Alcohols

Ethanol is the single most-examined compound in 0620 organic chemistry, because it sits at a junction: made two ways, used two ways, oxidised three ways. The compare-the-manufacturing-routes question is an Extended staple worth up to 4 marks, and it is won or lost on conditions: numbers, not adjectives.

What alcohols are

Alcohols have the general formula CnH2n+1OH and the −OH functional group. Name them to four carbons: methanol, ethanol, propanol, butanol. On the Extended paper, propanol and butanol have positional isomers (propan-1-ol and propan-2-ol), which connects to isomerism. The displayed formula of ethanol must show the O−H bond drawn out; a boxed “OH” loses the mark.

Ethanol’s two syllabus uses: a fuel and a solvent.

Making ethanol, route 1: fermentation

Mix aqueous glucose with yeast at about 25-35°C in anaerobic conditions (no oxygen). Enzymes in the yeast catalyse:

C6H12O6 → 2C2H5OH + 2CO2

Each condition has a reason, and “explain the conditions” is a frequent follow-up: the temperature is warm enough for a reasonable rate but cool enough that the enzymes are not denatured; air is excluded because oxygen allows the ethanol to be oxidised (and the yeast respires aerobically instead). Fermentation stops when the ethanol concentration kills the yeast, so the dilute product is concentrated by fractional distillation, linking forward to purification and separation methods.

Making ethanol, route 2: catalytic addition of steam (Supplement)

Ethene and steam react in an addition reaction over a phosphoric acid catalyst at 300°C and 60 atm:

C2H4 + H2O → C2H5OH

The Extended comparison, the way mark schemes split it:

	Fermentation	Hydration of ethene
Raw material	sugars (renewable)	ethene from petroleum (non-renewable)
Rate	slow	fast
Process type	batch (stop, empty, restart)	continuous
Purity	impure, needs fractional distillation	essentially pure product
Energy / conditions	low temperature, cheap to run	high temperature and pressure, energy-intensive

A 4-mark compare answer needs points from both columns, stated as comparisons (“fermentation is slow whereas hydration is fast”), not two separate lists.

Reactions of ethanol

Combustion. Ethanol burns with a clean flame, which is the basis of its use as a fuel:

C2H5OH + 3O2 → 2CO2 + 3H2O

Balancing trap: the oxygen inside the ethanol molecule counts. Students who write 3½O2 have forgotten it.

Oxidation to ethanoic acid. Three routes in the syllabus:

1. Combustion (complete oxidation to CO2; listed as oxidation, though it destroys the molecule).
2. Heating with acidified aqueous potassium manganate(VII): the purple oxidising agent is decolourised as ethanol is oxidised to ethanoic acid.
3. Bacterial oxidation: bacteria in air oxidise ethanol slowly; this is why open wine turns to vinegar.

The product, ethanoic acid, gets its own treatment under carboxylic acids.

Worked exam question

Ethanol is manufactured by fermentation and by the catalytic addition of steam to ethene. (a) State the conditions used for fermentation. [3] (b) Give one advantage and one disadvantage of the fermentation route compared with the ethene route. [2] (c) Name the organic product formed when ethanol is heated with acidified aqueous potassium manganate(VII). [1]

Model answer, mark by mark:

• (a) M1: yeast (as the source of enzymes). M2: 25-35°C (any value or range in that window). M3: anaerobic / absence of oxygen. (Aqueous glucose/sugar can substitute for one of these on most schemes.)
• (b) M4: advantage: renewable raw material (sugars) / lower energy use. M5: disadvantage: slow / batch process / impure product needing distillation. One of each, phrased as a comparison.
• (c) M6: ethanoic acid. (“Ethanoate” or “acetic acid”: the systematic name is the safe answer.)

The mistakes that cost marks

1. Conditions without numbers. “Warm” is not 25-35°C; “high pressure” is not 60 atm. The Extended mark scheme wants the figures.
2. Aerobic fermentation. Writing “with oxygen” reverses a key condition. The answer must state anaerobic, and be ready to say why.
3. Mixing the two routes’ conditions. Phosphoric acid catalyst belongs to hydration, yeast to fermentation. Swapped conditions score zero even when the numbers are right.
4. One-sided comparisons. “Fermentation is renewable” only earns the compare mark as “…whereas ethene comes from non-renewable petroleum.”
5. Combustion of ethanol balanced as a hydrocarbon. Count the ethanol oxygen: 3O2, not 3½.

How examiners want it phrased

Student wording	Mark-scheme wording
”Yeast makes sugar into alcohol"	"Enzymes in yeast convert aqueous glucose to ethanol and carbon dioxide at 25-35°C, anaerobically"
"The factory way is quicker"	"Catalytic addition of steam is a fast, continuous process giving a pure product"
"Fermentation is more eco-friendly"	"Fermentation uses a renewable raw material, whereas ethene is obtained from non-renewable petroleum"
"The purple stuff loses its colour"	"Acidified potassium manganate(VII) is decolourised as ethanol is oxidised to ethanoic acid”

Everything here feeds the parent organic chemistry pillar’s reaction map, with ethanol at its centre. Students who sketch that map once with a tutor tend to keep it; a free trial lesson is enough time to build the whole ethanol junction, conditions included.

Test yourself

Three quick checks. Do them without looking back and click to reveal each answer.

Q1 (2 marks). Write the balanced symbol equation for the fermentation of glucose.

Show answer

• correct formulae: glucose C6H12O6, ethanol C2H5OH, carbon dioxide CO2 [1] • balanced: C6H12O6 → 2C2H5OH + 2CO2 [1]

Q2 (2 marks). Write the balanced symbol equation for the complete combustion of ethanol.

Show answer

• correct products CO2 and H2O [1] • balanced: C2H5OH + 3O2 → 2CO2 + 3H2O (count the oxygen atom inside the ethanol molecule) [1]

Q3 (3 marks). (Supplement) Explain why air must be excluded during fermentation, and state why the reaction is run at about 25-35°C rather than at a higher temperature.

Show answer

• oxygen would oxidise the ethanol (to ethanoic acid) [1] • or the yeast would respire aerobically instead of fermenting (either reason for excluding air) [1] • above this range the enzymes in the yeast are denatured, so fermentation stops [1] (a third point: lower temperatures make the rate too slow)