ORGANIC CHEMISTRY - CHAPTER 20: CARBOXYLIC ACIDS DERIVATIVES

New Reactions

Important Concepts

1.  The electrophilic reactivity of the carbonyl carbon in carboxylic acid derivatives is weakened by good electron-donating substituents. This effect, measurable by IR spectroscopy, is responsible not only for the decrease in the reactivity with nucleophiles and acid, but also for the increased basicity along the series: acyl halides – anhydrides – esters – amides. Electron donation by resonance from the nitrogen in amides is so pronounced that there is hindered rotation about the amide bond on the NMR time scale.

2.  Carboxylic acid derivatives are named as acyl halides, carboxylic anhydrides, alkyl alkanoates, alkanamides, and alkanenitriles, depending on the functional group.

3.  Carbonyl stretching frequencies in the IR spectra are diagnostic of the carboxylic acid derivatives: Acyl chlorides absorb at 1790 – 1815 cm^-1, anhydrides at 1740 – 1790 and 1800 – 1850 cm^-1, esters at 1735 – 1750 cm^-1, and amides at 1650 – 1690 cm^-1.

4. Carboxylic acid derivatives generally react with water (under acid or base catalysis) to hydrolyze to the corresponding carboxylic acid; they combine with alcohols to give esters and with amines to furnish amides. With Grignard and other organometallic reagents, they form ketones; esters may react further to form the corresponding alcohols. Reduction by hydrides gives products in various oxidation states: aldehydes, alcohols, or amines.

5.  Long-chain esters are the constituents of animal and plant waxes.Triesters of glycerol are contained in natural oils and fats. Their hydrolysis gives soaps. Triglycerides containing phosphoric acid ester subunits belong to the class of phospholipids. Because they carry a highly polar head group and hydrophobic tails, phospholipids form micelles and lipid bilayers.

6.  Transesterification can be used to convert one ester into another.

7.  The functional group of nitriles is somewhat similar to that of the alkynes. The two component atoms are sp hybridized. The IR stretching vibration appears at about 2250 cm^-1. The hydrogens next to the cyano group are deshielded in ¹H NMR. The ¹³C NMR absorptions for nitrile carbons are at relatively low field (δ, 112 – 126 ppm), a consequence of the electronegativity of nitrogen.

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE