Carboxylic Acids

• The functional group of a carboxylic acid is a carboxyl group.

Structure of Carboxylic Acids

• The general formula of an aliphatic carboxylic acid is RCOOH.

• For an aromatic carboxylic acid, the general formula is ArCOOH.

Nomenclature of Carboxylic Acids

• IUPAC names: drop the -e from the parent alkane and add the suffix -oic acid.– If the compound contains a carbon-carbon double

bond, change the infix -an- to -en-.

• The carboxyl group takes precedence over most other functional groups.

• If the carboxyl group is bonded to a ring, name the ring compound and add the suffix -carboxylic acid.

• Dicarboxylic acids: add -dioic acid to the name of the parent alkane.

• There is no need to use numbers to locate the carboxyl groups; they can only be on the ends of the chain.

• Benzoic acid is the simplest aromatic carboxylic acid.

• Use numbers to show the location of substituents.

• Know the common names of formic and acetic acid.

H C

O

O H

formic acid

H3C C

O

O H

acetic acid

• In common nomenclature, keto indicates the presence of a ketone, and CH3CO– is named an aceto group.

• When common names are used, the letters , , , , , etc… are often used to locate substituents.

Physical Properties of Carboxylic Acids

• In the liquid and solid states, carboxylic acids are associated by hydrogen bonding into dimeric structures.

• 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 are alcohols, ethers, aldehydes, and ketones of comparable molecular weight.• They form hydrogen bonds with water

molecules through their C=O and OH groups.

• Water solubility decreases as the relative size of the hydrophobic portion of the molecule increases.

Acidity of Carboxylic Acids

• Carboxylic acids are stronger weak acids• Values of pKa 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.

• Electron-withdrawing substituents near the carboxyl group increase acidity through their inductive effect.

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

Summary of Reactions

• Reactions with bases• Reduction• Esterification (Fischer)

• Conversion to acid chloride (via SOCl2)

• Decarboxylation

• Carboxylic acids, whether soluble or insoluble in water, react with NaOH, KOH, and other strong bases to give water-soluble salts.

Reaction with Bases

• They also form water-soluble salts with ammonia and amines.

• 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

gas (bubbles) and water.

• Only a stronger weak acid (pKa < 7) will react with bicarbonate (carbonate); therefore, bubbling is confirmation of the presence of a carboxylic acid.

Reduction• The carboxyl group is very resistant to reduction.

• It is not affected by catalytic hydrogenation under conditions that easily reduce aldehydes and ketones and reduce alkenes and alkynes.

• Carboxyl groups are not reduced by NaBH4.

• Lithium aluminum hydride reduces a carboxyl group to a 1° alcohol.• Reduction is carried out in diethyl ether, THF, or

other nonreactive, aprotic solvents.

• we can use NaBH4 to reduce an aldehyde or ketone in the presence of a COOH group.

• Catalytic hydrogenation does not reduce a COOH group.• we can use H2/M to reduce an alkene in the presence

of a COOH group.

(Fischer) Esterification

• Esters can be prepared by treating a carboxylic acid with an alcohol in the presence of an acid catalyst, commonly H2SO4 or gaseous HCl.

• 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 (toward ester production).

• A key intermediate in Fischer esterification is a tetrahedral carbonyl addition intermediate (TCAI) formed by addition of ROH to the C=O group.

C OHRO

HOCH3H+

C OCH3

OH

OR

H

H+

C OCH3RO

HOH+ +

TCAI

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.• In older nomenclature, these compounds were known

as acyl halides.

• Acid chlorides are most often prepared by treating a carboxylic acid with thionyl chloride.

• Acid chlorides do not have many applications; however, they are very reactive; therefore they are extremely useful in the synthesis of molecules. (More next chapter)

Decarboxylation• Decarboxylation: loss of CO2 from a carboxyl

group.• Most carboxylic acids, if heated to a very high

temperature, undergo thermal decarboxylation.

• However, at lower temperatures, most are quite resistant to decarboxylation.

• Exceptions are carboxylic acids that have a carbonyl group beta to the carboxyl group.

• -ketoacids undergo decarboxylation on mild heating.

• Thermal decarboxylation of a -ketoacid involves rearrangement of six electrons in a cyclic six-membered transition state. (FYI only)