Organic -lec 17- Dr Naem

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Chapter 17: Carbonyl Compounds I

Reactions of Carboxylic Acids and Carboxylic

Acid Derivatives

Class I: Carbonyl Compounds with groups that can be

replaced by a nucleophile

Carbonyl group Acyl group

Class II: Carbonyl Compounds with that cannot bereplaced with a nucleophile


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Nomenclature of Carboxylic Acids

In systematic nomenclature, the carbonyl carbon is always

C-1. In common nomenclature, the carbon next to the

carbonyl is the a-carbon


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Naming Cyclic Carboxylic Acid

Acyl Halides


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Cyclic esters are known as lactones:

Esters

Acid Anhydrides


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If a substituent is bonded to the nitrogen, the name of the

substituent is stated first:

Amides

Cyclic amides are known as lactams:


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Nitriles


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Structures of Carboxylic Acids and Carboxylic Acid Derivatives

Amides have the highest boiling points


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Two major resonance contributors in esters, carboxylic acids,

and amides:


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The reactivity of carbonyl compounds

resides in the polarity of the carbonyl

group

General mechanism: The tetrahedral intermediate is a

transient species that eliminates the leaving group Y or

the nucleophile Z:

Nucleophilic acyl substitution reaction


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Z will be expelled if it is a much weaker base than Y;

that is, Z is a better leaving group than Y (k1>>k2):

Y will be expelled if it is a weaker base than Z; that

is, Y is a better leaving group than Z (k2>>k1):


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(a) the Nu is a weaker base

(b) the Nu is a stronger base

(c) the Nu and the leaving group have similar basicities

Reaction Coordinate Diagrams for

Nucleophilic Acyl Substitution Reactions


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The reactivity of a carboxylic acid derivative depends on

the basicity of the substituent attached to the acyl group:

Electron withdrawal increases the carbonyl carbons

susceptibility to nucleophilic attack:


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The weaker the basicity of Y, the greater the reactivity:

Weak bases are easier to expel when the tetrahedral

intermediate collapses:


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Reactivity of Acid Derivatives


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A carboxylic acid derivative can be converted only into a

less reactive carboxylic acid derivative:

If the nucleophile is neutral, a base is required :


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Reactions of Acyl Halides

R

CH2OH

K2Cr2O7,


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Reactions of Acid Anhydrides

i f


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Reactions of Esters

Alcohols that have low boiling points can be removed by

distillation as they are formed

Excess water will force the equilibrium to the right.


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Phenyl esters are more reactive than alkyl esters

because phenolate ions are weaker bases than

alkoxide ions:


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Acid catalyzes the reaction by


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Transesterification is also catalyzed by acid:

An acid catalyst can make a group a better leaving group:

Esters with tertiary alkyl groups undergo hydrolysis much

more rapidly than do others: Why?

Hydroxide ion increases the rate of formation as well as


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Hydroxide ion increases the rate of formation as well as

the collapse of the tetrahedral intermediate:


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Aminolysis does not need an acid catalysis. Why?

Long-chain carboxylate ions form micell


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Long chain carboxylate ions form micell

The nonpolar tails are buried in the hydrophobic

interior. The polar carboxylates are positioned at the

aqueous exterior.

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Organic -lec 17- Dr Naem