Alkanoic Acids/Carboxylic Acid – SS3 Chemistry Lesson Note (First Term)

Alkanoic acids (carboxylic acids) are organic compounds that contain the carboxyl functional group, -COOH, with the general molecular formula C_nH_2n+1COOH.

• Occurrence: they occur in nature as free acids and in fats and oils.
• Esters: they react with alkanols to form alkanoates (esters), which give flowers their smell and fruits their flavour.

Behavioural objectives

By the end of the lesson, students should be able to:

1. state the general molecular formula and functional group of alkanoic acids and name their natural sources;
2. describe the general properties of alkanoic acids;
3. describe the laboratory preparation of ethanoic acid and of ethyl ethanoate;
4. write equations for the reactions of ethanoic acid as an acid, in esterification and in reduction;
5. state the general formula, naming, reactions and uses of alkanoates (esters).

What are alkanoic acids?

Alkanoic acids are organic acids whose molecules contain the -COOH group, and the monocarboxylic ones have the formula C_nH_2n+1COOH.

• General formula: C_nH_2n+1COOH, where n starts from 0.
• Functional group: -COOH, the carboxyl group.
• Naming: the “-e” of the alkane name is replaced with “-oic acid”.

What are the first four alkanoic acids?

What are the general properties of monocarboxylic acids?

Their physical properties change regularly as the number of carbon atoms increases.

• Acids with 1 to 3 carbon atoms: colourless liquids with a sharp odour of vinegar.
• Acids with 4 to 8 carbon atoms: unpleasant odour, while the higher members are colourless and odourless.
• Boiling point and viscosity: both increase regularly as the relative molecular mass increases.
• Branching: among isomeric acids, the boiling point falls as branching increases, because surface area and intermolecular attraction decrease.
• Comparison with alkanols: the boiling point of an alkanoic acid is much higher than that of the corresponding alkanol.

What are the natural sources of alkanoic acids?

Aliphatic carboxylic acids occur in nature as free acids or in the form of fats and oils.

• Methanoic acid: injected by ants, bees and wasps when they sting, causing pain and irritation.
• Ethanoic acid: present in vinegar.
• Butanoic acid: found in human sweat.
• Citric acid: found in lemon.
• Palmitic acid: found in palm fruits.
• Lactic acid: found in sour milk.

How are alkanoic acids prepared in the laboratory?

Alkanoic acids are prepared in the laboratory by oxidising primary alkanols, among other methods.

• Oxidation of primary alkanols: a primary alkanol is oxidised first to an alkanal and then to the acid.
• Hydrolysis of esters in acid media: the ester splits into the acid and the alkanol.
• Distillation of anhydrous sodium ethanoate with conc. H₂SO₄: the stronger acid displaces ethanoic acid, which distils over.

How is ethanoic acid prepared by oxidation of ethanol?

Ethanol is refluxed with an acidified oxidising agent, either KMnO₄ or K₂Cr₂O₇.

Procedure

1. Add 30 cm³ of H₂SO₄ to 20 cm³ of water. Stir continually and allow to cool.
2. Dissolve 70 g of K₂Cr₂O₇ in the cooled acid and place the solution in a round-bottomed flask.
3. Mix 40 cm³ of ethanol with 20 cm³ of water, place the mixture in a dropping funnel and set up the apparatus for reflux.
4. Reflux the mixture (acidified K₂Cr₂O₇ and ethanol) as the ethanol is added from the dropping funnel.
5. When all the ethanol has been added, continue to reflux the mixture.
6. Rearrange the apparatus for distillation and collect the distillate, which contains ethanoic acid.

Equations

CH₃CH₂OH + [O] → CH₃CHO + H₂O

CH₃CHO + [O] → CH₃COOH

Notes

• All-glass apparatus: hot CH₃COOH vapours attack cork and rubber stoppers.
• Acid used: H₂SO₄ is used to acidify. HCl is not used because it is volatile and can itself be oxidised.
• Amount of oxidising agent: it is needed in larger amount than the ethanol for complete oxidation.
• Industrial method: ethyne is converted to ethanal using mercury(II) tetraoxosulphate(VI) as catalyst, and the ethanal is oxidised to ethanoic acid.

What are the chemical reactions of alkanoic acids?

The carboxyl group (-COOH) is responsible for the high reactivity of alkanoic acids, and ethanoic acid is used here as the typical example.

What are the reactions of ethanoic acid as an acid?

1. Ionisation in water: it ionises slightly to give hydronium ions.

CH₃COOH + H₂O ⇌ CH₃COO^– + H₃O⁺

2. With metals: it liberates hydrogen from strongly electropositive metals.

2CH₃COOH + 2Na → 2CH₃COONa + H₂

3. With bases: it neutralises a base to form a salt and water.

CH₃COOH + NaOH → CH₃COONa + H₂O

4. With carbonates: it liberates carbon(IV) oxide.

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

What is esterification?

Esterification is the reaction of a carboxylic acid with an alkanol, in the presence of conc. H₂SO₄, to form an ester and water.

• Catalyst: conc. H₂SO₄.
• Products: an ester and water only.
• Reversibility: the reaction is reversible.
• Observation: a fruity or fragrant smell of the ester is noticed.

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

The ester formed is ethyl ethanoate.

How are alkanoic acids reduced?

Alkanoic acids are reduced to primary alkanols by lithium tetrahydridoaluminate, LiAlH₄, a reducing agent that supplies hydrogen.

CH₃COOH + 4[H] → CH₃CH₂OH + H₂O

What are the uses of ethanoic acid?

Ethanoic acid is used as a solvent, as a food preservative and as a raw material for many products.

• Solvent: an important organic solvent.
• Food industry: vinegar, for preserving and flavouring food.
• Rubber: coagulating rubber latex.
• Vinyl ethanoate (vinyl acetate): used in vinyl resins.
• Ethanoic anhydride: used in making aspirin.
• Cellulose acetate: used in packing, in varnishes and in cigarette filter tips.
• Synthetic fibres: used in making rayon and artificial silk.

What are alkanoates?

Alkanoates are esters formed when the ionisable hydrogen of an alkanoic acid is replaced by an alkyl group.

• Fats and oils: animal and vegetable fats and oils are esters with high relative molecular masses.
• Simple esters: present in the fruits of plants, they give flowers their natural sweet smell and fruits their flavour.

What flavours do some esters give?

What is the general molecular formula of alkanoates?

The general molecular formula of alkanoates is C_nH_2nO₂, where n = 2, 3, 4 and so on.

• Representation: RCOOR¹.
• R: the alkyl group of the acid.
• R¹: the alkyl group that replaces the ionisable H atom of the acid.
• Functional group: -COOR¹.
• Saturated esters: when R and R¹ are both saturated, the esters are simply alkanoates.

How are alkanoates named?

In the systematic name, the alkanol part (R¹) is named first as an alkyl group, followed by the acid part (RCOO-) named as an alkanoate.

C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O

• Example: ethanol + ethanoic acid gives ethyl ethanoate + water.

How is ethyl ethanoate prepared in the laboratory?

Ethyl ethanoate is prepared by the esterification of glacial ethanoic acid with ethanol in the presence of conc. H₂SO₄.

Procedure

1. Add about 10 drops of conc. H₂SO₄ to about 2 cm³ of glacial ethanoic acid in a dry test tube.
2. Mix well, add 4 cm³ of ethanol and mix again.
3. Warm the mixture in water for about 25 minutes.
4. Pour the mixture into a beaker partially filled with sodium chloride or calcium chloride solution.
5. Observe the ethyl ethanoate floating on the mixture as oil globules.

C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O

What are the reactions of alkanoates?

Alkanoates are generally unreactive and undergo few reactions.

Hydrolysis

Hydrolysis is the reverse of esterification, and it produces the component acid and alkanol.

• With water alone: the reaction is extremely slow.
• With a catalyst: H⁺ or OH^– (a dilute acid or alkali) speeds it up.

CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH

• With an alkali: the acid formed reacts at once to give the sodium salt of the acid.

CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

Alkaline hydrolysis

Alkaline hydrolysis is the hydrolysis of an ester with an alkali, and it is not reversible.

• Soap making: when the ester is a fat or oil, the reaction forms soap.

Ammonolysis

Ammonolysis is the reaction of an ester with ammonia.

• Products: ethyl ethanoate reacts with aqueous ammonia to give ethanamide and ethanol.

CH₃COOC₂H₅ + NH₃ → CH₃CONH₂ + C₂H₅OH

What are the uses of esters?

Esters are used as solvents, in perfumes and as food flavours.

• Solvents: for cellulose nitrate and quick-drying substances.
• Perfumes and cosmetics: for their pleasant smell.
• Food: as artificial flavouring.

Evaluation

1. State the general molecular formula and the functional group of monocarboxylic acids.
2. Name two oxidising agents used to prepare ethanoic acid from ethanol, and write the two equations for the oxidation.
3. Write the equation for the reaction between ethanoic acid and ethanol, and name the ester formed.
4. State two reactions of ethyl ethanoate.
5. State two uses of esters.

Assignment

Ethanoic acid reacts with both sodium hydroxide and ethanol. Do the following:

1. Write the equation for each reaction.
2. Classify the products of each reaction.
3. Write the structural formula of ethyl ethanoate and of propyl ethanoate.