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  Alkanoic Acids, Chemistry tutorial

Introduction:

Alkanoic acids are the groups of organic compounds that have the -COOH group attached to a hydrogen atom or to the alkyl group. The old name for members of this homologous series is carboxylic acids. The names of members of the series are achieved by substituting the '- e' of the corresponding alkane through '- oic' and add the term 'acid'. The members of series can be symbolized through the general molecular formula of CnH2n+1 COOH here n ≥ 0. They are at times termed as fatty acids as some of them are found in the natural fats and oils.

The first five members of the alkanoic acid sequence are illustrated in the table shown below:

Molecular formula

Structural formula

IUPAC name

CH2O2

HCOOH

methanoic acid

C2H4O2

CH3COOH

ethanoic acid

C3H6O2

CH3CH2COOH

propanoic acid

C4H8O2

CH3CH2CH2COOH

butanoic acid

C5H10O2

CH3CH2CH2CH2COOH

pentanoic acid

Alkanoic acids are at times symbolized as R-COOH, here R = alkyl group. Older names specific alkanoic acids refer to the origin or use of the acid, example: methanoic acid was first made from ants (Formica = ant) and is termed as formic acid; ethanoic acid takes place in vinegar (acetus = sour) and is known as acetic acid.

Apart from the aliphatic acids, aromatic acids like benzoic acid are as well significant members of this series.

50_Benzoic acid.jpg

Fig: benzoic acid, an aromatic acid

Sources of Alkanoic Acids:

Natural Sources:

Some of the members of the alkanoic acid series are broadly distributed in the nature and found in living organism. For illustration, methanoic acid was first obtained from ants; ethanoic acid is present in vinegar and is responsible for its sour taste; butanoic acid has been isolated from the human sweat, that is, the higher alkanoic acids like citric acid. Tartaric acid, are known to be present in several kinds of fruits and vegetables.

General Methods of Preparation:

(a) Oxidation of primary alkanols:

Primary alkanols are oxidized through oxidizing agents, like potassium tetraoxomanganate (VII), potassium heptaoxochromate (VI) to alkanoic acids. The reaction is generally taken out through refluxing surplus of the oxidizing agent by the Alkanol.

R - CH2OH → ([O] heat) → R - COOH + H2O

Note: [O] symbolizes the Oxidizing agent

b) Hydrolysis of nitrile:

The acid catalyzed hydrolysis of nitrile with water, yield the alkanoic acid. For illustration ethanonitrile can be hydrolyses to ethanoic acid in the presence of small quantity of tetraoxosulphate (VI) acid.

CH3CN + 2H2O → (H+) → CH3COOH + NH3

Ethanonitrile

Characteristics of Alkanoic Acids:

Physical Properties:

a) The lower members of alkanoic acid series, like methanoic acid and ethanoic acid, are fully miscible in water. Solubility, though, reduces with higher members of the series. In common, the alkanoic acids are mere soluble in water, (that is, a polar solvent) than the corresponding alkanols having the similar number of carbon atoms. This is parity due to the high polarity of the Cδ+ - Oδ- and Oδ- - Hδ+ bonds that makes it simpler for the alkanoic acid molecule to bond readily by water molecules therefore increased solubility. By higher-members of the series, though, the hydrocarbon component (that is, R groups which are organic) becomes bigger than the polar component in the molecules, therefore the decrease in solubility.

b) Alkanoic acids encompass many higher bailing points than other compounds with comparable relative molecular mass example:

 

Relative Molecular Mass

Boiling point

Ethanoic acid

60

118 oC

Propan-l-ol

60

97 oC

Butane

58

-1 oC

The high boiling points of alkanoic acids are as an outcome of strong intermolecular hydrogen bonding which exist between the molecules. However, lower members of the series don't exist as single molecules however as dimers (as illustrated below), giving then an usually higher molecular mass than predicted.

1159_Alkanoic acid molecules and water molecules.jpg

Fig: Alkanoic acid molecules and water molecules

Hydrogen bonding between the alkanoic acid molecules and water molecules, as well partly illustrates their high solubility in the water.

Chemical properties:

a) Acidity: Due to the polar nature of bonds in the alkanoic acids they ionize in water as shown: 

2127_Alkanoic acids-Acidity.jpg

Fig: Alkanoic acids-acidity

Though, alkanoic acids are weaker acids than the mineral acids, like hydrochloric acids, however they are very much stronger than alkanols. The strength of any alkanoic acid based on the equilibrium position of the above equation. The stronger the acid the further to the right will be the equilibrium position in such a way that the acid strength will based on the readiness through which the -O-H bond will divide in the presence of water.

b) Formation of salts: The Alkanoic acids react by bases and trioxocarbonate (iv) to form salts. Therefore it,

- Reacts by trioxocarbonate (iv) and hydrogen trioxocarbonate (iv) to give salts of the acid with release of carbon (iv) oxide.

2RC00H + Na2CO3 → 2RCOONa +H20 + CO2 ↑

- Reacts by bases or alkalis to form salts of the acids and water. This is the acid-base neutralization.

RCOOH + NaOH → RCOO-Na+ + H20

In the above reaction, the alkanoic acids behave like mineral acids, showing their acidic property. Alkanols don't undergo these reactions as they are weaker acids.

c) Formation of esters (esterification): The Alkanoic acids react with the alkanols to form alkanoates (that is, esters); the H-atom of the carboxyl group is substituted through an alkyl group example:

R--COOH + R'-OH ↔ R-COOR' + H20

Alkanoic     Alkanol    Alkanoate   Water

acid

The reaction is generally catalyzed by mineral acids example: HCl or H2SO4. This reaction is generally termed to as esterification. Alkanoates or esters are a significant homologous series.

d) Reduction: The alkanoic acids can be reduced to primary alkanols by employing a special reducing agent termed as lithium aluminium hydride, LiAlH4.

R-COOH → (LiAlH4, HCl) → R - CH2OH

This reduction reaction is the reverse procedure of the oxidation of primary alkanols; used for generating alkanoic acids.

Laboratory Test for Alkanoic Acids:

The presence of alkanoic acid functionality is generally confirmed in the laboratory by using its acidic properties. We know that, alkanoic acids react with trioxocarbonate (iv) or hydrocarbonates to release carbon (iv) oxide. This is the laboratory test for the COOH group present in all the alkanoic acids. The evolution of the CO2 gas is employed as proof of the presence of the alkanoic acids.

R - COOH + NaHCO3 → R-COONa + H20 + CO2↑

Uses of Alkanoic Acids:

1) Methanoic acid is employed in dyeing electroplating, taming and in the coagulating rubber.

2) Ethanoic acid is employed as a raw material in the

  • Manufacture of cellulose ethanoate (utilized for packing, in varnishes, in the filter tips of cigarettes and in the synthetic fibres).
  • Prepare ethanoic anhydride that is employed in aspirin.

3) Ethanoic acid is the significant organic solvent.

4) Higher alkanoic acids are employed for making the detergents and soaps.

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