8.7.1 - Fission of a Covalent Bond
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Introduction to Bond Fission
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Today, we're going to learn about the fission of covalent bonds. Can anyone tell me why bond breaking is important in organic chemistry?
Maybe because it leads to the formation of new reactions and products?
Exactly! When bonds break, new entities are formed. We can classify bond fission into two types: heterolytic and homolytic.
What’s the difference between them?
Great question! In heterolytic cleavage, one atom keeps both electrons, resulting in charged species. Can anyone give me an example?
I think bromomethane can form a carbocation and a bromide ion?
Correct! Let's remember: in heterolysis, think 'charge' as a reminder of charged species.
So, what about homolytic cleavage?
In homolytic cleavage, each atom gets one electron, forming free radicals. We show this using half-headed arrows. Remember: 'neutral' for homolytic!
To summarize, heterolytic leads to charged species, whereas homolytic forms free radicals. Any questions?
Carbocation Stability
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Now let's dive deeper into carbocations. Who can explain how we classify them based on the carbon attached?
Aren't they classified as primary, secondary, and tertiary based on how many carbons are attached?
Exactly! Primary carbocations have one carbon attached, secondary have two, and tertiary have three. Tertiary ones are more stable. Can anyone explain why?
Is it because of inductive effect and hyperconjugation? More alkyl groups can stabilize the positive charge.
Yes! Remember: 'Tertiary Triumph' for stability due to the effect of nearby alkyl groups. What about the stability order?
C+H3 < CH3C+H2 < (CH3)2C+H < (CH3)3C+.
Perfect!! Let’s summarize: stability increases with the number of alkyl groups due to inductive effects.
Free Radicals
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Moving on to homolytic cleavage, what do we know about free radicals?
They form by homolytic cleavage and have an unpaired electron.
Exactly! Free radicals are important intermediates in many reactions. Do they have stability like carbocations?
I think they're also classified as primary, secondary, and tertiary like carbocations?
Yes! And we follow a similar stability trend, right? More alkyl groups mean more stability.
Right! So a tertiary free radical is more stable than a primary one.
Good! Keep in mind the term 'Free Radical Stability' as a memory aid.
In summary, free radicals are formed via homolytic cleavage with varying stability similar to carbocations.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
In this section, we explore the fission of covalent bonds, which can occur in two ways: heterolytic cleavage, resulting in charged carbocation and carbanion species, and homolytic cleavage, leading to neutral free radicals. Understanding these mechanisms is crucial for studying organic reactions and the stability of different intermediates.
Detailed
Fission of a Covalent Bond
In organic chemistry, the breaking of covalent bonds is a key process that leads to the formation of reactive intermediates. There are two primary ways in which a covalent bond can undergo fission: heterolytic cleavage and homolytic cleavage.
Heterolytic Cleavage
During heterolytic cleavage, the bond breaks in such a way that one of the bonded atoms retains both electrons from the bond, resulting in the formation of two charged species: a positively charged carbocation and a negatively charged carbanion. For example, when bromomethane (CH3Br) undergoes heterolytic cleavage, it yields a methyl cation (CH3+) and a bromide ion (Br-).
Carbocations can be classified as primary, secondary, or tertiary, based on the number of carbon atoms directly attached to the positively charged carbon atom. Their stability increases with the number of substituents due to inductive effects and hyperconjugation. The stability order is:
- C+H3 < CH3C+H2 < (CH3)2C+H < (CH3)3C+
Homolytic Cleavage
Conversely, in homolytic cleavage, each bonded atom retains one of the electrons from the shared pair, resulting in the formation of neutral free radicals. For instance, when butane (C4H10) undergoes homolytic cleavage, it generates free radicals such as CH3• and CH2•. The stability of free radicals follows a similar trend as carbocations, where tertiary radicals are more stable than secondary and primary ones. In reactions that proceed via homolytic cleavage, single electron movement is illustrated using half-headed arrows.
Conclusion
Overall, understanding the processes of bond fission is essential not only for grasping the chemistry of organic reactions but also for predicting the formation and reactivity of diverse intermediates in chemical pathways.
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Types of Bond Cleavage
Chapter 1 of 5
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Chapter Content
A covalent bond can get cleaved either by: (i) heterolytic cleavage, or by (ii) homolytic cleavage.
Detailed Explanation
Covalent bonds can break in two primary ways—heterolytic cleavage and homolytic cleavage. In heterolytic cleavage, the bond breaks, and one atom takes both of the electrons from the bond, leading to the formation of charged species—one with a full positive charge (carbocation) and the other with a full negative charge (carbanion). In contrast, homolytic cleavage results in each atom taking one of the shared electrons, forming two radicals, which have unpaired electrons and are usually neutral.
Examples & Analogies
Imagine a tug-of-war game. In heterolytic cleavage, one team yanks the rope all the way to their side, taking full control (like an atom taking both electrons), while in homolytic cleavage, each team pulls on the rope equally and ends up with half of it (each atom taking one electron).
Heterolytic Cleavage
Chapter 2 of 5
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Chapter Content
In heterolytic cleavage, the bond breaks in such a fashion that the shared pair of electrons remains with one of the fragments. After heterolysis, one atom has a sextet electronic structure and a positive charge and the other, a valence octet with at least one lone pair and a negative charge.
Detailed Explanation
Heterolytic cleavage results in two charged fragments. For example, in the cleavage of bromomethane, the bond between carbon and bromine breaks, where bromine takes both electrons, forming a bromide ion (Br–) and a carbocation (C+H3). The positively charged carbon atom now has six electrons instead of the usual eight, making it highly reactive, while the bromine, with extra electrons now, has a complete octet but carries a negative charge.
Examples & Analogies
Think of a teacher giving a prize to a student. If the teacher hands over the entire prize to one student (like Br taking both electrons), that student becomes the clear winner (carbocation) and now has all the attention, while the other students watch without participation (like the bromide ion).
Carbocations and Their Stability
Chapter 3 of 5
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Chapter Content
Carbocations are classified as primary, secondary, or tertiary depending on whether one, two, or three carbons are directly attached to the positively charged carbon. Carbocations are highly unstable and reactive species.
Detailed Explanation
The stability of a carbocation increases with the number of alkyl groups attached to the positively charged carbon atom. Primary carbocations have one alkyl group, secondary have two, and tertiary have three. Tertiary carbocations are the most stable because the surrounding alkyl groups can help disperse the positive charge through inductive and hyperconjugation effects.
Examples & Analogies
Imagine a person balancing on a seesaw. The more friends (alkyl groups) that person has sitting on their side to keep them balanced, the more stable they become. A tertiary carbocation is like a person with three supportive friends, making it much less likely to tip over precariously!
Homolytic Cleavage
Chapter 4 of 5
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Chapter Content
In homolytic cleavage, one of the electrons of the shared pair in a covalent bond goes with each of the bonded atoms, resulting in the formation of neutral species (free radicals) containing an unpaired electron.
Detailed Explanation
Homolytic cleavage occurs when the bond breaks such that each atom retains one of the electrons. This leads to the production of free radicals, which are highly reactive species because they have unpaired electrons. The movement of one electron is represented by a 'half-headed' arrow.
Examples & Analogies
Consider a pair of scissors that cuts a piece of string. Each blade of the scissors takes half of the string (one electron each). The resulting loose ends (free radicals) from the string are reactive, similar to how free radicals want to quickly bind with other atoms to find a stable state.
Reactivity of Free Radicals
Chapter 5 of 5
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Chapter Content
Alkyl radicals are classified as primary, secondary, or tertiary. Alkyl radical stability increases as we proceed from primary to tertiary.
Detailed Explanation
Just like carbocations, the stability of alkyl radicals also increases with the number of alkyl groups attached to the carbon atom that carries the unpaired electron. Tertiary radicals are the most stable due to the stabilization provided by the surrounding alkyl groups. This effect is vital in determining the products of reactions involving radicals.
Examples & Analogies
Imagine a skateboarder (the free radical) trying to land a trick. The more friends they have around to help steady them (like the surrounding alkyl groups), the more likely they are to land it smoothly—making tertiary radicals the expert skaters who have mastered their tricks with friends' support.
Key Concepts
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Heterolytic Cleavage: Results in the formation of charged species.
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Homolytic Cleavage: Results in the formation of free radicals.
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Carbocation Stability: Influenced by the number of alkyl groups attached.
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Free Radical Characteristics: Neutral with an unpaired electron, classified into stability levels.
Examples & Applications
Example of heterolytic cleavage: CH3Br → CH3+ + Br-
Example of homolytic cleavage: C2H6 → CH3• + CH3•
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
When bonds break, do take a look, hetero charges, homolytic book.
Stories
Once there were two friends, Hetero and Homo. Hetero took both electrons and formed a charged buddy, while Homo split evenly and danced as free radicals in the sunlight.
Memory Tools
Remember: HH – Heterolytic for Charge, Homolytic for Neutral.
Acronyms
Use 'CHARGE' to recall
Cleavage Heterolytic - A Reactive Glycation Entity.
Flash Cards
Glossary
- Covalent Bond
A chemical bond formed by the sharing of electrons between atoms.
- Heterolytic Cleavage
Bond breaking where one atom receives both shared electrons, forming charged species.
- Homolytic Cleavage
Bond breaking where each atom retains one electron, forming free radicals.
- Carbocation
A positively charged carbon species resulting from heterolytic cleavage.
- Carbanion
A negatively charged carbon species resulting from heterolytic cleavage.
- Free Radical
A neutral species with an unpaired electron formed from homolytic cleavage.
- Stability
A measure of a species' tendency to maintain its structure; in carbocations and free radicals, stability is influenced by substituents.
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