Chapter 10

Carboxylic Acids

Chemistry 20

A carboxylic acid contains a carboxyl group, which is a carbonyl group attach to a hydroxyl group.

carbonyl

group

O

CH3 — C—OH hydroxyl group or CH3COOH

carboxyl group

Carboxylic Acids

CH3CO2H

• In the IUPAC name of carboxylic acids, the “-e” in the name of the longest chain is replaced by “-oic acid”.

• The common names use prefixes “form-” and “acet-” for the first two carboxylic acids.

H-COOH methanoic acid formic acid

CH3-COOH ethanoic acid acetic acid

CH3-CH2-COOH propanoic acid

CH3-CH2-CH2-COOH butanoic acid

Naming Carboxylic Acids

Naming Carboxylic Acids

– Number the chain beginning with the carbon of the carboxyl group.

– Because the carboxyl carbon is understood to be carbon 1, there is no need to give it a number.

13

3-Methylbutanoic acid

OH

OOHH2N COOH15

4-Aminobenzoic acid5-Hydroxylhexanoic acid

4

CH3─CH2─CH─COOH

CH2 – CH3

2-Ethylbutanoic acid

2CH3─CH─CH2─COOH

CH3

1

1

Naming Dicarboxylic Acids

– Add the suffix “-dioic acid”dioic acid” to the name of the parent alkane that contains both carboxyl groups; thus, “-ane”ane” becomes “-anedioic acid”anedioic acid”.

– The numbers of the carboxyl carbons are not indicated because they can be only at the ends of the chain.

OHO OH

O

HO OH

O

OOH

O

OH

O

O

HOO

HO

1 1

1 1

2 3

4 6OH

O

HO15

O

Ehanedioic acid Propanedioic acid

Butanedioic acid Pentanedioic acid Hexanedioic acid

Physical properties of Carboxylic Acids

1- The carboxyl group contains three polar covalent bonds; C=O, C-O, and O-H. So they are so polar.

2- Carboxylic acids have higher boiling points than other types of organic compounds (with the same molecular weight) because of hydrogen bonding.

3- They are more soluble in water than alcohols, ethers, aldehydes, and ketones because of stronger hydrogen bonding.

4- Liquid carboxylic acids have sharp and disagreeable odors.

5- They taste sour (exist in pickle, lime, and lemon).

H3C CO

O

HCH3C

O

O

H- +

+ -

Hydrogen bondingbetween two molecules

Fatty Acids

• Long, unbranched chain carboxylic acids and they are found in animal fats, vegetable oils, or phospholipids of biological membranes.

• Most have between 12 and 20 carbons in an unbranched chain.

• In most unsaturated fatty acids, the cis isomer is usually existed and the trans isomer is rare.

• Unsaturated fatty acids have lower melting points than their saturated counterparts.

COOH

COOH

COOH

COOH

Stearic acid (18:0)(mp 70°C)

Oleic acid (18;1)(mp 16°C)

Linoleic acid (18:2)(mp-5°C)

Linolenic acid (18:3)(mp -11°C)

COOH

COOH

COOH

COOH

Stearic acid (18:0)(mp 70°C)

Oleic acid (18;1)(mp 16°C)

Linoleic acid (18:2)(mp-5°C)

Linolenic acid (18:3)(mp -11°C)

Cis

Fatty Acids

Saturated fatty acids are solids at room temperature.

COOHCOOHCOOHCOOHCOOH

Packed together Maximum London dispersion forces

Fatty Acids

Unsaturated fatty acids are liquids at room temperature.

Can not packed together London dispersion forces

COOHCOOH

COOH

COOH

COOH

Cis

In an ester, the H in thecarboxyl group is replacedby an alkyl group.

O

CH3 — C—O —CH3

ester group

Esters

• Natural soaps are sodium or potassium salts of fatty acids.

• They are prepared from a blend of tallow and coconut oils (triglycerides).

• Triglycerides are triesters of glycerol.

• the solid fats are melted with steam and the water insoluble triglyceride layer that forms on the top is removed.

Soaps

CH2 – CH – CH2

OH OHOH1,2,3-Propanetriol(glycerol, glycerin)

• Preparation of soaps begins by boiling the triglycerides with NaOH. The reaction that takes place is called saponification.

• Boiling with KOH gives a potassium soap.

Soaps

Sodium soaps1,2,3-Propanetriol(Glycerol; glycerin)

A triglyceride( a triester of glycerol)

+saponification+

CH2OCR

CH2OCRCHOHCH2OH

CH2OHRCOCH 3NaOH 3RCO-Na+

OO

OO

Sodium soaps1,2,3-Propanetriol(Glycerol; glycerin)

A triglyceride( a triester of glycerol)

+saponification+

CH2OCR

CH2OCRCHOHCH2OH

CH2OHRCOCH 3NaOH 3RCO-Na+

OO

OO

Sodium soaps1,2,3-Propanetriol(Glycerol; glycerin)

A triglyceride( a triester of glycerol)

+saponification+

CH2OCR

CH2OCRCHOHCH2OH

CH2OHRCOCH 3NaOH 3RCO-Na+

OO

OO

Soaps

Hydrophobic part: nonpolar

Hydrophilic part: polar (remains in contact with environment)

When soap is mixed with dirt (grease, oil, and …), soap micelles “dissolve” these nonpolar, water-insoluble molecules.

Soaps

• Natural soaps form water-insoluble salts in hard water.• Hard water contains Ca(II), Mg(II) and Fe(III) ions.

Soaps

++

A sodium soap(soluble in water as micelles)

Calcium salt of a fatty acid (insoluble in water)

2CH3(CH2)1 4COO-Na+ Ca2 + [CH3 (CH2)14COO-]2 Ca2+ 2Na+

Solution:

Using Synthetic detergents.

-SO3- (sulfonate) instead of COO- (carboxylate)

Chemical properties of Carboxylic Acids

1- They are weak acids.

Substituents of high electronegativity, especially -OH, -Cl, and -NH3+, near

the carboxyl group increase the acidity of carboxylic acids.

CH3COOH ClCH2COOH Cl2CHCOOH Cl3CCOOHFormula:

pKa:

Name:

Increasing acid strength2.86

Chloroaceticacid

0.70

Trichloroaceticacid

1.48

Dichloroacetic acid

Acetic acid4.76

They react with NaOH, KOH, NH3, and other strong bases to form water-soluble salts.

COOH NaOHH2O COO- Na+ H2O+ +

Benzoic acid(slightly soluble in water)

Sodium benzoate(60 g/100 mL water)

COOH NH3H2O

COO- NH4++

Ammonium benzoate(20 g/100 mL water)

Benzoic acid(slightly soluble in water)

2- Reaction with bases:

Chemical properties of Carboxylic Acids

3- Reduction:

Chemical properties of Carboxylic Acids

Resistant to reduction

Using a powerful reducing agent: LiAlH4 (Lithium aluminum hydride).

1° alcohol

COH

=OLiAlH4, ether

H2OCH2OH

3-cyclopentene-carboxylic acid

4-Hydroxymethyl-cyclopentene

Chemical properties of Carboxylic Acids

3- Fischer Esterification:

- A carboxylic acid reacts with an alcohols to form an ester.

- Using an acid catalyst such as concentrated sulfuric acid.

CH3C-OHO

H-OCH2CH3

H2SO4CH3COCH2CH3

OH2O

Ethanoic acid(Acetic acid)

++Ethyl ethanoate(Ethyl acetate)

Ethanol(Ethyl alcohol)

The best way to prepare an ester.

5- Decarboxylation:

Chemical properties of Carboxylic Acids

Loss of CO2 from a carboxyl group.

decarboxylation +O

RCOH RH CO2Heat