Carboxylic Acids and their Derivatives

Carboxylic Acids and their Derivatives

Physical properties

Examples:

Carboxylic Acids and their Derivatives

The simple aliphatic acids are liquids at room temperature. Aromatic acidsare solids at room temperature. All carboxylic acids are highly associated with H-bonding. This explains why no gaseous acids exist at room temperatureand their miscibility in water.

The Carboxylic GroupThis group contains both a carbonyl (C=O) group and a hydroxyl group (O-H).Both bonds in isolation, are quite strongly polar (C=O and O-H).

They interact when combined together so that the characteristics of the bondsin carboxylic acid is less susceptible to attack by nucleophiles than in aldehydesand ketones (the C atom is less positively charged) and the O-H bond in carboxylicacids splits and releases an H+ ion much more readily than in alcohols. 

Chemical properties

Acidity All organic carboxylic acids are weak acids. They are however, much stronger than the corresponding alcohol. In solution the equation is:

Carboxylic Acids and their Derivatives

The loss of the proton is enhanced by the electronegativity of the adjacentoxygen, and the charge separation in the C=O. The latter produces a slight+ve charge on the carbon which pulls the bonding electrons in O-H to itself:

Carboxylic Acids and their Derivatives

In the alcohol there is only the inductive effect of the electronegativeoxygen in the O-H bond. Therefore, less tendency for H+ to be released.The acid equilibrium is encouraged to move to the right by the stability of the ion formed. The ion is stabilized because the â�"ve charge can be delocalised throughout the system:

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i.e. cloud extends on O-C-O 

1. Ethanoic acid is less acidic than methanoic acid due to the inductive effect of the CH3 group.

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2. Introduction of a electronegative Cl atom produces an increase in acidity due to the electron withdrawing power of Cl.

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3.Similarly, with an NO2 group. This also produces a greater acidity, i.e.

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4. With CH3CH2CH2COOH: inductive effect of CH3 group reduced as it is transmitted through 2 CH2groups.

Methods of preparation and manufacture

a) By oxidation of primary alcohols or aldehydesb) By hydrolysis of nitriles

This hydrolysis can be brought about both in acids and alkaline solution.In acid solution, using dilute sulphuric acid:

2CH3CN(aq) + H2SO4(aq) + H2O(l)

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2CH3COOH(aq) + (NH4)2SO4(aq) 

Chemical reactions

a) Formation of saltsCarboxylic acids will react with bases and carbonates to form salts:

b) Formation of EstersCarboxylic acids react with alcohols to form esters, the H atom of the carboxylgroup is replaced by an alkyl group.

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CH3COOH(aq) + C2H5OH(aq)CH3COOC2H5(aq)

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Concentrated H2SO4 is often used as it catalyses thereaction and removes the water. This removal of water drives the equilibrium towards the right.

c) Formation of Acyl Chlorides

These can be prepared by the reaction of sulphur oxide dichloride SOCl2,phosphorus pentachloride (PCl5) or phosphorous trichloride (PCl3)on the acid.

Carboxylic Acids and their Derivatives

Acyl chloride

Hydrolysis of acyl chloridesAcyl chlorides react with water, often violently, to give the corresponding carboxylic acids and hydrogen chloride.

The mechanism is a simple variation of the general addition-elimination reaction.

Carboxylic Acids and their Derivatives

Reactions of Acyl ChloridesAcyl chlorides react with nucleophiles, the Cl atom being replaced by othergroups.

Carboxylic Acids and their Derivatives

a) With alcohols

Carboxylic Acids and their Derivatives

b) With phenol

Carboxylic Acids and their Derivatives

c) With primary amines

Carboxylic Acids and their Derivatives

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