Carboxylic Acids

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Physical Properties of Carboxylic Acids

Carboxylic molecules dimerise forming two hydrogen bonds between each pair of molecules, so they have relatively high melting and boiling temperatures.

The carboxylic acids C1 to C4 are soluble in water. 

e.g. ethanoic acid is soluble in water.

Reason:  Ethanoic acid can form hydrogen bonds with water molecules because it has a polar -COOH group. The -COOH group can form hydrogen bonds with water molecules.

At C5 and thereafter, the solubility decreases 

Reason: As the molar mass increases the non-polar hydrocarbon chain which is insoluble becomes larger

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Preparation of Carboxylic Acids

1. Oxidation of primary alcohols and aldehydes

e.g. CH3-CH2-CH2OH + 2[O] ---------------> CH3-CH2-COOH + H2O

 Reagents: K2Cr2O7 solution and dilute H2SO Conditions: heat under reflux

Name of organic product: carboxylic acid Type of reaction: Oxidation


2. Hydrolysis of nitriles

e.g.  CH3-CH2-CN + 2H2O + HCl ---------------> CH3-CH2-COOH + NH4Cl

 Reagents: dilute hydrochloric acid Conditions: heat under reflux

Name of organic product: carboxylic acid Type of reaction: Hydrolysis

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Chemical reactions of carboxylic acids pt 1

Carboxylic acids are weak acids which ionise to a small extent in solution. 

It is important to learn the following 5 equations before learning the reactions with carboxylic acids:

1. metal + acid ----> salt + hydrogen

2. acid + alkali ----> salt + water

3. acid + base ----> salt + water

4. acid + carbonate ----> salt + water + carbon dioxide

5. acid + hydrogencarbonate ----> salt + water + carbon dioxide

Since it is dangerous to use the first equation, therefore the last four equations can be used for Neutralisation.

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Chemical reactions of carboxylic acids pt 2

1. Neutralisation reactions

Carboxylic acids react with alkalis, carbonates and hydrogencarbonates to produce ethanoate salts.

Reagent: aq NaOH/aq Na2CO3/aq NaHCO3 Conditions: room temperature 

 Name of organic product: sodium salt of a carboxylic acid Type of reaction: Neutralisation

Test for the -COOH group

Add aqueous sodium carbonate (or aqueous NaHCO ) to the compound. Fizzing/Effervescence is observed.A colourless gas evolves which turns limewater cloudy. - CO2

2RCOOH + NaCO ---------------> 2RCOO-Na+  + H2O + CO2

2RCOOH + NaHCO ---------------> 2RCOO-Na+  + H2O + CO2

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Chemical reactions of carboxylic acids pt 3

2. Reaction of carboxylic acids with PCl - Nucleophilic Substitution

R-COOH + PCl5 -------> R-COCl + POCl3 +HCl

Reagent: phosphorus pentachloride, PCl5 Conditions: room temperature  

Name of organic product: acyl chloride Type of reaction: Nucleophilic Substitution

3. Reduction of carboxylic acids with LiAlH - Reduction

R-COOH + 4[H] -------> R-CH2OH + H2O

Reagent: LiAlH and dry ethoxyethane, followed by dilute HCl Conditions: LiAlH is used as a solution in dry ethoxyethane 

Name of organic product: primary alcohol Type of reaction: Reduction

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Chemical reactions of carboxylic acids pt 4

4. The reaction of carboxylic acids with alcohols - Esterification

When a carboxylic acid is heated under reflux wiith an alcohol in the prescence of concentrated sulphuric acid as a catalyst, an ester is formed. (It has a fruity smell).

The reaction between carboxylic acids and alochols is a reversible reaction and results in an equilibrium. Therefore the reaction gives a poor yield of the ester.

Reagents: an alcohol and sulphuric acid Conditions: heat under reflux   

Name of organic product: an ester Type of reaction: Esterification

Functions of concentrated sulphuric acid

  • The concentrated sulphuric acid is a catalyst for the reaction
  • The concentrated sulphuric acid absorbs the water formed, driving the equilibrium to the right an increases the yield of the ester.
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