Premed · Premed · Organic Chemistry 2
Lecture 9: Carboxylic Acid Derivatives: Esters, Amides, Anhydrides
Organic Chemistry II
Learning Objectives
By the end of this lecture, students will be able to:
- Identify and name the four major carboxylic acid derivatives: acid chlorides, anhydrides, esters, and amides
- Rank carboxylic acid derivatives in order of reactivity toward nucleophilic acyl substitution
- Describe the structural features that account for the differences in reactivity
- Explain the physical properties and biological significance of esters and amides
- Describe key synthetic methods for each derivative
- Identify carboxylic acid derivatives by spectroscopic methods
Lecture Content
I. Overview of Carboxylic Acid Derivatives
All carboxylic acid derivatives share the general structure R-C(=O)-L, where L is a leaving group bonded to the carbonyl carbon. The four main classes are acid chlorides (RCOCl, where L = Cl), acid anhydrides ((RCO)2O, where L = OCOR), esters (RCOOR', where L = OR'), and amides (RCONR'2, where L = NR'2, NH2, or NHR'). These derivatives differ dramatically in reactivity, following the order acid chlorides > anhydrides > esters (approximately equal to carboxylic acids) > amides, from most to least reactive. This order reflects both the leaving group ability of L and the degree of resonance stabilization of the carbonyl.
II. Structural Basis for Reactivity Differences
The key to understanding the reactivity differences among acid derivatives lies in resonance donation from the leaving group atom into the carbonyl. The lone pair on the atom attached to the C=O donates electron density into the carbonyl pi system, which decreases the electrophilicity of the carbonyl carbon. The greater the resonance donation, the less reactive the compound.
Amides are the least reactive because nitrogen is an excellent resonance donor. Its lone pair, housed in a 2p orbital, overlaps efficiently with the carbonyl pi system, imparting significant C-N double-bond character (approximately 40%). This resonance lowers the C=O stretching frequency to about 1650 cm-1 and creates a substantial barrier to rotation about the C-N bond. Esters exhibit moderate resonance stabilization from oxygen donation, though oxygen is more electronegative and therefore less willing to share its electrons than nitrogen. The ester C=O stretch appears near 1735 cm-1. Anhydrides show moderate stabilization from the carboxylate leaving group and display two characteristic C=O stretching frequencies near 1800 and 1750 cm-1. Acid chlorides are the most reactive because chlorine is a poor resonance donor -- its 3p orbital overlaps inefficiently with the 2p orbital of carbon, providing minimal resonance stabilization. The C=O stretch of acid chlorides appears at approximately 1800 cm-1, the highest frequency among the derivatives, reflecting the strongest (least delocalized) carbonyl bond.
<image>Panel A: Four carboxylic acid derivatives drawn with their resonance structures showing the degree of lone pair donation into the carbonyl. Acid chloride shows minimal donation (small curved arrow), ester shows moderate donation, and amide shows extensive donation (large curved arrow). Panel B: A reactivity scale (arrow from left to right) ordering the four derivatives from most reactive (acid chloride) to least reactive (amide), with C=O IR stretching frequencies listed below each structure and the relative electrophilicity of the carbonyl carbon indicated by partial positive charge size.</image>
III. Acid Chlorides
Acid chlorides are most commonly prepared by treating a carboxylic acid with thionyl chloride (SOCl2), which produces the acid chloride along with gaseous SO2 and HCl as byproducts. Alternative reagents include oxalyl chloride ((COCl)2) and phosphorus trichloride or pentachloride. Acid chlorides are low-boiling liquids with pungent, irritating odors. They are extremely moisture-sensitive, reacting vigorously with water to regenerate the carboxylic acid.
Because of their high reactivity, acid chlorides serve as the "universal" starting material for interconversion among acid derivatives. They react with water to give carboxylic acids, with alcohols to give esters, with amines to give amides (using two equivalents of amine or a base such as pyridine to neutralize HCl), and with carboxylate salts to give anhydrides. They can also be reduced with LiAlH4 to primary alcohols, partially reduced to aldehydes via the Rosenmund reduction (H2/Pd on BaSO4), or treated with Gilman reagents (R'2CuLi) to give ketones.
IV. Acid Anhydrides
Acid anhydrides can be prepared by dehydration of two equivalents of carboxylic acid using P2O5, or by reaction of an acid chloride with a carboxylate salt. Cyclic anhydrides form readily from dicarboxylic acids such as succinic acid, glutaric acid, and phthalic acid upon heating. Acetic anhydride is commercially important and inexpensive, serving as a common acetylating agent in industry.
The reactions of anhydrides parallel those of acid chlorides, but one equivalent of carboxylic acid is released as a byproduct with each transformation. Reaction with an alcohol gives an ester plus a carboxylic acid, and reaction with an amine gives an amide plus a carboxylic acid. The synthesis of aspirin (acetylsalicylic acid) from salicylic acid and acetic anhydride is a classic example of anhydride chemistry.
V. Esters
Esters can be synthesized by several routes. Fischer esterification treats a carboxylic acid with an alcohol under acid catalysis. Alternatively, an acid chloride or anhydride can be reacted with an alcohol in the presence of a base. Diazomethane converts carboxylic acids to methyl esters under very mild conditions with nitrogen gas as the only byproduct. An SN2 reaction between a carboxylate anion and a primary or methyl alkyl halide also produces esters.
Many esters have pleasant, fruity odors and are responsible for characteristic flavors and fragrances: ethyl butanoate evokes pineapple, isoamyl acetate smells of bananas, and methyl salicylate is the essence of wintergreen. Esters have lower boiling points than the corresponding carboxylic acids because they lack hydrogen-bond donor capability, though they can act as hydrogen-bond acceptors and exhibit moderate water solubility.
The key reactions of esters include acid hydrolysis (reversible, giving the carboxylic acid and alcohol), base hydrolysis or saponification (irreversible, giving the carboxylate salt and alcohol), reduction with LiAlH4 to give two alcohols, partial reduction with DIBAL-H at -78 degrees C to give an aldehyde, Grignard reaction (two equivalents of R''MgX give a tertiary alcohol), and transesterification (exchange of the alkoxy group under acid or base catalysis). Triglycerides, the triesters of glycerol with fatty acids, are the principal form of energy storage in fats and oils.
VI. Amides
The most reliable laboratory synthesis of amides involves reaction of an acid chloride with two equivalents of an amine (or with one equivalent of amine plus a base like triethylamine). Anhydrides can also be used. Heating the ammonium salt formed from a carboxylic acid and an amine drives off water to produce the amide. Aminolysis of esters is thermodynamically favorable but kinetically slow. For peptide synthesis, coupling reagents such as DCC, EDC, and HATU are used to activate the carboxyl group and form the amide bond efficiently.
Primary and secondary amides exhibit very high boiling points due to extensive N-H hydrogen bonding. N,N-Disubstituted (tertiary) amides lack N-H bonds but are still highly polar; DMF (dimethylformamide) and NMP (N-methylpyrrolidone) are common polar aprotic solvents.
A defining property of amides is restricted rotation about the C-N bond, a consequence of its partial double-bond character. The rotation barrier is approximately 75 kJ/mol. In NMR spectroscopy, this restricted rotation manifests as two distinct signals for groups on nitrogen; DMF at room temperature shows two separate N-CH3 peaks. The trans configuration about the C-N bond is generally preferred over cis.
Amide hydrolysis requires vigorous conditions -- strong acid or base with heating. Reduction with LiAlH4 converts amides to amines. The Hofmann rearrangement converts primary amides to primary amines with loss of one carbon (as CO2) upon treatment with Br2 and NaOH. The peptide bond that links amino acids in proteins is fundamentally an amide bond, making amide chemistry central to biochemistry.
<image>Panel A: Diagram showing the restricted rotation in N,N-dimethylformamide (DMF), with the two rotational isomers (cis and trans methyl groups relative to the carbonyl oxygen) drawn, and a double-headed arrow indicating slow interconversion. A simulated 1H NMR spectrum shows two separate N-CH3 peaks. Panel B: The peptide bond in a dipeptide shown with resonance structures indicating partial double bond character of the C-N bond, planarity of the amide unit, and the trans configuration of the C=O and N-H groups.</image>
VII. Spectroscopic Identification of Acid Derivatives
The carbonyl stretching frequency in IR spectroscopy is the most diagnostic feature for identifying and distinguishing among carboxylic acid derivatives. Acid chlorides absorb near 1800 cm-1, anhydrides show two bands near 1800 and 1750 cm-1, esters absorb near 1735 cm-1 (with a strong C-O stretch at 1000-1300 cm-1), and amides absorb near 1650 cm-1 (the amide I band), often accompanied by an N-H bending absorption near 1550 cm-1 (the amide II band). The trend is clear: greater resonance donation from the substituent lowers the C=O stretching frequency.
In proton NMR, ester -OCH3 or -OCH2 groups appear at delta 3.5 to 4.5 ppm, while amide N-H protons resonate broadly at delta 6 to 9 ppm and are exchangeable. Carbon-13 NMR places the carbonyl carbon of acid chlorides near delta 170 ppm, esters at delta 170 to 175 ppm, and amides at delta 165 to 175 ppm.
