Alcohols

The functional group and the family. An alcohol is any organic compound in which a hydrogen atom of a hydrocarbon chain has been replaced by a hydroxyl group, –OH. As a homologous series, alcohols follow the general formula

where: $n$ is the number of carbon atoms in the chain ($n = 1, 2, 3, \dots$). The first member is methanol, $CH_3OH$ ($n=1$); the second — and the one ICSE studies in depth — is ethanol, $C_2H_5OH$, more explicitly written as $CH_3\text{-}CH_2\text{-}OH$ ($n=2$). Every member differs from the next by one $CH_2$ unit, the usual mark of a homologous series, and the –OH group sitting at the end of the chain is what gives every alcohol its family of shared reactions.

Two routes to ethanol. Industry and biology reach the same molecule by different paths. Fermentation breaks down sugar biologically. Molasses or starch is first converted to glucose, and then zymase, an enzyme present in yeast, converts glucose to ethanol in the absence of air, at a warm temperature of roughly $25\text{–}35\ ^\circ C$:

where: $C_6H_{12}O_6$ is glucose, and the reaction proceeds with no external heating or catalyst — the yeast's own enzyme does the work, and carbon dioxide gas bubbles off as a visible by-product. Hydration of ethene is the industrial route. Ethene gas, an unsaturated hydrocarbon with a $C=C$ double bond, is reacted with steam over a catalyst (commonly phosphoric acid) under controlled temperature and pressure:

where: the $H$ and $OH$ of water add directly across the double bond, with no by-product formed — a cleaner, faster route than fermentation, but one that depends on ethene supply from the petrochemical industry rather than on agriculture.

Physical properties. Ethanol is a colourless, volatile liquid with a sharp, characteristic smell. It is miscible with water in all proportions — the –OH group forms the same kind of attraction with water molecules that water molecules form with each other — and it burns with a nearly non-luminous blue flame, a useful test for identifying it.

The three signature reactions. All three trace back to the reactivity of the –OH group. With sodium metal, ethanol behaves like a mild version of water: sodium displaces the hydrogen of the –OH group, liberating hydrogen gas and leaving sodium ethoxide behind:

On oxidation (by an oxidising agent, or slowly by air over time), ethanol is converted to ethanoic acid — the same souring that turns an open bottle of wine into vinegar:

where: $[O]$ stands for oxidation supplied by an oxidising agent, written this way because the syllabus does not require the full oxidising-agent equation. In esterification, ethanol reacts with a carboxylic acid in the presence of a little concentrated sulphuric acid (acting as catalyst and dehydrating agent) to give a sweet-smelling ester plus water — the reaction responsible for the fragrance of many fruits and perfumes.

Denatured spirit and uses. Ethanol destined for industrial use is deliberately made unfit to drink by adding a poisonous or foul-tasting substance such as methanol, pyridine, or a dye — this is denaturing, and it stops industrial alcohol being diverted as a beverage while leaving it fully usable as a solvent, fuel additive, and starting material for making other chemicals such as ethanoic acid.

The two preparation routes and the three signature reactions all follow from a single structural fact: ethanol has an –OH group riding on a two-carbon chain that is otherwise fully saturated. Understanding why each reaction happens the way it does — rather than memorising the equations in isolation — is what lets you predict products for a molecule you have not seen before.

Step 1. Fermentation works because zymase, an enzyme in yeast, recognises glucose and breaks it apart into ethanol and carbon dioxide as the yeast releases energy anaerobically. The "no air" condition matters: with oxygen present, yeast would instead fully respire glucose to $CO_2$ and water, giving no ethanol — this is why fermentation vats are sealed.

Step 2. Hydration of ethene works because the $C=C$ double bond is attacked when water is present under a catalyst's influence; the bond opens, one carbon gains $H$, the other gains $OH$. Because two atoms are added across the double bond with nothing removed, this is an addition reaction — the same class alkenes undergo with $H_2$ and with halogens such as $Br_2$.

Step 3. The reaction with sodium works because the O–H bond in ethanol, like in water, is polarised, leaving the hydrogen reactive toward a metal that gives up electrons readily. Sodium displaces only that one hydrogen — not the whole –OH group — which is why the product is sodium ethoxide, $C_2H_5ONa$, and not sodium hydroxide.

Step 4. Oxidation and esterification both target the same –OH group: oxidation removes hydrogen from it to give the acid product, while esterification uses the –OH's oxygen to bond with a carboxylic acid's carbon, releasing water. Both confirm that the –OH group, not the carbon chain, is the seat of an alcohol's chemistry — why the whole family is classified by that one functional group.

Question: $180\ g$ of glucose (one mole, since the molar mass of $C_6H_{12}O_6$ is $180\ g/mol$) is completely fermented using yeast. Using the fermentation equation, calculate (a) the mass of ethanol produced, and (b) the volume of carbon dioxide released at STP (take the molar volume of a gas at STP as $22.4\ L/mol$, molar mass of $C_2H_5OH = 46\ g/mol$).

Step 1. Write the balanced equation and read off the mole ratio: $C_6H_{12}O_6 \xrightarrow{\text{zymase}} 2\,C_2H_5OH + 2\,CO_2$. One mole of glucose yields two moles of ethanol and two moles of carbon dioxide.

Step 2. Convert the given mass of glucose to moles: $180\ g \div 180\ g/mol = 1\ mol$ glucose. By the mole ratio from Step 1, this produces $2\ mol$ of ethanol and $2\ mol$ of $CO_2$.

Step 3. Find the mass of ethanol: $2\ mol \times 46\ g/mol = 92\ g$. Find the volume of $CO_2$ at STP: $2\ mol \times 22.4\ L/mol = 44.8\ L$.

Answer: fermenting $180\ g$ of glucose gives $92\ g$ of ethanol and $44.8\ L$ of carbon dioxide gas at STP. Notice the mass check: $180\ g$ of glucose becomes $92\ g$ ethanol $+ (2 \times 44\ g) = 88\ g$ of $CO_2$, and $92 + 88 = 180\ g$ — mass is conserved, a quick way to verify any stoichiometry answer.

• Confusing fermentation (biological, uses yeast/zymase, produces $CO_2$ as a visible by-product) with hydration of ethene (industrial, uses a catalyst, gives ethanol directly with no $CO_2$) — these are two separate routes with different starting materials and conditions.
• Thinking denatured spirit is just diluted alcohol — denaturing means adding a poisonous or foul-tasting substance to make it undrinkable, not lowering its concentration.
• Assuming ethanol's reaction with sodium releases sodium hydroxide — it actually forms sodium ethoxide ($C_2H_5ONa$), since only the hydrogen of the O–H bond is displaced, not the whole –OH group.
• Writing esterification as just "ethanol + acid gives ester" without naming the concentrated sulphuric acid catalyst or recognising that water is also formed alongside the ester.

Ask the tutor: Want to practise writing balanced equations for all three of ethanol's signature reactions, or work through another fermentation stoichiometry problem with different starting masses? Ask the AI tutor to walk you through it.