Copyright © 2010 Pearson Education, Inc.

Organic Chemistry, 7th Edition L. G. Wade, Jr.

Carboxylic Acids

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Introduction is an organic compound that contains a carboxyl group (C=O)OH).

The general formula of a carboxylic acid is R–COOH.

The functional group of carboxylic acids consists of a C═O with —

OH bonded to the same carbon.

Aliphatic acids have an alkyl group bonded to —COOH.

Aromatic acids have an aryl group.

Fatty acids are long-chain aliphatic acids.

Nomenclature

Functional group suffix = -oic acid

If the acid is substituted onto a ring the suffix -carboxylic acid is used.

The root name is based on the longest chain including the carboxylic acid group.

Since the carboxylic acid group is at the end of the chain, it must be C1.

The carboxylic acid suffix is appended after the hydrocarbon suffix minus the "e"

: e.g. -ane + -oic acid = -anoic acid etc.

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Common Names Many aliphatic

acids have

historical names.

Positions of

substituents on

the chain are

labeled with

Greek letters

starting at the

carbon attached

to the carboxylic

carbon.

Carbon

atomsCommon name IUPAC name Chemical formula

Common location or

use

1 Carbonic acid Carbonic acid OHCOOH

Blood and tissues

(bicarbonate buffer

system)

1 Formic acid Methanoic acid HCOOH Insect stings

2 Acetic acid Ethanoic acid CH3COOH Vinegar

5 Valeric acid Pentanoic acid CH3(CH2)3COOH Valerian

6 Caproic acid Hexanoic acid CH3(CH2)4COOH Goat fat

8 Caprylic acid Octanoic acid CH3(CH2)6COOHCoconuts and breast

milk

10 Capric acid Decanoic acid CH3(CH2)8COOHCoconut and Palm

kernel oil

16 Palmitic acid Hexadecanoic acid CH3(CH2)14COOH Palm oil

18 Stearic acid Octadecanoic acid CH3(CH2)16COOHChocolate, waxes,

soaps, and oils

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Unsaturated Acids Remove the final -e from alkene name, add the ending -oic acid.

Stereochemistry is specified.

Aromatic Acids• Aromatic acids are named as derivatives of benzoic acid.• Ortho-, meta- and para- prefixes are used to specify the location of a

second substituent.• Numbers are used to specify locations when more than 2 substituents are

present.

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Naming Dicarboxylic Acids

Aliphatic diacids are usually called

by their common names.

For IUPAC name, number the chain

from the end closest to a substituent.

Naming Carboxylic Acid

Salts

First name the cation.

Then name the anion by replacing

the -ic acid with -ate.

3-bromohexanedioic acid

-bromoadipic acid

HOOCCH2CHCH2CH2COOH

Br

CH3CH2CHCH2COO-K

+

Cl

potassium 3-chloropentanoate

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Structure of Formic Acid

The sp2 hybrid carbonyl carbon atom is planar, with nearly trigonal bond angles.

The O—H bond also lies in this plane, eclipsed with the C═O bond.

The sp3 oxygen has a C—O—H angle of 106°.

Bond angles are close to 120.

O—H eclipsed with C═O, to get

overlap of orbital with orbital of

lone pair on oxygen

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physical properties Carboxylic acids have higher boiling points than similar

alcohols, due to the formation of a hydrogen-bonded dimer.

Aliphatic acids with more than 8 carbons are solids at room

temperature.

Double bonds (especially cis) lower the melting point. The

following acids all have 18 carbons:

Stearic acid (saturated): 72C

Oleic acid (one cis double bond): 16C

Linoleic acid (two cis double bonds): -5C

Water solubility decreases with the length of the carbon chain.

With up to 4 carbons, acid is miscible in water.

Very soluble in alcohols.

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Acidity of Carboxylic Acids A carboxylic acid may

dissociate in water to give a proton and a carboxylate ion.

The equilibrium constant Ka

for this reaction is called the acid-dissociation constant.

The acid will be mostly dissociated if the pH of the solution is higher than the pKa of the acid.

Energy Diagram of Carboxylic Acids and Alcohols <<<<<<<<<<<

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Substituent Effects on Acidity

1. Substituent effect depends on its distance from the carboxyl group.

2. Electron-withdrawing groups enhance the acid strength and electron-donating groups decrease the acid strength.

3. Effects are strongest for substituents in the ortho and para positions.

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Deprotonation of Carboxylic Acids

The hydroxide ion

deprotonates the

acid to form the

carboxylate salt.

Adding a strong

acid, like HCl,

regenerates the

carboxylic acid.

The hydroxide ion

deprotonates the

acid to form the acid

salt.

Adding a mineral

acid regenerates the

carboxylic acid.

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Properties of Acid Salts

Usually solids with no odor.

Carboxylate salts of Na+, K+, Li+, and NH4+ are soluble in water.

Soap is the soluble sodium salt of a long chain fatty acid.

Salts can be formed by the reaction of an acid with NaHCO3,

releasing CO2.

Hydrolysis of

Fats and OilsThe basic

hydrolysis of fat

and oils produces

soap (this reaction

is known as

saponification).

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Synthesis Review

Oxidation of primary alcohols and aldehydes with chromic acid.

Cleavage of an alkene with hot KMnO4 produces a carboxylic

acid if there is a hydrogen on the double-bonded carbon.

Alkyl benzene oxidized to benzoic acid by hot KMnO4 or hot

chromic acid.

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Oxidation of Primary Alcohol to

Carboxylic Acids

Primary alcohols and aldehydes are commonly oxidized to acids by chromic acid (H2CrO4 formed from Na2Cr2O7 and H2SO4).

Potassium permanganate is occasionally used, but the yields are often lower.

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Cleavage of Alkenes Using KMnO4

Warm, concentrated permanganate solutions oxidize

the glycols, cleaving the central C═C bond.

Depending on the substitution of the original double

bond, ketones or acids may result.

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Side Chain Oxidation of

Alkylbenzenes

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Conversion of Grignards to

Carboxylic Acids

Grignard reagent react with CO2 to produce, after protonation, a

carboxylic acid.

This reaction is sometimes called “CO2 insertion” and it

increases the number of carbons in the molecule by one.

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Carboxylic Acids Reactions LiAlH4 or BH3 Reduction of Carboxylic Acids

LiAlH4 reduces carboxylic acids to primary alcohols.

The intermediate aldehyde reacts faster with the reducing agent than the carboxylic acid.

BH3•THF (or B2H6) can also reduce the carboxylic acid to the alcohol