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7.3 Acid Halides
Acid halides are the most reactive of the carboxylic acid derivatives because they produce the most stable leaving groups.
We can prepare any of the other carboxylic acid derivatives.
It's important that you know how to make acid halide.
A synthesis problem where you will need to make an acid halide at some point in the synthesis is very common.
HO- is less stable than Cl-, so this would be a challenge.
HO- is not going to be expelled.
We need to convert the OH group into a different group that can be expelled.
An acid halide can be obtained by treating a carboxylic acid with thionyl chloride.
The reagent converts the OH group into a better leaving group and serves as a source of chloride ion that will attack the carbonyl group and expel the newly formed leaving group.
The carboxylic acid is a nucleophilic.
After expelling a chloride ion, the bond is re- formed.
The first objective of the three steps is to convert an OH group into a better leaving group.
SO is formed as a by-product.
We are ready to explore reactions of acid halides now that we have a method for preparing them.
There will be many reactions.
Don't try to memorize them.
They all follow the same rules.
There are two core steps, attack and re-form the carbonyl.
It's much easier with acid halides because they only have protons at the end of the mechanism.
Attack, re-form, and deprotonate are the three steps of this mechanism.
Many of the reactions we are about to see are repeating the same order of events.
The by-products of the reaction are Cl- and H O+.
Pyridine reacts with HCl to form pyridinium chloride.
The process traps the HCl so that it can't be used for any other reactions.
pyridine is not used in this case.
We use two equivalents of ammonia, which is twice as much ammonia as acid chloride.
The process has three steps: attack, re-form, and deprotonate.
When an acid halide is treated with H- or C-, what happens?
We have an obvious question.
The starting acid halide will give an alcohol if it reacts with two equivalents of the nucleophile.
Some of the acid will get attacked twice, and others will not get attacked at all, if we just try to use one equivalent of the Grig halidenard reagent.
There is a class of compounds that will do that.
They are called R CuLi.
In the first semester of chemistry, you might have been introduced to these compounds.
The cuprates are less reactive than the Grignard reagents.
The cuprates will react with acid halides but not with ketones.
The starting material is acid chloride.
There is a plausible mechanism for each of the reactions.
We are starting with an acid halide.
The reagent is a dialkyl cuprate.
The reagent does not attack twice.
The carboxylic acid we are starting with is alcohol.
The carbonyl group must be reduced to an alcohol.
This sounds like a Grignard reaction.
A carboxylic acid cannot be used in a Grignard reaction.
If there are acidic protons available, the Grignard reagent will be destroyed.
We must first convert the carboxylic acid into an acid halide to start.
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