Enrol to start learning
Reading is open to everyone. Enrolling is free, and it is what unlocks the audio lessons, practice tests and progress tracking.
8.9.4. Substitution Reactions in the Hydrocarbon Part
Interactive Audio Lesson
Unlock the classroom podcast
The transcript is above and free to read. A free account plays the conversation back.
Create a free account
Today, we will explore the exciting world of halogenation of carboxylic acids. Specifically, we'll focus on how carboxylic acids with an alpha-hydrogen can react with halogens.
What exactly happens during this reaction, and why is it important?
Great question! When we treat a carboxylic acid with chlorine or bromine in the presence of red phosphorus, a halogen substitutes the alpha-hydrogen, forming alpha-halocarboxylic acids. This is called the Hell-Volhard-Zelinsky reaction.
Why is red phosphorus used specifically?
Red phosphorus serves as a catalyst that facilitates this reaction by creating a polar environment conducive to halogenation. Remember, this reaction modifies acidity and enhances reactivity.
Can you summarize the importance of this reaction?
Certainly! Halogenation modifies the properties of carboxylic acids, making them more versatile in organic synthesis.
Unlock the classroom podcast
The transcript is above and free to read. A free account plays the conversation back.
Create a free account
Now, let’s shift gears to electrophilic substitution reactions involving aromatic carboxylic acids. Who can tell me what that entails?
Is it where an electrophile replaces a hydrogen atom on the aromatic ring?
Exactly! However, carboxylic acids are unique in that they act as deactivating and meta-directing groups. This means they slow down the reaction rate compared to other substituents.
Why do they deactivate the ring?
The carboxyl group has a strong electronegative oxygen that pulls electron density away from the aromatic ring, making it less reactive to incoming electrophiles.
Does this mean we can’t use Friedel-Crafts reactions with them?
You’re on the right track! Friedel-Crafts reactions are not suitable for carboxylic acids because the catalyst will bond to the carboxyl group, which hinders the reaction.
Can you recap the significance of these reactions?
Sure! Understanding how carboxylic acids undergo electrophilic substitutions helps chemists predict reaction pathways in benzene derivatives and design new synthetic routes.
Overview
Short Summary
This section focuses on the substitution reactions involving carboxylic acids, detailing halogenation and electrophilic substitution.
Medium Summary
The section provides insight into the substitution reactions of carboxylic acids, detailing processes such as halogenation at the alpha position and electrophilic substitution involving aromatic carboxylic acids, emphasizing the reactivity differences of the functional groups involved.
Detailed Summary
Substitution Reactions in the Hydrocarbon Part
This section emphasizes two significant types of substitution reactions pertaining to carboxylic acids, specifically halogenation and aromatic electrophilic substitution.
1. Halogenation
Carboxylic acids with an alpha-hydrogen can undergo halogenation, where chlorine or bromine reacts in the presence of red phosphorus, leading to the formation of alpha-halocarboxylic acids. This process is formally recognized as the **Hell-Volhard-
Reference YouTube Videos
Audio Book
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountAromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group. They however, do not undergo Friedel-Crafts reaction (because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group).
Detailed Explanation
Aromatic carboxylic acids can participate in electrophilic substitution reactions, where an electrophile replaces a hydrogen atom on the benzene ring. However, the carboxyl group (-COOH) is electron-withdrawing, making the ring less reactive towards electrophiles. This property means that when reactions do occur, new substituents will attach in a meta position relative to the carboxyl group. The presence of the carboxyl group also prevents the Friedel-Crafts reactions from occurring, which typically require a reactive aromatic system and a good electrophile.
Examples & Analogies
Imagine a family dinner with a strict parent (the carboxyl group) who sets the rules about who can come to dinner (the electrophiles coming in to substitute on the benzene ring). Because the parent is strict, they only allow certain friends (the substituents) to sit at the table in specific places (meta position). The parent won't let anyone else come in that disrupts the family dynamics (the Friedel-Crafts reaction is stopped because the strict parent won’t allow it).
--
Key Concepts
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
When chloroacetic acid is formed via halogenation of acetic acid, it exhibits increased acidity compared to acetic acid.
In the example of benzoic acid undergoing nitration, the reaction favors the meta position due to the presence of the carboxyl group.
Memory Aids
Interactive tools to help you remember key concepts