Chapter 19 Carboxylic Acids and Derivatives

Question 1

Production of Carboxylic Acids

Answer 1

• Carboxylic acids are compounds with a -COOH functional group
• They can be prepared by a series of different reactions

Question 2

Oxidation of primary alcohols & aldehydes

Answer 2

• Carboxylic acids can be formed from the oxidation of primary alcohols and aldehydes by either acidified K2Cr2O7 or acidified KMnO4 and reflux
• The oxidising agents themselves get reduced causing the solutions to change colour
• -In K2Cr2O7 the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions
• –In KMnO4 the purple manganate ions (MnO4–) are reduced to colourless Mn2+ ions

Question 3

Hydrolysis of nitriles

Answer 3

-Carboxylic acids can also be prepared from the hydrolysis of nitriles using either dilute acid or dilute alkali followed by acidification

• –Hydrolysis by dilute acid results in the formation of a carboxylic acid and ammonium salt
• –Hydrolysis by dilute alkali results in the formation of a sodium carboxylate salt and ammonia; Acidification is required to change the carboxylate ion into a carboxylic acid

-The -CN group at the end of the hydrocarbon chain is converted to a -COOH group

Question 4

Hydrolysis of esters

Answer 4

• Esters are formed from the condensation reaction between an alcohol and carboxylic acid
• Hydrolysis of esters by dilute acid or dilute alkali and heat followed by acidification will reform the alcohol and carboxylic acid
• –Hydrolysis by dilute acid, is a reversible reaction and an equilibrium is established
• –Hydrolysis by dilute alkali is an irreversible reaction as all the ester is broken down to form a sodium carboxylate salt and an alcohol; acidification is required to change the carboxylate ion into a carboxylic acid

Question 5

Reactions of Carboxylic Acids

Answer 5

• Carboxylic acids are weak acids as they do not completely dissociate in water
• This means that the position of the equilibrium lies to the left and that the concentration of H+ is much smaller than the concentration of the carboxylic acid
• The solution has a pH value of less than 7

Question 6

Carboxylic acids are reactive compounds which can undergo many types of reactions including:

Answer 6

• Redox reactions with reactive metals
• Neutralisation reactions with alkali
• Acid-base reactions with carbonates
• Esterification with alcohols
• Reduction by LiAlH4

Question 7

Production of Esters

Answer 7

• Esters are compounds with an -COOR functional group and are characterised by their sweet and fruity smells
• They are prepared from the condensation reaction between a carboxylic acid and alcohol with concentrated H2SO4 as catalyst
• –This is also called esterification
• The first part of the ester’s name comes from the alcohol and the second part of the name comes from the carboxylic acid
• –E.g. Propanol and ethanoic acid will give the ester propyl ethanoate

Question 8

Hydrolysis of Esters

Answer 8

• Esters can be hydrolysed to reform the carboxylic acid and alcohol by either dilute acid or dilute alkali and heat
• When an ester is heated under reflux with dilute acid (eg. sulfuric acid) an equilibrium mixture is established as the reaction is reversible
• However, heating the ester under reflux with dilute alkali (eg. sodium hydroxide) is an irreversible reaction as the ester is fully hydrolysed
• This results in the formation of a sodium carboxylate salt which needs further acidification to turn into a carboxylic acid
• —The sodium carboxylate (-COO–) ion needs to get protonated by an acid (such as HCl) to form the carboxylic acid (-COOH)