Home › Learning Hub › IGCSE Chemistry › 11b Alcohols, acids and polymers
Topic 11 · 11.6–11.8

Alcohols, carboxylic acids and polymers

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Name and draw ethanol and ethanoic acid; name and draw unbranched alcohols, carboxylic acids and esters up to four carbons EXTENDED.
  • Describe the manufacture of ethanol by fermentation and from ethene, its combustion and its uses; compare the two methods EXTENDED.
  • Describe the reactions of ethanoic acid with metals, bases and carbonates; its formation by oxidising ethanol, and esterification EXTENDED.
  • Define polymers and monomers; describe addition polymerisation of ethene and the environmental problems of plastics.
  • Deduce repeat units of addition and condensation polymers; describe nylon, PET and proteins EXTENDED.

📚The chemistry

Alcohols and carboxylic acids

Displayed formulae. Ethanol, CH3CH2OH: two carbons with an O-H group on the end. Ethanoic acid, CH3COOH: a CH3 group joined to a carbon that has a double-bonded O and an O-H. Propan-1-ol, CH3CH2CH2OH: the O-H on the end carbon. Propan-2-ol, CH3CH(OH)CH3: the O-H on the middle carbon.
Alcohols have the –OH functional group; carboxylic acids have –COOH. The carbon of the –COOH group is counted in the name: ethanoic acid has two carbons.

The acids in the series are methanoic (HCOOH), ethanoic, propanoic and butanoic acid. EXTENDED For alcohols from three carbons up, the number shows the position of the –OH: propan-1-ol and propan-2-ol, butan-1-ol and butan-2-ol.

Making ethanol

Fermentation: aqueous glucose with yeast, at 25–35 °C, in the absence of oxygen
Hydration of ethene: ethene + steam, at 300 °C and 6000 kPa (60 atm), with an acid catalyst
\[ \mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2} \]
\[ \mathrm{C_2H_4 + H_2O \rightarrow C_2H_5OH} \]
A conical flask containing aqueous glucose and yeast, sealed with a bung. A delivery tube leads into a test tube of limewater, which turns milky as carbon dioxide bubbles through. Notes: 25 to 35 degrees Celsius, warm not hot; no oxygen, anaerobic; the air is kept out.
Too cold and the yeast works slowly; too hot and its enzymes are denatured. Oxygen is kept out so the yeast makes ethanol rather than respiring aerobically.

EXTENDED Comparing the two methods:

  • Fermentation: uses a renewable raw material (sugar from crops) and needs little energy, but it is slow, is a batch process, and gives a dilute solution that must be distilled.
  • Hydration of ethene: fast, continuous, and gives pure ethanol, but ethene comes from crude oil (non-renewable) and the high temperature and pressure need a lot of energy.

Ethanol burns cleanly and is used as a fuel and as a solvent:

\[ \mathrm{C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O} \]

Reactions of ethanoic acid

Ethanoic acid is a weak acid but has the typical acid reactions. Its salts are ethanoates:

+ metal: magnesium → magnesium ethanoate, (CH3COO)2Mg, + hydrogen
+ base: sodium hydroxide → sodium ethanoate, CH3COONa, + water
+ carbonate: sodium carbonate → sodium ethanoate + water + carbon dioxide

EXTENDED Ethanoic acid is made by oxidising ethanol, either by heating it with acidified aqueous potassium manganate(VII) (purple to colourless) or by bacterial oxidation, which is how vinegar is made.

Esters EXTENDED

A carboxylic acid reacts with an alcohol, using an acid catalyst, to form an ester and water. The name starts with the alcohol part and ends with the acid part: ethanol + ethanoic acid gives ethyl ethanoate, CH3COOCH2CH3.

Displayed formulae for ethanoic acid plus ethanol, with an acid catalyst, giving ethyl ethanoate plus water. The OH of the acid and the H of the alcohol's OH group are highlighted: they leave as water, and the remaining O links the two parts in the ester.
The ester link is –COO–. The acid keeps its C=O; the alcohol keeps its oxygen. EXTENDED

Polymers

Polymers are large molecules built up from many smaller molecules called monomers. Plastics are made from polymers. Poly(ethene) forms by addition polymerisation: the double bonds of many ethene molecules open and join into one long chain.

Top: many ethene monomers become part of a poly(ethene) chain of single-bonded carbons, each with two hydrogens. Bottom: repeat units. Ethene gives the repeat unit of two carbons with four hydrogens in brackets with subscript n; propene, with a CH3 group on one carbon, gives the repeat unit of poly(propene) with the CH3 on the second carbon, also in brackets with n.
EXTENDED To go from monomer to repeat unit, keep the groups on each carbon exactly where they are, make the C=C single, and add continuation bonds through the brackets. Work backwards to find the monomer.

Plastics and the environment. Most plastics are not biodegradable, so they

  • take up space in landfill sites for a very long time;
  • accumulate in the oceans, harming marine life;
  • form toxic gases when burned.

Condensation polymers EXTENDED

In condensation polymerisation two different functional groups react, and each link formed also releases a small molecule (usually water). Addition polymerisation uses one kind of monomer with a C=C and gives only the polymer.

  • Polyamides (such as nylon) form from a dicarboxylic acid and a diamine: amide links, –CONH–.
  • Polyesters (such as PET) form from a dicarboxylic acid and a diol: ester links, –COO–. PET can be converted back into its monomers and re-polymerised, which makes it easier to recycle.
  • Proteins are natural polyamides made from amino acid monomers, H2N–CHR–COOH, where R is a side chain that varies.
Block diagrams of three condensation polymers, with grey blocks for the carbon chains. Nylon: blocks joined alternately by C(=O) and N-H groups, forming amide links CONH. PET: blocks joined by C(=O) and O, forming ester links COO. Protein: repeating N-H, block, C(=O), with amide links.
Nylon: the amide links alternate in direction. Protein: every link points the same way, because each amino acid has one –NH2 and one –COOH. EXTENDED

✏️Worked example EXTENDED

(a) Ethanol reacts with propanoic acid with an acid catalyst. Name the ester and give its structural formula. [2] (b) Chloroethene, CH2=CHCl, forms an addition polymer. Draw its repeat unit and name the polymer. [2] (c) State one difference between the formation of this polymer and the formation of nylon. [1]

(a) Ethyl propanoate: CH3CH2COOCH2CH3. The acid part (propanoate, three carbons including the C=O) is written first in the formula; the alcohol part (ethyl) comes first in the name.

(b) Two carbons joined by a single bond, with H, H on the first and H, Cl on the second, continuation bonds through square brackets, and n outside: poly(chloroethene).

(c) Addition polymerisation gives only the polymer; forming nylon (condensation) also releases water.

Check it. Count atoms: ethanol C2H6O + propanoic acid C3H6O2 − H2O = C5H10O2, the formula of ethyl propanoate ✓.
Naming esters backwards. “Propyl ethanoate” is a different compound (from propanol and ethanoic acid). The name is alcohol-yl then acid-oate.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) What are the products when ethanoic acid reacts with calcium carbonate? A: calcium ethanoate and hydrogen. B: calcium ethanoate, water and carbon dioxide. C: calcium oxide and ethanol. D: ethyl ethanoate and water.
B. Acid + carbonate → salt + water + carbon dioxide.
2. (Theory.) Name the compound CH3CH2CH2COOH and state the homologous series it belongs to. [2]
Butanoic acid (four carbons, including the one in –COOH); carboxylic acids.
3. (Theory.) Give the name and formula of the salt formed when ethanoic acid reacts with sodium hydroxide. [2]
Sodium ethanoate, CH3COONa.
4. (Theory.) EXTENDED A country with plenty of sugar cane but no oil wants to make ethanol. Suggest which method it should use, and give one disadvantage of that method. [2]
Fermentation: sugar is a renewable raw material it has, and it needs no ethene from oil. Disadvantage: it is slow (batch process) and the product is impure, so it must be distilled.
5. (Theory.) EXTENDED Name the ester made from methanol and butanoic acid, and give its structural formula. [2]
Methyl butanoate, CH3CH2CH2COOCH3.
6. (Theory.) Explain what is meant by a monomer and a polymer, using poly(ethene) as the example. [2]
A monomer is a small molecule (ethene) that joins with many others to form a polymer, a large molecule (poly(ethene)).
7. (Theory.) Describe two environmental problems caused by disposing of plastics. [2]
Any two: they are not biodegradable and fill landfill sites; they accumulate in the oceans and harm marine life; burning them releases toxic gases.
8. (Theory.) EXTENDED The repeat unit of a polymer is –CH2–CH(CH3)–. Name and give the structural formula of its monomer. [2]
Propene, CH2=CHCH3.
9. (Theory.) EXTENDED Name the type of monomers used to make a polyester, and name the link formed. [2]
A dicarboxylic acid and a diol; ester links (–COO–).
10. (Theory.) EXTENDED Proteins and nylon are both polyamides. Name the monomers of proteins and give their general structure. [2]
Amino acids, H2N–CHR–COOH (R is a side chain that differs between amino acids).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Royal Society of Chemistry — making esters from alcohols and acids, microscale
  • Royal Society of Chemistry — the “nylon rope trick” demonstration of condensation polymerisation