Alkanoates

What Are Alkanoates?

Alkanoates, commonly called esters, are organic compounds that smell nice. If you’ve ever noticed the sweet smell of ripe pineapple, banana, or apple, you’ve experienced esters. These compounds are formed when alkanoic acids (carboxylic acids) react with alkanols (alcohols).

Think of esterification like mixing two ingredients to make a new dish. Just as mixing flour and water creates dough (something different from both ingredients), mixing an acid and alcohol creates an ester with completely different properties.

Formation of Alkanoates: Esterification

What is Esterification?

Esterification is the chemical reaction between an alkanoic acid and an alkanol to produce an alkanoate (ester) and water. The reaction requires a catalyst – usually concentrated tetraoxosulphate(VI) acid (H₂SO₄) or concentrated hydrochloric acid (HCl).

General equation:
Alkanoic acid + Alkanol → Alkanoate + Water

RCOOH + R’OH → RCOOR’ + H₂O

The reaction needs heat (around 150°C) and the acid catalyst to speed up the process. Without the catalyst and heat, the reaction would be very slow.

Example: Formation of Ethyl Ethanoate

The most common example you’ll see in WAEC exams is the formation of ethyl ethanoate (a common solvent).

Reactants:
Ethanoic acid (CH₃COOH) + Ethanol (C₂H₅OH)

Conditions:
Heat to 150°C in the presence of concentrated H₂SO₄

Products:
Ethyl ethanoate (CH₃COOC₂H₅) + Water (H₂O)

Equation:
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

The ethyl ethanoate produced has a pleasant, fruity smell – very different from the sharp, vinegar smell of ethanoic acid.

Naming Alkanoates

The name of an alkanoate comes from two parts:

1. First part: From the alkanol (alcohol) – this becomes “alkyl”
2. Second part: From the alkanoic acid – change the ending from “-oic acid” to “-oate”

Examples of Naming:

Naming Pattern:

Alcohol part + Acid part = Ester name
Methanol + Ethanoic acid = Methyl ethanoate
Ethanol + Propanoic acid = Ethyl propanoate

General Formula and Structure

The general molecular formula for alkanoates is RCOOR’

Where:

• R = alkyl group from the acid (can be H in methanoates)
• COO = the ester functional group (this is the key part)
• R’ = alkyl group from the alcohol

The ester functional group (-COO-) is the characteristic part of all alkanoates. It consists of a carbonyl group (C=O) connected to an oxygen atom that’s bonded to another carbon.

Structure of Ethyl Ethanoate:

CH₃-CO-O-C₂H₅

• CH₃ is from the ethanoic acid
• CO-O is the ester functional group
• C₂H₅ is from the ethanol

Physical Properties of Alkanoates

1. Smell

Most small esters have pleasant, fruity smells. This is completely different from their parent acids (which smell sharp and sour) and alcohols (which may have unpleasant smells).

Examples:

• Ethyl methanoate: Smells like rum
• Ethyl ethanoate: Smells like pears or nail polish remover
• Pentyl ethanoate: Smells like bananas
• Octyl ethanoate: Smells like oranges
• Methyl butanoate: Smells like pineapples

2. Boiling Points

Esters have lower boiling points than carboxylic acids of similar molecular mass. This is because carboxylic acids form strong hydrogen bonds with each other, while esters cannot form hydrogen bonds among themselves (though they can with water).

Example: Ethyl ethanoate boils at 77°C, while butanoic acid (similar mass) boils at 164°C.

3. Solubility

Small esters (like ethyl ethanoate) are slightly soluble in water. They can form hydrogen bonds with water molecules through their oxygen atoms. As the carbon chain gets longer, esters become less soluble in water but more soluble in organic solvents.

4. State at Room Temperature

Most small esters are volatile liquids at room temperature. They evaporate easily, which is why you can smell them quickly when a bottle is opened.

Chemical Properties of Alkanoates

1. Hydrolysis

Hydrolysis is the reverse of esterification. It means breaking down the ester using water to get back the original acid and alcohol.

There are two types of hydrolysis:

a) Acid Hydrolysis

When you heat an ester with dilute acid (like dilute H₂SO₄) and water, it breaks down slowly into the parent alcohol and acid.

Equation:
Alkanoate + Water → Alkanoic acid + Alkanol

RCOOR’ + H₂O → RCOOH + R’OH

Example:
CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH
(Ethyl ethanoate + Water → Ethanoic acid + Ethanol)

This reaction is slow and reversible – it can go both ways.

b) Alkaline Hydrolysis (Saponification)

When you heat an ester with sodium hydroxide (NaOH) or potassium hydroxide (KOH), it breaks down quickly into the sodium/potassium salt of the acid and the alcohol.

Equation:
Alkanoate + Sodium hydroxide → Sodium alkanoate + Alkanol

RCOOR’ + NaOH → RCOONa + R’OH

Example:
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
(Ethyl ethanoate + Sodium hydroxide → Sodium ethanoate + Ethanol)

This reaction is faster than acid hydrolysis and is not reversible. It is called saponification because this is the same process used to make soap from fats and oils.

2. Reduction

When esters are reduced using lithium aluminium hydride (LiAlH₄) or hydrogen gas with a catalyst, they form two alcohols.

Comparing Esterification and Hydrolysis

Uses of Alkanoates

1. Food and Beverage Industry

Esters are widely used as artificial food flavourings and fragrances. Their pleasant fruity smells make them perfect for:

• Soft drinks and juices (to add fruit flavours)
• Sweets and candies (banana, apple, strawberry flavours)
• Ice cream and desserts
• Chewing gum

The esters used in food are safe and approved by NAFDAC in Nigeria.

2. As Solvents

Ethyl ethanoate is an excellent solvent used in:

• Nail polish and nail polish remover
• Paint thinners and cleaning products
• Glues and adhesives
• Printing inks
• Dissolving cellulose nitrate

3. Perfumes and Cosmetics

Many esters have pleasant floral or fruity scents. They are used in:

• Perfumes and colognes
• Body lotions and creams
• Shampoos and soaps
• Air fresheners

4. Industrial Applications

• Manufacturing plastics (polyesters)
• Making synthetic fibres for clothing
• Pharmaceutical industry (some medicines are esters)
• Biodiesel production

Common Exam Mistakes

Fats and Oils as Esters

Natural fats and oils are also esters. They are formed from glycerol (a trihydric alcohol – has three -OH groups) and long-chain carboxylic acids called fatty acids.

When fats/oils react with sodium hydroxide, they undergo saponification to produce:

• Soap: Sodium salts of fatty acids
• Glycerol: A thick, sweet liquid used in cosmetics

This is why the process is called saponification – “sapo” means soap in Latin.

Practice Questions

Multiple Choice Questions

1. What is the product when ethanoic acid reacts with methanol in the presence of concentrated H₂SO₄?
a) Ethyl methanoate
b) Methyl ethanoate ✓
c) Methyl methanoate
d) Ethyl ethanoate

2. Which of the following is NOT a use of alkanoates?
a) Food flavouring
b) Perfumes
c) Solvents
d) Fertilizers ✓

3. The process of breaking down an ester using water is called:
a) Esterification
b) Hydrolysis ✓
c) Oxidation
d) Polymerization

4. What is the general formula for alkanoates?
a) RCOOH
b) ROH
c) RCOOR’ ✓
d) RCHO

5. Which catalyst is used for esterification?
a) Dilute H₂SO₄
b) Concentrated H₂SO₄ ✓
c) Sodium hydroxide
d) Platinum

Theory Questions

1. (a) Define esterification. (2 marks)
(b) Give two uses of alkanoates. (2 marks)
(c) Write a balanced equation for the formation of ethyl ethanoate from ethanoic acid and ethanol. State the conditions required. (4 marks)
(d) State two physical properties of esters. (2 marks)

Examiner’s Tip: For part (c), don’t forget to state the conditions: heat to 150°C in the presence of concentrated H₂SO₄. Show the structural formulas clearly.

2. (a) Explain the difference between acid hydrolysis and alkaline hydrolysis of esters. (4 marks)
(b) Write equations for the alkaline hydrolysis of ethyl ethanoate. (3 marks)
(c) Why is alkaline hydrolysis preferred over acid hydrolysis in soap making? (3 marks)

Examiner’s Tip: Explain that acid hydrolysis is reversible and slow, while alkaline hydrolysis is irreversible and faster. Use the word “saponification” when discussing alkaline hydrolysis.

3. (a) Name the esters formed when the following pairs react:
(i) Methanoic acid and ethanol (1 mark)
(ii) Propanoic acid and methanol (1 mark)
(iii) Ethanoic acid and butanol (1 mark)
(b) Write the structural formula for ethyl propanoate. (2 marks)
(c) An ester has a pleasant banana smell. Suggest what it might be used for. Give two examples. (2 marks)

Examiner’s Tip: Remember the naming pattern: alcohol name first (changed to alkyl), then acid name (changed to alkanoate). For structure, show all bonds clearly.

4. A student carried out an experiment to prepare an ester. She mixed 10cm³ of ethanol with 10cm³ of ethanoic acid in a test tube. She added 2 drops of concentrated H₂SO₄ and heated the mixture in a water bath for 10 minutes. A pleasant smell was produced.
(a) Name the ester formed. (2 marks)
(b) Write a balanced equation for the reaction. (3 marks)
(c) What is the role of concentrated H₂SO₄ in this reaction? (2 marks)
(d) Why was the mixture heated in a water bath rather than directly with a flame? (2 marks)
(e) How could the student test that an ester was formed? (1 mark)

Examiner’s Tip: For (c), state that H₂SO₄ acts as both catalyst and dehydrating agent. For (d), explain safety – ethanol is flammable. For (e), the pleasant fruity smell confirms ester formation.

Memory Aids

Mnemonics to Remember:

“ESTER = Easy Sweet-Smelling Things Everyone Remembers”

For naming: “Alcohol Always Comes First” – AACF
(Ethanol + Ethanoic acid = Ethyl ethanoate – alcohol name first)

For esterification: “Acids And Alcohols Add = Esters”
(A + A = E + W: Acid + Alcohol = Ester + Water)

For hydrolysis: “Water Wrecks Esters”
(Hydrolysis breaks esters back to acid and alcohol)

For catalyst: “Concentrated Cat” – Concentrated H₂SO₄ is the Catalyst

For uses: “FIPS” – Flavourings, Industrial solvents, Perfumes, Soap making

Quick Comparison:

Esterification: Acid + Alcohol → Ester + Water (needs concentrated H₂SO₄, heat)
Hydrolysis: Ester + Water → Acid + Alcohol (needs dilute acid OR NaOH, heat)

Related Topics

• Alkanoic Acids (Carboxylic Acids) – parent compounds that form esters
• Alkanols (Alcohols) – react with acids to form esters
• Saponification – alkaline hydrolysis used in soap making
• Fats and Oils – naturally occurring esters
• Functional Groups in Organic Chemistry