Carboxylic acids

The carboxylic group is a combination of a carbonyl group and a hydroxyl group(OH): the name carboxylic group.

Nomenclature

1. Their names end with suffic “oic” followed by the word acid.
2. The carboxylic group must always be at the end of the chain and therefore it’s position need not to be specified. Take longest chain that contain the carboxylic group and start numbering from the carboxylic carbon

Examples	Systematic name	Common name
HCOOH	Mechanoic acid	Formic acid
CH3COOH	Ethanoic	Acetic acid
CH3CH2COOH	Propanoic acid	Propionic acid
CH2CH2CH2COOH	Butanoic acid	Butyric acid
CH3CH(CH3)COOH	2-methylpropanoic acid	Isobutyric acid

Physical properties

(1) Lower members are liquids and higher members are waxy solids. With boiling points which are higher than expected, this because, their molecules are associated with hydrogen bonds.

(2) Lower members are soluble in water but the solubility decreases with the increasing molecular mass of carboxylic acid. This is because carboxylic acids are capable of the formation of hydrogen bonds with water.

(3) Melting points

Compounds	Mpt (0C)	Bpt (0C)	Ka
HCOOH	8	100.5	1.7 x 10-4
CH3COOH	16.6	118	1.77 x 10-5
CH3CH2COOH	21	141	1.34 x 10-5
CH3(CH2)2COOH	-6	164	1.54 x 10-5
CH3(CH2)3COOH	-34	187	1.52 x 10-5
CH3(CH2)4COOH	-3	205
CH3(CH2)5COOH	16	259

Generally the melting point increase with the molecular mass. However, the melting points of lower carboxylic acids are relatively higher than those of higher carboxylic acids because they form stronger hydrogen bonds.

4. The acidity of carboxylic acids decreases with the increase in molecular mass due to the positive inductive effect of alkyl groups. Electron-withdrawing groups make carboxylic acid more acidic.

For example, CCl₃COOH is more acidic than CH₃COOH.

Methods of preparation

1. By oxidation of primary alcohols using K₂Cr₂O₇/H⁺, Na₂Cr₂O₇/H⁺, or KMnO₄/H⁺

2. Oxidation of aldehyde using K₂Cr₂O₇/H⁺, Na₂Cr₂O₇/H⁺, or KMnO₄/H⁺

3. Hydrolysis of nitrile in presence of a mineral acid

4. Hydrolysis of acid amide with a mineral acid or alkali

5. Reaction of carbon dioxide with a grignard reagent followed with hydrolysis.

6. Preparation of methanoic acid

By heating a solution of ethane dioic acid in propane-1,2,3-triol

7. Oxidation of alkenes using hot alkaline KMnO₄.

8. Oxidation of methyl ketone

Chemical properties

1. Carboxylic acid react with base to form salts and even liberate caarbon dioxide from carbonates.

Examples

2. Reduction of carboxylic acid

Carboxylic acids are resistant to reduction by mild reducing agents. However, can be reduced through aldehydes to primary alcohols by LiAlH4 in presence of dry ether.

Example

3. Decarboxylation

This is a reaction in which a molecule of CO2 is removed from carboxylic acids.

(i) Simple carboxylic acids are not easily decarboxylated but their salts are easily decarboxylated in presence of soda lime.

(ii) Acids with electron-withdrawing groups on α-carbon are easily decarboxylated on heating.

Example

(iii) Esterification: Carboxylic acids react with alcohol in presence of mineral acids to form es

Easter have got sweet smell; this is why this reaction is used in identification of carboxylic acids

Reactivity of alcohols is in order 1⁰ ›2⁰ ›3⁰, and the reactivity of carboxylic acids is in order HCOOH› CH₃COOH › RCH₂COOH › R₂CHCOOH › R₃CCOOH, due to steric hindrance. i.e. the presence of bulky groups near the site of reaction, whether in the alcohol or in acid slows esterification.

5. Formation of acid halides

Carboxylic acids (except methanoic acid) react with phosphorus halides to form acid halides

   5. Halogenation of aliphatic acids

(a) Carboxylic acids (except methanoic acid) reacts with chlorine in presence of sunlight or u.v light, thereby a chlorine atom replacing an α-hydrogen.

(b) Bromine replaces an α-hydrogen in presence of red phosphorous (hell-vohlard zelensky reaction)

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