Chemistry · Lesson 16

Alcohols, acids, esters, fats, carbohydrates & polymers

JAMBWAECNECO

What the syllabus asks

JAMB: alkanols, alkanoic acids, alkanoates, fats and oils, soaps and detergents, carbohydrates, proteins, polymers

WAEC: alkanols (classification, preparation by fermentation and hydration, properties, uses), alkanoic acids (ethanoic acid), esters and esterification, fats and oils, soaps and detergents (saponification), carbohydrates (glucose, sucrose, starch, cellulose), proteins, synthetic and natural polymers (addition and condensation)

NECO: alkanols (classification, preparation by fermentation and hydration, properties, uses), alkanoic acids (ethanoic acid), esters and esterification, fats and oils, soaps and detergents (saponification), carbohydrates (glucose, sucrose, starch, cellulose), proteins, synthetic and natural polymers (addition and condensation)

What you will learn

Name, classify and draw simple alkanols; describe the preparation of ethanol by fermentation and hydration.
Describe the reactions of ethanol: combustion, sodium, oxidation, dehydration, esterification.
Describe the properties of ethanoic acid and esterification.
Describe fats and oils, hardening of oils, and saponification; compare soaps and detergents.
Describe carbohydrates (glucose, sucrose, starch, cellulose) and their tests.
Describe proteins, amino acids and tests for proteins.
Distinguish addition and condensation polymers, with examples and uses.

Key points

Alkanols CₙH₂ₙ₊₁OH: primary, secondary or tertiary by carbons on the C–OH carbon; hydrogen bonding raises b.p.
Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (zymase, no air); hydration: C₂H₄ + H₂O (H₃PO₄, 300 °C, 60 atm).
Ethanol burns, reacts with Na (H₂), oxidises (dichromate green) to ethanal then ethanoic acid, dehydrates to ethene.
Ethanoic acid is a weak acid with typical acid reactions.
Acid + alkanol ⇌ ester + water (conc. H₂SO₄); esters are named alkyl alkanoate and smell fruity.
Fats/oils are glyceryl triesters; oils are unsaturated; Ni + H₂ hardens oil to margarine.
Saponification: fat + NaOH → soap + glycerol; salt out the soap; detergents work in hard water.
Carbohydrates: glucose (reducing), sucrose (non-reducing), starch (iodine blue-black), cellulose.
Proteins: amino acids joined by peptide bonds; Biuret test purple.
Addition polymers from C=C monomers (polythene, PVC); condensation polymers lose water (nylon, terylene, starch, proteins).

Start here: Alkanols: structure, naming and classification

Imagine taking an alkane and replacing one hydrogen with an –OH (hydroxyl) group. You get an alkanol (alcohol). The –OH group gives alkanols their characteristic properties.

Alkanols have the general formula CₙH₂ₙ₊₁OH (or ROH). Name: replace the final "-e" of the alkane with "-ol", and give the position of the –OH.

n	Name	Formula	b.p./°C
1	Methanol	CH₃OH	65
2	Ethanol	C₂H₅OH	78
3	Propan-1-ol	CH₃CH₂CH₂OH	97
4	Butan-1-ol	C₄H₉OH	118

Alkanols boil much higher than alkanes of similar mass (ethane boils at −89 °C) because of hydrogen bonding between –OH groups. The small ones mix completely with water for the same reason.

Classification

Look at the carbon carrying the –OH and count how many other carbons are joined to it.

Methanol is counted as primary. Alkanols with two –OH groups are diols (ethane-1,2-diol, antifreeze); with three, triols (propane-1,2,3-triol, glycerol).

Worked example 16.1

Classify (a) CH₃CH(OH)CH₃, (b) (CH₃)₃COH, (c) CH₃CH₂CH₂CH₂OH.

(a) The C–OH carbon is attached to two CH₃ groups: secondary (propan-2-ol).
(b) It is attached to three CH₃ groups: tertiary.
(c) It is attached to one carbon (CH₂ chain): primary (butan-1-ol).

(a) 2° (b) 3° (c) 1°

⚠️ Common mistake: thinking alkanols are alkaline because of "OH". The –OH is covalently bonded; alkanols are neutral.

Try it yourself. Write the formula of the alkanol with 5 carbons.

Show solution

C₅H₁₁OH (n = 5: 2n+1 = 11) — pentanol.

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Practice questions

1. The functional group of alkanols is

–OH
–COOH
–CHO
C=C

Show answer

A: –OH
The hydroxyl group.

2. Ethanol is produced industrially by

cracking
saponification
the Haber process
fermentation of sugars or hydration of ethene

Show answer

D: fermentation of sugars or hydration of ethene
Both routes are used.

3. The functional group of alkanoic acids is

–COO–
–NH₂
–COOH
–OH

Show answer

C: –COOH
The carboxyl group.

4. An alkanol reacts with an alkanoic acid to form

a polymer only
an ester and water
a salt and hydrogen
an alkene

Show answer

B: an ester and water
Esterification.

5. Esters are known for their

fruity smells
sour taste
bleaching action
acidic nature

Show answer

A: fruity smells
Used in flavourings.

Key words

Functional group: Atom or group of atoms that determines the characteristic reactions of a compound.
Alkanol: Organic compound with the –OH group attached to a saturated carbon.
Fermentation: Breakdown of sugars by yeast enzymes in the absence of air, giving ethanol and CO₂.
Esterification: Reaction of an acid with an alkanol to form an ester and water.
Saponification: Alkaline hydrolysis of fats or oils to form soap and glycerol.
Hydrolysis: Breaking down a compound by reaction with water.
Soap: Sodium or potassium salt of a long-chain fatty acid.
Detergent (soapless): Synthetic cleansing agent, e.g. sodium alkylbenzene sulphonate; works in hard water.
Monomer: Small molecule that joins with others to form a polymer.
Polymer: Very large molecule made of many repeating units.
Addition polymerisation: Joining of unsaturated monomers with no by-product.
Condensation polymerisation: Joining of monomers with loss of a small molecule such as water.

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