Carboxylic Acid Nomenclature. © 2013 Pearson Education, Inc.Chapter 202 Introduction The...

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Carboxylic Acid Carboxylic Acid

NomenclatureNomenclature

Introduction The functional group of carboxylic acids

consists of a C═O with —OH bonded to the same carbon.

Carboxyl group is usually written —COOH. Aliphatic acids have an alkyl group bonded

to —COOH. Aromatic acids have an aryl group. Fatty acids are long-chain aliphatic acids.

Table 18.1

Systematic NameO

CH3COH

CH3(CH2)16COH

methanoic acid

ethanoic acid

octadecanoic acid

Systematic IUPAC names replace "-e" ending of alkane with "oic acid"

Table 18.1

Systematic Name Common Name

methanoic acid formic acid

ethanoic acid acetic acid

octadecanoic acid stearic acid

Common names are based on natural origin rather than structure.

CH3COH

CH3(CH2)16COH

Table 18.1

Systematic Name Common Name

2-hydroxypropanoic acid lactic acid

(Z)-9-octadecenoic acidor (Z)-octadec-9-enoic acid

oleic acid

CH3CHCOH

(CH2)7COH

CH3(CH2)7

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

two substituents are present.

Structure of the Carboxyl Group

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°.

Boiling Points

Carboxylic acids boil at considerably higher temperatures than do alcohols, ketones, or aldehydes of similar molecular weights.

The high boiling points of carboxylic acids result from formation of a stable, hydrogen-bonded dimer.

Carboxylic acids are similar to alcohols in respect to their solubility in water.

They form hydrogen bonds to water.

Solubility in Water

Solubility

Water solubility decreases with the length of the carbon chain.

Acids with more than 10 carbon atoms are nearly insoluble in water.

Very soluble in alcohols. Also soluble in relatively nonpolar solvents like

chloroform because the hydrogen bonds of the dimer are not disrupted by the nonpolar solvent.

Acidity of Carboxylic AcidsAcidity of Carboxylic Acids

Most carboxylic acids have a pMost carboxylic acids have a pKKaa close to 5. close to 5.

But carboxylic acids are far more acidic than alcohols.

Carboxylic Acids are Weak Acids

CH3COH

CH3CH2OH

pKa = 4.7 pKa = 16

Energy Diagram of Carboxylic Acids and Alcohols

Acetate Ion Structure

Each oxygen atom bears half of the negative charge. The delocalization of the negative charge over the

two oxygens makes the acetate ion more stable than an alkoxide ion.

Substituent Effects on Acidity

The magnitude of a substituent effect depends on its distance from the carboxyl group.

Aromatic Carboxylic Acids

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

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

Hybridization Effect

H2C CH COH

sp2-hybridized carbon is more electron-withdrawing than sp3, and sp is more electron-withdrawing than sp2.

Salts of Carboxylic Acids

Deprotonation of Carboxylic Acids

The hydroxide ion completely deprotonates the acid to form the carboxylate salt.

Protonation of Carboxylic Acids Salts

Adding a strong acid, like HCl, regenerates the carboxylic acid.

Naming Carboxylic Acid Salts

First name the cation. Then name the anion by replacing the

-ic acid with -ate.

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.

Basic Hydrolysis of Fats and Oils

• The basic hydrolysis of fat and oils produces soap (this reaction is known as saponification).

Unbranched carboxylic acids with 12-18 carbonsgive carboxylate salts that form micelles in water.

Micelles O

ONasodium stearate

(sodium octadecanoate)

CH3(CH2)16CO

Na+–

Micelles O

polarnonpolar

Micelles O

polarnonpolar

Sodium stearate has a polar end (the carboxylate end) and a nonpolar "tail“.

The polar end is hydrophilic ("water-loving”).

The nonpolar tail is hydrophobic ("water-hating”).

In water, many stearate ions cluster together to form spherical aggregates; carboxylate ions are on the outside and nonpolar tails on the inside.

Figure 18.5: A micelle

Micelles

The interior of the micelle is nonpolar and has the capacity to dissolve nonpolar substances.

Soaps clean because they form micelles, which are dispersed in water.

Grease (not ordinarily soluble in water) dissolves in the interior of the micelle and is washed away with the dispersed micelle.

Dicarboxylic AcidsDicarboxylic Acids

Dicarboxylic Acids

One carboxyl group acts as an electron-withdrawing group toward the other; effect decreases with increasing separation.

Oxalic acid

Malonic acid

Heptanedioic acid

HOCCH2COH

HOC(CH2)5COH

side-chain oxidation of alkylbenzenes (Section 11.12)

oxidation of primary alcohols (Section 15.9)

oxidation of aldehydes (Section 17.15)

Synthesis of Carboxylic Acids: Review

Side Chain Oxidation of Alkylbenzenes

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.

Oxidation of Aldehydes

Aldehydes are easily oxidized to carboxylic acids.

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.

Alkyne Cleavage Using Ozone or KMnO4

With alkynes, either ozonolysis or a vigorous permanganate oxidation cleaves the triple bond to give carboxylic acids.

Synthesis of Carboxylic AcidsSynthesis of Carboxylic Acidsby theby the

Carboxylation of Grignard ReagentsCarboxylation of Grignard Reagents

Carboxylation of Grignard Reagents

Grignard reagents 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.

Example: Alkyl Halide

CH3CHCH2CH3

(76-86%)

1. Mg, diethyl ether

2. CO2

3. H3O+

CH3CHCH2CH3

Cl CO2H

2-methylbutanoic acid2-methylbutanoic acid

Example: Aryl Halide

1. Mg, diethyl ether

2. CO2

3. H3O+

CO2HBr

Synthesis of Carboxylic AcidsSynthesis of Carboxylic Acidsby theby the

Preparation and Hydrolysis of NitrilesPreparation and Hydrolysis of Nitriles

Hydrolysis of Nitriles

Basic or acidic hydrolysis of a nitrile (—CN) produces a carboxylic acid.

The overall reaction, starting from the alkyl halide, adds an extra carbon to the molecule.

A limitation is that the halide must be reactive toward substitution by SN2 mechanism.

Example

DMSO(92%)

CH2COH

Example: Dicarboxylic Acid

BrCH2CH2CH2Br

NaCN H2O

(77-86%)NCCH2CH2CH2CN

H2O, HCl heat

(83-85%)HOCCH2CH2CH2COH

via Cyanohydrin

1. NaCN

CH3CCH2CH2CH3

(60% from 2-pentanone)

HCl, heat

CH3CCH2CH2CH3

Reactions of Carboxylic Acids:Reactions of Carboxylic Acids:

A Review and a PreviewA Review and a Preview

Reduction with LiAlH4 (Section 15.3)

Formation of acyl chlorides (Section 12.7)

Esterification (Section 15.8)

Reactions already discussed

Reactions of Carboxylic Acids

LiAlH4 Reduction of Carboxylic Acids

LiAlH4 reduces carboxylic acids to primary alcohols.

Synthesis of Acid Chlorides

The best reagents for converting carboxylic acids to acid chlorides are thionyl chloride (SOCl2) and oxalyl chloride (COCl2).

They form gaseous by-products that do not contaminate the product.

Mechanism of Acid Chloride Formation

Step 1

Step 2

Step 3

Fischer Esterification

Reaction of a carboxylic acid with an alcohol under acidic conditions produces an ester.

Reaction is an equilibrium; the yield of ester is not high. To drive the equilibrium toward the formation of products, use a large

excess of alcohol.

Mechanism of the Fischer Esterification

Step 1: The carbonyl oxygen is protonated to activate the carbon

toward nucleophilic attack. The alcohol attacks the carbonyl carbon. Deprotonation of the intermediate produces the ester

hydrate.

Mechanism of the Fischer Esterification (Continued)

Step 2: Protonation of one of the hydroxide groups creates a good

leaving group. Water leaves. Deprotonation of the intermediate produces the ester.

Protonation of carbonyl group activates carbonyl oxygen.

Nucleophilic addition of alcohol to carbonyl groupforms tetrahedral intermediate.

Elimination of water from tetrahedral intermediate restores carbonyl group.

Key Features of Mechanism

Intramolecular Ester Formation:Intramolecular Ester Formation:

LactonesLactones

Lactones are cyclic esters.

Formed by intramolecular esterification in acompound that contains a hydroxyl group anda carboxylic acid function

Lactones

Examples

IUPAC nomenclature: replace the -oic acid ending of the carboxylic acid by –olide.

Identify the oxygenated carbon by number.

HOCH2CH2CH2COH

O+ H2O

4-hydroxybutanoic acid 4-butanolide

Examples

HOCH2CH2CH2COH

O+ H2O

4-hydroxybutanoic acid 4-butanolide

HOCH2CH2CH2CH2COH

5-hydroxypentanoic acid 5-pentanolide

Common names O

-butyrolactone -valerolactone

Ring size is designated by Greek letter corresponding to oxygenated carbon

A lactone has a five-membered ring.

A lactone has a six-membered ring.

Reactions designed to give hydroxy acids often yield the corresponding lactone, especially if theresulting ring is 5- or 6-membered.

Lactones

Example

5-hexanolide (78%)

CH3CCH2CH2CH2COH

1. NaBH4

2. H2O, H+

via:via:

CHCH33CHCHCHCH22CHCH22CHCH22COHCOH

OOOOHH

Decarboxylation of Malonic AcidDecarboxylation of Malonic Acid

and Related Compoundsand Related Compounds

Decarboxylation of Carboxylic Acids

Simple carboxylic acids do not decarboxylatereadily.

RH + CO2RCOH

But malonic acid does.

150°CCH3COH

+ CO2HOCCH2COH

Mechanism of Decarboxylation of Malonic Acid

This compound is the enol form of acetic acid.

One carboxyl group assists the loss of the other.

HOCCH3

These hydrogens play no role. H

HOCCHR'

Groups other than H may be present. R

R R’

185°C

Decarboxylation is a general reactionfor 1,3-dicarboxylic acids

160°C

(74%) CH(CO2H)2

(96-99%)

CH2CO2H

This OH group plays no role. R

R R’

HOCCHR'

Mechanism of Decarboxylation of Malonic Acid O O

Groups other than OH may be present.

R"CCHR'

Mechanism of Decarboxylation of Malonic Acid O O

This kind of compoundis called a -keto acid.

R"CCHR'

Decarboxylation of a -keto acid gives a ketone.

Decarboxylation of a -Keto Acid

CO2H25°C

Some Important Acids

Acetic acid is in vinegar and other foods, used industrially as a solvent, catalyst, and reagent for synthesis.

Fatty acids from fats and oils. Benzoic acid is found in drugs and

preservatives. Adipic acid is used to make nylon 66. Phthalic acid is used to make polyesters.

Spectroscopic Analysis ofSpectroscopic Analysis of

Carboxylic AcidsCarboxylic Acids

IR Bands of Carboxylic Acids

There will be two features in the IR spectrum of a carboxylic acid: the intense carbonyl stretching absorption (1710 cm–1) and the OH absorption (2500–3500 cm–1).

Conjugation lowers the frequency of the C═O band.

IR Spectroscopy

NMR of Carboxylic Acids

Carboxylic acid protons are the most deshielded protons we have encountered, absorbing between 10 and 13.

The protons on the carbon atom absorb between 2.0 and 2.5.

NMR Spectroscopy

13C NMR

Carbonyl carbon is at low field ( 160-185 ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone ( 190-215 ppm).

Aliphatic carboxylic acids undergo a varietyof fragmentations.Aromatic carboxylic acids first form acylium ions,which then lose CO.

Mass Spectrometry

••O •

•+O •

ArC O ••

Carboxylic Acid Nomenclature. © 2013 Pearson Education, Inc.Chapter 202 Introduction The...

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