14.6a Fragmentation Patterns of Alkanes, Alkenes, and Aromatic Compounds | Organic Chemistry

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20 Sept 201806:19

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TLDRThe provided script discusses the fragmentation patterns of different functional groups, focusing on alkanes, alkenes, and aromatics. It emphasizes the formation of the most stable carbocations during fragmentation, highlighting the importance of substituents on carbons. For alkanes, fragmentation tends to occur between more substituted carbons to form stable secondary or tertiary carbocations and primary radicals. The script also covers the loss of specific carbon groups, such as methyl, ethyl, propyl, and butyl, which correspond to mass losses of 15, 29, 43, and 57, respectively. In the case of alkenes, fragmentation leads to the formation of resonance-stabilized allylic carbocations, while aromatics often result in the formation of a tropylium ion after a rearrangement of an initially formed benzylic carbocation. The summary provides a clear insight into the common fragmentation patterns and the types of stable intermediates formed, which are crucial for understanding the mass spectrometry of these organic compounds.

Takeaways

🧩 When fragmenting alkanes, prioritize forming the most stable carbocations.
🔍 In alkene fragmentation, predict where the typical bond break occurs by considering the substitution of carbons.
✂️ For alkene fragmentation, cut between the most substituted carbons to form stable carbocations and radicals.
📉 Look for the loss of specific carbon groups (methyl, ethyl, propyl, butyl) to help identify fragmentation patterns.
🔢 Common fragment losses are 15 (methyl), 29 (ethyl), 43 (propyl), and 57 (butyl) in mass units.
💥 For alkene fragmentation, avoid cutting between the allylic carbon and its alkene to preserve resonance stabilization.
⚡ Allylic carbocations in alkenes are stabilized by resonance, so aim to form these during fragmentation.
🔬 In aromatic compounds, like benzene, fragmentation often leads to the formation of a tropylium ion, a seven-membered carbocation with an m/z ratio of 91.
⛓ Benzylic carbocations initially formed in aromatic fragmentation can rearrange to form a more stable tropylium ion.
📚 Keep in mind that while the most stable fragments are common, less stable fragments can also occur but are less likely.
📈 Recognize that fragmentation patterns can be identified by both the mass of the fragments and the mass lost during fragmentation.

Q & A

What is the primary goal when considering fragmentation patterns of alkanes?
-The primary goal is to form the most stable carbocations possible.
How does the substitution level of carbons affect fragmentation in alkenes?
-The more substituted the carbons are, the more stable the carbocations and radicals formed as a result of fragmentation, making them more likely to occur.
What is the term for the carbocation formed when an alkene fragments?
-The carbocation formed is called an allylic carbocation.
What is the preferred fragmentation pattern in alkenes?
-The preferred fragmentation pattern is to cut between the most substituted carbons, forming a more substituted carbocation and a less substituted radical.
What is the significance of the M over Z values in fragmentation patterns?
-M over Z values are used to identify the mass-to-charge ratio of ions, which can indicate the presence or loss of specific functional groups in the fragmentation process.
How does the loss of a methyl group in alkanes affect the M over Z value?
-The loss of a methyl group, which weighs 15, would result in an M minus 15 peak in the fragmentation pattern.
What is the term for the rearranged carbocation formed from a seven-carbon carbocation in aromatic compounds?
-The rearranged carbocation is called a tropilium ion.
What is the M over Z ratio of the tropilium ion?
-The M over Z ratio of the tropilium ion is 91.
Why is it important to identify the allylic carbons in alkene fragmentation?
-Identifying allylic carbons is important because they can form resonance-stabilized carbocations, which are more stable and thus more likely to occur during fragmentation.
What is the common fragmentation pattern for aromatic compounds?
-The common fragmentation pattern for aromatic compounds involves the formation of a resonance-stabilized carbocation, specifically a tropilium ion, from a benzylic carbocation.
How does the stability of fragments in alkane fragmentation relate to the number of carbons lost?
-The stability of fragments is related to the number of carbons lost because the loss of fewer carbons typically results in more substituted and thus more stable carbocations and radicals.
What is the role of resonance in stabilizing carbocations formed during fragmentation of alkenes and aromatics?
-Resonance plays a crucial role in stabilizing carbocations by delocalizing the positive charge over multiple atoms, which lowers the overall energy of the ion and makes it more stable.

Outlines

00:00

🧩 Fragmentation Patterns in Organic Chemistry

This paragraph discusses the fragmentation patterns of alkanes, alkenes, and aromatics, focusing on the formation of stable carbocations and radicals. It emphasizes the importance of the degree of substitution when predicting fragmentation, with more substituted carbons leading to more stable carbocations. The paragraph also covers the concept of fragmentation in alkenes, where the formation of allylic carbocations is highlighted, and in aromatics, where the formation of a tropilium ion from a benzylic carbocation is explained. The discussion includes the identification of primary, secondary, and tertiary carbons and the common fragmentation patterns that result from these structures. Additionally, the paragraph touches upon the calculation of mass-to-charge ratios (m/z) for different fragments, such as methyl, ethyl, propyl, and butyl groups, and how these can be used to identify the structure of the original molecule.

05:08

🔍 Fragmentation of Alkenes and Aromatics

The second paragraph delves into the fragmentation of alkenes and aromatic compounds, particularly how they form resonance-stabilized carbocations. For alkenes, the focus is on identifying allylic carbons, which are carbons attached to the sp2 carbons of the alkene, and the importance of not separating these from the alkene for stabilization. The paragraph explains that fragmentation should occur on the side opposite to the allylic carbon to maintain resonance stabilization. In the case of aromatic compounds, the concept of a benzylic carbocation is introduced, which undergoes a rearrangement to form a tropilium ion. This ion is characterized by an m/z ratio of 91 and is a common fragment in aromatic compounds. The summary underscores the common fragmentation patterns for alkenes and aromatics, which involve the formation of stable carbocations through resonance stabilization.

Mindmap

Keywords

💡Fragmentation

Fragmentation refers to the process in which larger molecules break down into smaller ones. In the context of the video, it's about how different functional groups in organic compounds, such as alkanes, alkenes, and aromatics, undergo fragmentation to form stable carbocations or radicals. The video discusses how fragmentation patterns are influenced by the stability of the resulting carbocations or radicals, which is a key theme in understanding organic chemistry and mass spectrometry.

💡Alkanes

Alkanes are a class of hydrocarbons with the general formula CnH2n+2, where all the carbon-carbon bonds are single bonds. The video talks about the fragmentation of alkanes and emphasizes the formation of the most stable carbocations during this process. For example, the script mentions that fragmentation tends to occur to form the most stable carbocations, which are typically more substituted ones.

💡Alkenes

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. The video discusses the fragmentation of alkenes, highlighting the formation of allylic carbocations that are stabilized by resonance. The script specifically mentions that fragmentation occurs in a way that one of the allylic carbons becomes either a radical or a carbocation, leading to a more stable molecule.

💡Aromatics

Aromatics are a class of organic compounds that have a planar, unsaturated ring structure with delocalized π electrons. The video explains that aromatics, like alkenes, commonly form resonance-stabilized carbocations upon fragmentation. It details how benzylic carbocations can rearrange to form a tropilium ion, which is a specific type of aromatic carbocation with an m/z ratio of 91.

💡Carbo Cations

Carbo cations are ions that carry a positive charge on a carbon atom. They are a key concept in the video, as they are the stable products formed during the fragmentation of alkanes, alkenes, and aromatics. The video emphasizes that the more substituted the carbocation, the more stable it is, which is a fundamental principle in organic chemistry.

💡Radicals

Radicals are species with an unpaired electron, often resulting from the fragmentation of organic molecules. In the context of the video, radicals are formed alongside carbocations during the fragmentation process. The video explains that radicals are typically formed on the less substituted side of a molecule to ensure the overall stability of the resulting fragments.

💡Substitution

Substitution refers to the replacement of an atom or a group of atoms in a molecule with another atom or group. The video discusses how the degree of substitution affects the stability of carbocations and radicals formed during fragmentation. More substituted carbons lead to more stable carbocations, which is why fragmentation tends to favor the formation of such species.

💡Mass Spectrometry

Mass spectrometry is an analytical technique used to identify and quantify compounds by measuring the mass-to-charge ratio of ions. The video's discussion on fragmentation patterns is directly related to mass spectrometry, as it explains how different functional groups in organic molecules fragment to form ions that can be detected and analyzed using this technique.

💡Methane Loss

Methane loss, denoted as 'M minus 15' in the video, refers to the specific fragmentation event where a methane group (CH3) is lost from a molecule. This is a common fragmentation pattern observed in mass spectrometry of organic compounds, and the video uses it as an example to illustrate how the loss of different groups can be indicative of the structure of the original molecule.

💡Ethyl Group

An ethyl group is a functional group with the formula C2H5, which is derived from ethane by removing a hydrogen atom. The video mentions the ethyl group in the context of fragmentation, where the loss of an ethyl group (M minus 29) is a common peak observed in mass spectrometry, indicating the presence of this group in the original molecule.

💡Tropilium Ion

The tropilium ion is a specific type of carbocation with a seven-membered ring structure, resulting from the rearrangement of a benzylic carbocation in aromatic compounds. The video explains that this ion has a characteristic m/z ratio of 91 and is a common fragment in the mass spectrometry of aromatic compounds.

Highlights

Fragmentation of alkanes involves forming the most stable carbocations possible.

In fragmentation of alkenes, predict where the typical bond break is by looking at carbon substitution.

More substituted carbocations and radicals formed from fragmentation are more stable.

Label carbons as primary, secondary, tertiary to identify most stable fragmentation points.

Cutting between most substituted carbons (secondary-tertiary) yields most stable fragments.

Fragmentation of alkenes can yield common fragment losses like -15 (methyl), -29 (ethyl), -43 (propyl), -57 (butyl).

Look for fragment weights and losses to identify what is present and what has been lost.

Fragmentation of aromatics commonly forms resonance-stabilized carbocations.

In alkene fragmentation, identify allylic carbons attached to sp2 carbons for resonance stabilization.

Avoid cutting between an allylic carbon and its alkene to maintain resonance stabilization.

In aromatic fragmentation, identify the benzylic position attached to the sp2 carbons of the benzene ring.

Cutting just outside the benzylic position on the molecular ion leads to formation of the Trillium ion.

The Trillium ion is a 7-membered ring carbocation with an m/z ratio of 91.

Common fragmentation patterns for alkenes involve allylic carbocation stabilization.

For aromatics, fragmentation initially appears to yield benzylic carbocations, but rearranges to form the Trillium ion.

Focus on identifying the most stable fragments formed during alkane, alkene, and aromatic fragmentation.

Use the concept of carbocation and radical stability to predict and interpret fragmentation patterns.

Fragmentation patterns can provide valuable information about the structure and functional groups present in a molecule.

Transcripts

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Takeaways

Q & A

What is the primary goal when considering fragmentation patterns of alkanes?

How does the substitution level of carbons affect fragmentation in alkenes?

What is the term for the carbocation formed when an alkene fragments?

What is the preferred fragmentation pattern in alkenes?

What is the significance of the M over Z values in fragmentation patterns?

How does the loss of a methyl group in alkanes affect the M over Z value?

What is the term for the rearranged carbocation formed from a seven-carbon carbocation in aromatic compounds?

What is the M over Z ratio of the tropilium ion?

Why is it important to identify the allylic carbons in alkene fragmentation?

What is the common fragmentation pattern for aromatic compounds?

How does the stability of fragments in alkane fragmentation relate to the number of carbons lost?

What is the role of resonance in stabilizing carbocations formed during fragmentation of alkenes and aromatics?