Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction to Organic Chemistry 2 ed William H. Brown

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. CarboxylicAcids Chapter 12

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Structure The functional group of a carboxylic acid is a carboxyl group The general formula of an aliphatic carboxylic acid is RCO 2 H; that of an aromatic carboxylic acid is ArCO 2 H

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - IUPAC e oic acidIUPAC names: drop the -e from the parent alkane and add the suffix -oic acid anenif the compound contains a carbon-carbon double bond, change the infix -an- to -en-

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - IUPAC the carboxyl group takes precedence over most other functional groups

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - IUPAC Dicarboxylic acids: add the suffix -dioic acid to the name of the parent alkane containing both carboxyl groups

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - IUPAC carboxylic acidIf the carboxyl group is attached to a ring, name the ring compound and add the suffix -carboxylic acid

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - IUPAC The simplest aromatic carboxylic acid is benzoic acid derivatives are named using numbers to show the location of substituents relative to the carboxyl group

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - IUPAC Aromatic dicarboxylic acids are named by adding the words “dicarboxylic acid” to “benzene”

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature-Common When common names are used, the letters   etc. are often used to locate substituents

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Physical Properties In the liquid and solid states, carboxylic acids are associated by hydrogen bonding into dimeric structures

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Physical Properties Carboxylic acids are polar compounds and form very strong intermolecular hydrogen bonds through both their C=O and OH groups they have significantly higher boiling points than other types of organic compounds of comparable molecular weight they are more soluble in water than alcohols, ethers, aldehydes, and ketones of comparable molecular weight

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Physical Properties Water solubility decreases as the relative size of the hydrophobic portion of the molecule increases

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Acidity Carboxylic acids are weak acids values of pK a for most aliphatic and aromatic carboxylic acids fall within the range 4 - 5

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Acidity The greater acidity of carboxylic acids relative to alcohols is due to two factors: the significant resonance stabilization of a carboxylate anion compared with an alkoxide anion

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Acidity the electron-withdrawing inductive effect of the adjacent carbonyl group on the O-H bond

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Acidity electron-withdrawing substituents near the carboxyl group increase acidity through their inductive effect

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Acidity multiple substitution of electron-withdrawing groups further increases acidity

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Acidity The inductive effect of an electron-withdrawing substituent falls off rapidly with its distance from the carboxyl group

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Reaction with Bases Carboxylic acids, whether soluble or insoluble in water, react with NaOH, KOH, and other strong bases to give water-soluble salts

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Reaction with Bases they also form water-soluble salts with ammonia and amines

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Reaction with Bases Carboxylic acids react with sodium bicarbonate and sodium carbonate to form water-soluble salts and carbonic acid carbonic acid, in turn, breaks down to carbon dioxide and water

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Reduction The carboxyl group is one of the organic functional groups most resistant to reduction it is not affected by catalytic hydrogenation under conditions that easily reduce aldehydes and ketones to alcohols, and reduce alkenes and alkynes to alkanes it is not reduced by NaBH 4, a reagent that readily reduces the carbonyl groups of aldehydes and ketones

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Reduction by LiAlH 4 Lithium aluminum hydride reduces a carboxyl group to a 1° alcohol reduction is most commonly carried out in diethyl ether or THF

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Selective Reduction The carboxyl group is not affected by catalytic hydrogenation under conditions that easily reduce aldehydes and ketones to alcohols.

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Selective Reduction it is also possible to selectively reduce the carbonyl group of an aldehyde or ketone using the less reactive NaBH 4

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Fischer Esterification Esters can be prepared by treatment of a carboxylic acid with an alcohol in the presence of an acid catalyst, most commonly H 2 SO 4 or gaseous HCl

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Fischer Esterification Fischer esterification is an equilibrium reaction by careful control of experimental conditions, it is possible to prepare esters in high yield if the alcohol is inexpensive relative to the carboxylic acid, it can be used in excess to drive the equilibrium to the right if water can be removed from the reaction mixture, the equilibrium is also driven to the right

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Fischer Esterification A key intermediate in Fischer esterification is the tetrahedral carbonyl addition intermediate formed by addition of ROH to the C=O group

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Decarboxylation DecarboxylationDecarboxylation: loss of CO 2 from a carboxyl group most carboxylic acids, if heated to a very high temperature, undergo thermal decarboxylation most carboxylic acids, however, are quite resistant to moderate heat and melt or even boil without decarboxylation

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Decarboxylation Exceptions are carboxylic acids that have a carbonyl group beta to the carboxyl group this type of carboxylic acid undergoes decarboxylation on mild heating

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Decarboxylation thermal decarboxylation of a  -ketoacid involves rearrangement of six electrons in a cyclic six- membered transition state

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Decarboxylation Decarboxylation occurs if there is any carbonyl group beta to the carboxyl malonic acid and substituted malonic acids, for example, also undergo thermal decarboxylation

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Decarboxylation thermal decarboxylation of malonic acids also involves rearrangement of six electrons in a cyclic six- membered transition state

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. CarboxylicAcids End Chapter 12