Alcohols, Carboxylic Acids, Esters & Biological Polymers

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Alcohols Homologous Series (-OH)

Contain the hydroxyl functional group (-OH). First four members: Methanol (CH₃OH), Ethanol (C₂H₅OH), Propanol (C₃H₇OH), Butanol (C₄H₉OH). Reactions: 1. Burn in air (complete combustion) to produce CO₂ and H₂O. 2. Dissolve in water to form neutral solutions (pH 7). 3. React with sodium metal to produce a metal alkoxide (e.g., sodium ethoxide) and bubbles of hydrogen gas (2C₂H₅OH + 2Na -> 2C₂H₅ONa + H₂). 4. Oxidised by chemical oxidising agents (acidified potassium dichromate) or microbes in air to form carboxylic acids.

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2

Ethanol Manufacture: Fermentation vs Ethene Hydration

Fermentation: Glucose -> Ethanol + Carbon Dioxide (C₆H₁₂O₆(aq) -> 2C₂H₅OH(aq) + 2CO₂(g)) using yeast enzymes at ~30°C anaerobically. Uses renewable sugar cane crops and low energy, but is a slow batch process producing impure dilute ethanol (~15%) requiring fractional distillation. Hydration of ethene: C₂H₄(g) + H₂O(g) -> C₂H₅OH(g) at 300°C, 60-70 atm, H₃PO₄ catalyst. Fast continuous process with 100% atom economy and pure product, but uses non-renewable crude oil and high energy.

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3

Carboxylic Acids (-COOH)

Contain the carboxyl functional group (-COOH). First four: Methanoic acid (HCOOH), Ethanoic acid (CH₃COOH), Propanoic acid (C₂H₅COOH), Butanoic acid (C₃H₇COOH). They dissolve in water to form weakly acidic solutions (pH 3 to 5) because they only partially ionise in water (CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)). They react with metal carbonates to produce a carboxylate salt (ending in -anoate, e.g., sodium ethanoate), water, and carbon dioxide gas.

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4

Esters (-COO-)

Formed when an alcohol reacts reversibly with a carboxylic acid in the presence of a strong acid catalyst (concentrated H₂SO₄): Ethanol + Ethanoic acid ⇌ Ethyl ethanoate + Water (CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O). Esters are volatile compounds with distinctive fruity smells, used in perfumes and food flavourings.

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5

Amino Acids & Proteins (Higher Tier)

Amino acids have two functional groups in the same molecule: an amine group (-NH₂) and a carboxyl group (-COOH). For example, glycine is H₂NCH₂COOH. Glycine polymerises by condensation polymerisation to form the polypeptide poly(glycine): n H₂NCH₂COOH -> -(-HN-CH₂-CO-)ₙ- + n H₂O. Different amino acids combine in specific chains to form proteins.

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6

DNA & Natural Carbohydrates

DNA (deoxyribonucleic acid) is a large molecule essential for life that encodes genetic instructions. It consists of two polymer chains made from four different monomers called nucleotides, twisted around each other in a double helix. Other natural polymers include starch and cellulose (both made by condensation polymerisation of glucose monomers).

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1. Q1

   Ethanol can be manufactured either by the fermentation of glucose from sugar cane or by the catalytic hydration of ethene from crude oil. Evaluate these two industrial methods of producing ethanol. (Triple Science) [6 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Raw materials (sustainability): Fermentation uses glucose from sugar cane/plants, which is a renewable resource, whereas hydration uses ethene obtained from cracking crude oil, which is a finite (non-renewable) resource [1 mark]. 2. Energy and reaction conditions: Fermentation takes place at a low temperature (~30°C) and atmospheric pressure (low energy cost), whereas hydration requires a high temperature (300°C) and high pressure (60-70 atm), requiring large amounts of energy [1 mark]. 3. Rate and type of process: Fermentation is a slow batch process (taking days, after which tanks must be emptied and cleaned), whereas hydration of ethene is a fast continuous process [1 mark]. 4. Purity of product: Fermentation produces a dilute impure aqueous solution (~15% ethanol) that requires expensive fractional distillation to purify, whereas hydration produces pure ethanol directly [1 mark]. 5. Atom economy: Hydration of ethene is an addition reaction with a 100% atom economy (no waste products), whereas fermentation produces carbon dioxide gas as a waste product (lower atom economy of 51%) [1 mark]. 6. Overall judgement: Countries rich in land and sunlight (like Brazil) favour renewable fermentation, whereas oil-rich countries favour fast continuous hydration [1 mark].

2. Q2

   Compare the observations and products when ethanol (C₂H₅OH) and ethanoic acid (CH₃COOH) are tested separately with: (a) Universal Indicator, (b) solid sodium carbonate, and (c) magnesium ribbon. (Triple Science) [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. (a) Universal Indicator: Ethanol is neutral so Universal Indicator stays green (pH 7), whereas ethanoic acid is a weak acid so Universal Indicator turns orange/yellow (pH 3-5) [1 mark]. 2. (b) Sodium carbonate: Ethanol has no reaction with sodium carbonate [1 mark]. 3. Ethanoic acid reacts with sodium carbonate with effervescence (fizzing), producing sodium ethanoate, water, and carbon dioxide gas (which turns limewater cloudy) [1 mark]. 4. (c) Magnesium ribbon: Ethanol does not react with magnesium [1 mark]. 5. Ethanoic acid reacts slowly with magnesium ribbon (fizzing), producing magnesium ethanoate and hydrogen gas (which gives a squeaky pop with a lighted splint) [1 mark].

3. Q3

   Describe how a student could prepare a small sample of the ester ethyl ethanoate in a school laboratory, and explain why a water bath is used instead of a naked Bunsen burner flame. (Triple Science) [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Place equal small volumes of ethanol and ethanoic acid into a boiling tube [1 mark]. 2. Add a few drops of concentrated sulfuric acid, which acts as a catalyst for the reversible esterification reaction [1 mark]. 3. Warm the mixture gently in a hot water bath (~75-80°C) for 5 minutes, then pour into a beaker of sodium carbonate solution to neutralise excess acid and smell the fruity ester layer floating on top [1 mark]. 4. Safety: A water bath is used instead of a Bunsen flame because ethanol (and the ester) are highly volatile and flammable liquids that could easily catch fire near a naked flame [1 mark].

4. Q4

   Explain how amino acids polymerise to form polypeptides and proteins, and compare this with how glucose monomers form starch and cellulose. (Triple Higher Tier) [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Amino acids contain two different functional groups: a basic amine group (-NH₂) and an acidic carboxyl group (-COOH) [1 mark]. 2. Amino acids join together by condensation polymerisation: the -NH₂ group of one amino acid reacts with the -COOH group of another to form a peptide link (-CONH-), eliminating one molecule of water (H₂O) for every link formed [1 mark]. 3. Different amino acids can be joined in the same chain to form complex proteins, whereas a single amino acid forms a simple polypeptide [1 mark]. 4. Similarly, glucose monomers join together by condensation polymerisation (losing water molecules) to form the natural carbohydrate polymers starch and cellulose [1 mark].

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