1. Fermentation of sugars

The yeast cells contain enzymes (biological catalysts) which break down the sugars to produce energy needed for the cells to live and reproduce. These enzymes, unlike inorganic catalysts, can be deactivated/denatured by overheating so it is essential to keep the fermenting solution warm, but not hot.

Ethanol is produced when yeast respires anaerobically, i.e. breaks down sugars to produce energy without using oxygen. The fermentation process is more efficient if air is excluded.

C₆H₁₂O_6(aq) à 2 CH₃CH₂OH_(aq) + 2 CO_2(g)

High concentrations of ethanol can’t be produced by fermentation because once the fermentation gets to a certain value, the yeast is killed. This is an example of a BATCH process, i.e. the reactants are assembled in a container. The reaction proceeds and then the products are separated from the unused reactants and impurities. Many of the industrial processes we have seen so far are CONTINUOUS, i.e. the reactants meet a catalyst for a short time, the product is separated, and unused reactants are recirculated.

More concentrated solutions of ethanol can be obtained from the fermented mixture by distillation.

However, ethanol molecules interact so strongly with water molecules that it is impossible to produce 100% pure ethanol in the lab

Ethanol is also made in industry by the direct hydration of ethene

Conditions :

300ºC, 60 atmospheres, H₃PO₄ catalyst

Pure ethanol can be extracted from the reacting mixture by cooling it so ethanol condenses as an aqueous solution and then distilling. This is a continuous process, and we need to be able to contrast and compare the different ways of producing ethanol in these terms.

Some reaction of alcohols

Alcohols will react with the reactive metals to make ethoxide salts and hydrogen gas

2 CH₃CH₂OH + 2 Na à 2 CH₃CH₂O⁺Na⁺ + H_{2 (g)}

The reaction with alcohol is more gentle than that of water. Ethanol can be oxidised by oxygen in the air to form ethanoic acid, which is present in vinegar and gives it its smell. This is why wine exposed to air for a long time goes sour.

Spirits don’t go sour because the alcohol concentration is so high that the bacteria necessary for the operation can’t function

N.B. Ethanoic acid is part of a homologous series called the carboxylic acids. These have the functional group

Alcohols will react with carboxylic acids when they are boiled together with sulphuric acid catalysts to make esters. In general,

carboxylic acid + alcohol à ester + water

in particular,

ethanoic acid + ethanol « ethyl ethanoate + water

Carboxylic Acid

These form a hermologous series with the functional group COOH

The first three numbers of the series are

Properties (some)

1. Carboxylic acids are typically acids, except that they are WEAK, i.e. they are only partially dissociated into ions in aqueous solution

CH₃COOH_(aq) « CH₃COO^-_(aq) + H⁺_(aq)

i.e. the position of the equilibrium above is well to the left. This means that the hydrogen ion concentration is relatively small and the typical reaction of acids are slow

H⁺_(aq) + OH^-_(aq) à H2O_(l)

acid + alkali à salt + water

CH₃CH₂COOH + NaOH à CH₃CH₂COO^-Na⁺ + H₂O

ethanoic acid + sodium à sodium propanate + water

hydroxide

2. Reaction with carbonate or hydrogencarbonate

2H⁺ + CO₃^2- à H₂O + CO₂

H⁺ + HCO₃^- à H₂O + CO₂

e.g.

methanoic acid + calcium carbonate à calcium methanoate + water + carbon dioxide

propanoic acid + sodium à sodium propanoate + water + carbon dioxide

hydrogencarbonate