the carbon bonded to the halogen, forming a carbon-bromine double bond and a carbon radical.
  • 🔁 Resonance stabilization is a key factor in the stability of the cations formed during both types of fragmentation in alkyl halides.
  • 💧 In alcohols, a common fragmentation pattern is the loss of water, resulting in an alkene and a mass peak known as the M minus 18 peak.
  • 🚫 Alcohols often do not have a molecular ion peak due to the high likelihood of losing water to form a stable fragment.
  • 🔄 Alpha cleavage in alcohols also leads to the formation of a carbon-oxygen double bond and a resonance-stabilized cation.
  • 🔵 For amines, alpha cleavage is similar to that in alkyl halides and alcohols, resulting in a nitrogen-containing double bond and a carbon radical.
  • ⚖️ The positive charge resulting from alpha cleavage in amines is shared between nitrogen and carbon, leading to a resonance-stabilized cation.
  • 🔬 Mass spectrometry is a valuable tool for identifying and characterizing organic compounds by analyzing their fragmentation patterns.
Q & A

  • What are the two primary ways of fragmentation of alkyl halides?

    -The two primary ways of fragmentation of alkyl halides are cleavage of the halogen itself and alpha cleavage.

  • Which electrons are most likely to be knocked off during the cleavage of the halogen in alkyl halides?

    -The most likely candidates for being knocked off are the nonbonding electrons because they are the highest energy electrons in the molecule.

  • What is the result of the cleavage of the carbon-bromine bond in alkyl halides?

    -The result is the formation of a carbocation and a bromine atom with three lone pairs and an unpaired electron, which is a radical.

  • What peak is observed when there is a loss of bromine in the fragmentation of alkyl halides?

    -The loss of bromine, which is essentially 79 units in mass, results in what is called the M minus 79 peak.

  • How does alpha cleavage in alkyl halides occur?

    -Alpha cleavage occurs by breaking the bond next to the carbon bonded to the bromine, forming a carbon-bromine double bond and a two-carbon radical, with a positive charge stabilized by resonance.

  • What is the common fragmentation pattern for alcohols?

    -The common fragmentation pattern for alcohols is the loss of water, leading to what's known as the M minus 18 peak, and also alpha cleavage, similar to that seen in alkyl halides.

  • Why is the loss of water from alcohols so common?

    -The loss of water is so common because it forms a stable fragment, an alkene, and is such a favored process that alcohols often do not have a molecular ion peak in their mass spectrum.

  • What is the significance of the alpha cleavage in alcohols?

    -Alpha cleavage in alcohols is significant because it forms a resonance-stabilized cation with a carbon-oxygen double bond and a positive charge shared between the oxygen and the carbon.

  • What is the typical fragmentation pattern observed for amines?

    -The typical fragmentation pattern for amines is alpha cleavage, which results in a double bond with nitrogen and a positive charge, along with a two-carbon radical.

  • Why is alpha cleavage favored in the fragmentation of alkyl halides, alcohols, and amines?

    -Alpha cleavage is favored because it leads to the formation of a resonance-stabilized cation, which is a relatively stable and common fragmentation pattern for these types of compounds.

  • What is the term used to describe the peak resulting from the loss of water in alcohols?

    -The peak resulting from the loss of water in alcohols is referred to as the M minus 18 peak.

  • How does the formation of a resonance-stabilized cation contribute to the stability of the fragments formed during alpha cleavage?

    -The formation of a resonance-stabilized cation contributes to the stability of the fragments by distributing the positive charge over multiple atoms, reducing the overall energy of the system and making the fragmentation pattern more common.

Outlines
00:00
🧪 Fragmentation of Alkyl Halides and Cleavage Mechanisms

This paragraph delves into the fragmentation processes of alkyl halides, focusing on two primary pathways: cleavage of the halogen itself and alpha cleavage. The discussion begins with the cleavage of the halogen, where a nonbonding electron is removed, leading to the formation of a carbocation and a halogen radical. This process results in a mass spectrometry peak at M minus 79, indicative of the loss of bromine. The paragraph then transitions to alpha cleavage, where the bond adjacent to the carbon bonded to the bromine is broken. This results in the formation of a carbon-bromine double bond and a carbon radical, both of which are stabilized through resonance, leading to a common and stable fragmentation pattern for alkyl halides.

Mindmap
Keywords
💡Fragmentation
Fragmentation refers to the process where a molecule breaks down into smaller parts or fragments. In the context of the video, it is a key concept as it describes how alkyl halides, alcohols, and amines break down during mass spectrometry, leading to the formation of specific ions that can be detected and analyzed.
💡Alkyl Halides
Alkyl halides are organic compounds in which an alkyl group is covalently bonded to a halogen atom. The video discusses the fragmentation of alkyl halides, focusing on two primary pathways: cleavage of the halogen and alpha cleavage. These processes are crucial for understanding the mass spectrometry of these compounds.
💡Cleavage of the Halogen
This term describes the first pathway of fragmentation for alkyl halides, where the carbon-halogen bond is broken, and the halogen atom receives both electrons, resulting in a carbocation and a halide ion. This process is significant as it leads to the formation of a stable carbocation, which is often observed in mass spectrometry.
💡Alpha Cleavage
Alpha cleavage is the second fragmentation pathway discussed in the video, where a bond adjacent to the carbon bonded to the halogen is broken. This results in the formation of a carbon-bromine double bond and a radical on the adjacent carbon. Alpha cleavage is common in alkyl halides and leads to the formation of resonance-stabilized cations, which are important for mass spectral analysis.
💡Carbo Cation
A carbo cation is a carbon-centered cation with a positive charge. In the video, the formation of carbo cations is a result of both the cleavage of the halogen and alpha cleavage in alkyl halides. These carbo cations are resonance-stabilized, making them relatively stable and significant in the mass spectrometry of these compounds.
💡Resonance Stabilization
Resonance stabilization is a phenomenon where the true state of a molecule is an average of two or more contributing structures. In the context of the video, it is mentioned in relation to the formation of stable cations from alkyl halides, alcohols, and amines. The stabilization of these cations through resonance is crucial for understanding their stability and reactivity in mass spectrometry.
💡Mass Spectroscopy
Mass spectroscopy is an analytical technique used to identify and quantify compounds by measuring the mass-to-charge ratio of ions. The video script discusses how different types of molecules fragment in mass spectroscopy, which is essential for interpreting the resulting mass spectra and identifying the compounds.
💡M Minus 79 Peak
The 'M minus 79 peak' refers to a specific peak in a mass spectrum that results from the loss of a bromine atom (which has a mass of approximately 79) from the molecular ion. This peak is indicative of the cleavage of the carbon-bromine bond and is a common observation in the mass spectrometry of alkyl halides.
💡Alcohols
Alcohols are a class of organic compounds containing a hydroxyl (-OH) functional group. The video discusses the fragmentation patterns of alcohols, particularly the loss of water (M minus 18) and alpha cleavage, which are common and significant for the mass spectrometry of these compounds.
💡M Minus 18 Peak
The 'M minus 18 peak' is observed in the mass spectra of alcohols and results from the loss of a water molecule (which has a mass of approximately 18) from the molecular ion. This peak is indicative of a common fragmentation pathway for alcohols and is often the base peak in the mass spectrum of alcohols due to the stability of the resulting alkene.
💡Amines
Amines are a class of organic compounds containing a nitrogen atom with a lone pair of electrons. The video briefly touches on the fragmentation pattern of amines, noting that alpha cleavage is also common for these compounds, leading to the formation of a resonance-stabilized cation involving the nitrogen atom.
Highlights

Two primary fragmentation pathways for alkyl halides: cleavage of the halogen and alpha cleavage

Cleavage of the halogen involves breaking the carbon-bromine bond and forming a carbocation and bromine radical

Alpha cleavage involves breaking the bond next to the carbon bonded to the bromine, forming a carbon-bromine double bond and a two-carbon radical

Resonance stabilization of the positive charge in both cleavage pathways contributes to the stability of the resulting fragments

Common fragmentation pattern for alcohols is the loss of water (M-18 peak) due to the high stability of the resulting alkene

Alcohols often do not have a true parent peak in their mass spectrum due to the very common and favorable loss of water

Alpha cleavage in alcohols involves breaking the bond to the carbon bonded to the hydroxyl group, forming a carbon-oxygen double bond and a radical

The positive charge in alpha-cleaved alcohols is resonance-stabilized, shared between the oxygen and carbon

Amines undergo a similar alpha cleavage as alkyl halides and alcohols, forming a resonance-stabilized cation with a positive charge on nitrogen

The high energy non-bonding electrons are most likely to be lost during fragmentation

Loss of bromine (M-79 peak) is a common fragmentation pattern for alkyl halides

Alpha cleavage in alkyl halides is a common and stable fragmentation pathway

The resulting fragments from alpha cleavage in alcohols and alkyl halides are resonance-stabilized cations

The loss of water from alcohols is so common that it often does not have a true parent peak in the mass spectrum

The alpha carbon is the key focus during alpha cleavage in alcohols and amines

The resulting fragments from alpha cleavage are relatively stable due to resonance stabilization

The fragmentation patterns discussed have broad applicability to alkyl halides, alcohols, and amines

Transcripts

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Related Tags
Chemical FragmentationAlkyl HalidesAlcoholsAminesAlpha CleavageResonance StabilizationMolecular IonsMass SpectrometryOrganic ChemistryStable FragmentsRadical Formation