4.8 CARBOXYLIC ACIDS

General Formula C_nH_2nO₂ n ³ 1

Nomenclature R—C=O

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OH

Name	Molecular formula	Structural formula	b.pt. /^oC	Density /g cm-^3
methanoic acid	CH2O2	H—C=O ï OH
ethanoic acid	C2H4O2	CH3—C=O ï OH
benzoic acid	C7H6O2	H—C=O ï
ethandioic	C2H2O4	HO—C=O ï HO—C=O

Alkanoic acids contain both the carbonyl (C=O) group and the hydroxy (—OH) group, together called the carboxyl group —C=O

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OH

These groups are so close together that they modify each others properties. Therefore alkanoic acids have many properties that are different from those of both alcohols and carbonyl compounds. The properties of the —COOH group are modified only slightly by the presence of a benzene ring. Therefore aromatic carboxylic acids have many properties in common with aliphatic ones

Formation from primary alcohols, aldehydes and esters.

Preparation of an aqueous solution of the acid from the alcohol.

Preparation of alkanoic acids ( 3 methods)

1 Oxidation of primary alcohols.

These undergo oxidation using acidified potassium dichromate(VI)

3C₂H₅OH + Cr₂O₇^2- + 8H⁺ 3CH₃COOH + 2Cr³⁺ + 4H₂O

or more simply Cr₂O₇^2-/H⁺

C₂H₅OH + 2[O] CH₃COOH + H₂O

ethanol ethanoic acid

2 Oxidation of aldehydes

These undergo oxidation using acidified potassium dichromate(VI)

3CH₃CHO + Cr₂O₇^2- + 8H⁺ 3CH₃COOH + 2Cr³⁺ + 4H₂O

or simply

Cr₂0₇^2-/H⁺

CH₃CHO + [O] CH₃CO0H

ethanal ethanoic acid

3 Hydrolysis of esters

Esters undergo acid hydrolysis, on refluxing with a dilute acid, forming a carboxylic acid and an alcohol.

H⁺

CH₃COOC₂H₅ + H₂O CH₃COOH + C₂H₅OH

ethylethanoate ethanoic acid ethanol

Reference to the effect of hydrogen bonding on boiling point and miscibility with water.

Physical properties

Early members (C₁-C₄) are miscible with water in all proportions due to hydrogen bonding.

O d+

R—C d- H

O—H O

d- d+ H d+

Carboxylic acids have higher than expected melting and boiling points because of hydrogen bonding, forming dimers. They also dimerise in non polar solvents.

d+ d-

R—C=O H—O

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O—H O= C—H

Acidity. Formation of salts using bases such as sodium carbonate, sodium hydroxide and ammonia.

Acidity

Carboxylic acids are weak acids, dissociating slightly in water, although the strength of the acid depends on the nature of the group attached to the carboxyl (—COOH) group.

R—C=O � R—C=O + H⁺

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OH O^-

alkanoate ion

X-ray diffraction studies show the two carbon - oxygen bonds in the alkanoate ion to be equal in length suggesting delocalisation of charge.

O

R—C -

O

ACID	FORMULA	Ka (dissociation constant)
methanoic	HCOOH	1.6 x 10^-4
ethanoic	CH3COOH	1.7 x 10^-5
propanoic	CH3CH2COOH	1.3 x 10^-5
chloroethanoic	ClCH2COOH	1.3 x 10^-3
dichloroethanoic	Cl2CHCOOH	5.0 x 10^-2
trichloroethanoic	Cl3CCOOH	2.3 x 10^-1

The greater the K_a value, the greater the degree of dissociation in water, and the stronger the acid. The nature of the group attached to the —COOH has a considerable effect on the acid strength. When the groups are electron donating alkyl groups the —OH bond is less polar and so it does not dissociate as readily.

O d- O

// //

H— C � H— C + H⁺

\ \

O—H O-

d+

O d- O

// //

CH₃® C � CH₃— C + H⁺

\

O—H O-

d+

HCOOH is a stronger acid than CH₃COOH. Since the electron donating effect of all alkyl groups is roughly the same, ethanoic acid, propanoic acid etc. have similar acidity. The converse is true for electron withdrawing groups. e.g. chlorine makes the —O—H bond more polar and therefore dissociates more readily.

Relative acid strengths of substituted acids

Cl Cl H H

½ ï

Cl¬C—COOH > H—C—COOH > H—C—COOH > H—C®COOH

¯ ¯ ¯ ï

Cl Cl Cl H

Reaction with bases forming salts

Carboxylic acids react in a similar way to mineral acids with bases.

CH₃COOH + NaOH CH₃COO^-Na⁺ + H₂O

sodium ethanoate

CH₃COOH + NH₃ CH₃COO^-NH₄⁺

ammonium ethanoate

2CH₃COOH + Na₂CO₃ 2CH₃COO^-Na⁺ + CO₂ + H₂O

Reaction with alcohols , PCl₅ SOCl₂ and LiAlH₄.

Reaction with alcohols

In the presence of an acid catalyst, carboxylic acids react with alcohols to form esters. Water is eliminated. The reaction is reversible.

H⁺

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

Concentrated sulphuric acid is used for two reasons

(i) it acts as a,catalyst and

(ii) it is a dehydrating agent removing water as it is formed and thus promoting the forward reaction.

Conditions: warm, add conc. sulphuric acid, distil.

Reaction with phosphorous(V) chloride

Carboxylic acids react forming alkanoyl ch!orides. This is a halogenation reaction - replacing the -OH group with a halogen, usually chlorine, forming a group of very reactive compounds called alkanoyl chlorides (acyl chlorides).

This reaction,occurs at room temperature, evolving fumes of hydrogen chloride.

CH₃—C=O + PCl₅ CH₃—C=O + HCl + POCl₃

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OH Cl

ethanoic ethanoyl phosphorous

acid chloride oxychloride

CH₃CH₂—C=O + PCl₅ CH₃CH₂—C=O + HCl + POCl₃

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OH Cl

propanoic propanoyl phosphorous

acid chloride oxychloride

Reaction with thionyl chloride, SOCl₂

Thionyl chloride reacts with carboxylic acids in similar way to phosphorus (V) chloride. Again the reaction is rapid at room temperature.

R—C=O + SOCl₂ R—C=O + HCl + SO₂

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OH Cl

This reaction has the advantage over that with phosphorus (V) chloride as the by-products are gases and easily separated from the reaction mixture.

Reaction with lithium tetrahydridoaluminate(IV), LiAlH₄

This is a powerful reducing agent and reduces the acid to an alcohol.

LiAlH₄

CH₃COOH + 4[H] CH₃CH₂OH + H₂O

An aldehyde is not obtained because LiAlH₄ is a powerful reducing agent and reduces the acid completely to an alcohol.