William H. Brown Thomas Poon Chapter Fourteen Carboxylic Acids

14-2 Structure The functional group of a carboxylic acid is a carboxyl group. The general formula of an aliphatic carboxylic acid is RCOOH. That of an aromatic carboxylic acid is ArCOOH.

14-3 Nomenclature 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-.

14-4 Nomenclature The carboxyl group takes precedence over most other functional groups. Don’t worry about R or S… we didn’t learn about that.

14-5 Nomenclature dioic acid  Dicarboxylic acids: add -dioic acid to the name of the parent alkane containing both carboxyl groups. There is no need to use numbers to locate the carboxyl groups; they can only be on the ends of the chain.

14-6 Nomenclature carboxylic acidIf the carboxyl group is bonded to a ring, name the ring compound and add the suffix -carboxylic acid. Don’t worry about rings with multiple –COOHs!

14-7 Nomenclature Benzoic acid is the simplest aromatic carboxylic acid. Use numbers to show the location of substituents. Don’t worry about benzene rings with multiple –COOHs!

14-8 Nomenclature When common names are used, the letters  etc. are often used to locate substituents. keto aceto groupIn common nomenclature, keto indicates the presence of a ketone, and CH 3 CO- is named an aceto group.

14-9 Physical Properties In the liquid and solid states, carboxylic acids are associated by hydrogen bonding into dimeric structures.

14-10 Physical Properties Carboxylic acids have significantly higher boiling points than other types of organic compounds of comparable molecular weight. They are polar compounds and form very strong intermolecular hydrogen bonds. Carboxylic acids are more soluble in water than alcohols, ethers, aldehydes, and ketones of comparable molecular weight. They form hydrogen bonds with water molecules through their C=O and OH groups.

14-11 Physical Properties Water solubility decreases as the relative size of the hydrophobic portion of the molecule increases.

14-12 Acidity Carboxylic acids are weak acids Values of pK a for most aliphatic and aromatic carboxylic acids fall within the range 4 to 5. The greater acidity of carboxylic acids relative to alcohols, both of which contain an OH group, is due to resonance stabilization of the carboxylate anion.

14-13 Acidity Electron-withdrawing substituents near the carboxyl group increase acidity through their inductive effect.

14-14 Acidity The acid-strengthening effect of a halogen substituent falls off rapidly with increasing distance from the carboxyl group.

14-15 Reaction with Bases Carboxylic acids, whether soluble or insoluble in water, react with NaOH, KOH, and other strong bases to give water-soluble salts. They also form water-soluble salts with ammonia and amines.

14-16 Reaction with Bases Carboxylic acids react with sodium bicarbonate and sodium carbonate to form water-soluble salts and carbonic acid. Carbonic acid (H 2 CO 3 ), in turn, breaks down to carbon dioxide and water.

14-17 Reaction with Bases Figure 14.1 Flowchart for separation of benzoic acid from benzyl alcohol.

14-18 Reduction The carboxyl group is very 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.

14-19 Reduction Lithium aluminum hydride reduces a carboxyl group to a 1° alcohol. Reduction is carried out in diethyl ether, THF, or other nonreactive, aprotic solvents. (Notice that the alkene double bond is unaffected!) OH

14-20 Selective Reduction Catalytic hydrogenation does not reduce a COOH group! we can use H 2 /Metal to reduce an alkene in the presence of a COOH group. we can use NaBH 4 to reduce an aldehyde or ketone in the presence of a COOH group.

14-21 Fischer Esterification Esters can be prepared by treating a carboxylic acid with an alcohol in the presence of an acid catalyst, commonly H 2 SO 4 or gaseous HCl.

14-22 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. A key intermediate in Fischer esterification is the tetrahedral carbonyl addition intermediate formed by addition of ROH to the C=O group.

14-23 Acid Chlorides The functional group of an acid halide is a carbonyl group bonded to a halogen atom. Among the acid halides, acid chlorides are by far the most common and the most widely used.

14-24 Acid Chlorides Acid chlorides are most often prepared by treating a carboxylic acid with thionyl chloride.

14-25 Decarboxylation Decarboxylation:Decarboxylation: loss of CO 2 from a carboxyl group. Most carboxylic acids, if heated to a very high temperature, undergo thermal decarboxylation. Most, however, are quite resistant to moderate heat and melt or even boil without decarboxylation.

14-26 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.

14-27 Decarboxylation Thermal decarboxylation of a  -ketoacid involves rearrangement of six electrons in a cyclic six-membered transition state.

14-28 Decarboxylation Thermal decarboxylation of malonic acids also involves rearrangement of six electrons in a cyclic six-membered transition state.

14-29 Decarboxylation Problem:Problem: Draw the product of decarboxylation. Problem:Problem: Draw the  -ketoacid that undergoes decarboxylation to give this ketone.

14-30 Carboxylic Acids End Chapter 14