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General Reactivity
Carboxylic acid derivatives are reactive in one way.
- They are electrophilic at the carbon of the carbonyl.
Wize Concept
The order of reactivity is based on leaving group ability of the R group! The order is:
acid halide > acid anhydride > ester > amide
There are some other (less important!) factors effecting the reactivity of the carboxylic acid derivatives. They are:
- steric effects: the smaller leaving groups (Cl-, in acid chlorides) will depart faster than larger leaving groups (-OC(O)R, in acid anhydrides) leading to greater reactivity
- electronic effects: larger bond dipoles between the carbon of the carbonyl and the leaving group will lead to greater reactivity
- ring strain: cyclic derivatives (lactones which are cyclic esters and lactams which are cyclic amides) react faster than their linear analogues (especially when they are in 3- or 4-membered rings.
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General Mechanism at the Carbonyl
The carbon at the base of the carbonyl is electrophilic because of a bond dipole between the carbon and the oxygen atom.
- The nucleophile attacks the carbon atom.
- The tetrahedral intermediate is formed.
- The leaving group departs, reforming the carbonyl.
Wize Tip
This is called a nucleophilic substitution reaction (aldehydes and ketones undergo addition reactions).
Wize Concept
Since the tetrahedral intermediate has a good leaving group, the carbonyl is able to reform with departure of the leaving group.
Wize Tip
It will seem like there are lots of reactions for the carboxylic acid derivatives! However, they are all just variations of this reaction with different leaving groups and nucleophiles!
Rank the following species in order of decreasing reactivity:
I: ester
II: anhydride
III: amide
IV: acyl chloride
Summary Sheet: Carboxylic Acid Derivative Reactivity
