Carboxylic acids

Splash a little vinegar on your fingers and the sharp, sour smell that hits you is ethanoic acid — the ICSE Class 10 face of an entire family of organic compounds called carboxylic acids. Every member of this family carries the same signature group, –COOH, bolted onto a carbon chain, and that one group decides almost everything the molecule does: it turns litmus red, fizzes with carbonates, dissolves reactive metals with a hiss of hydrogen, and — most memorably — teams up with an alcohol to make sweet-smelling esters. This article stays tightly inside the prescribed scope: the structure and general formula of the carboxylic acid series, the laboratory preparation of ethanoic acid by oxidising ethanol, its physical properties (including the famous glacial acid), its four acid-type reactions, esterification, and the simple smell-plus-fizz test used to spot it. No reaction mechanisms, no acids beyond ethanoic acid — just the reactions you are expected to write and balance, tied together with one stoichiometric calculation so the chemistry also becomes a number you can check.

The core insight

Carboxylic acids are the homologous series $C_nH_{2n+1}COOH$ built on the –COOH functional group, and ethanoic acid ( $CH_3COOH$ ), made by oxidising ethanol, behaves as a weak acid with metals, carbonates and bases while also undergoing esterification with alcohols.

The core idea

The functional group. A carboxylic acid molecule contains the carboxyl group, –COOH — a carbonyl carbon (C=O) and a hydroxyl (–OH) fused onto the same carbon atom. Stacking –COOH onto successive alkyl chains generates a homologous series with the general formula

C_nH_{2n+1}COOH

where: $n$ is the number of carbons in the attached alkyl chain (so $n=0$ gives methanoic acid, $HCOOH$ , and $n=1$ gives the ICSE-prescribed example, ethanoic acid). Because every member differs from the next by a $-CH_2-$ unit, they share the same chemical behaviour and only gradually shift in physical properties — the classic mark of a homologous series.

Preparation. Ethanoic acid is made in the laboratory (and forms naturally when wine or cider is left exposed to air) by oxidising ethanol:

C_2H_5OH + 2[O] \xrightarrow{\text{oxidising agent}} CH_3COOH + H_2O

where: $[O]$ stands for "oxygen supplied by a suitable oxidising agent," a shorthand used at this level instead of naming a specific reagent or mechanism. Notice the atom count balances only if two oxygen atoms are supplied — one completes the carbonyl, one appears in the water byproduct — which is exactly the bookkeeping the worked example below turns into a mass calculation.

Physical properties. Ethanoic acid is a colourless liquid with a pungent, vinegar-like smell. Pure, water-free ethanoic acid is called glacial acetic acid because it freezes into ice-like crystals just below room temperature (around 16–17 degrees C) — a name that describes its freezing behaviour, not a different substance. It is completely miscible with water in all proportions.

Chemical properties — acid behaviour plus esterification. As a weak acid (only partially ionised in water), ethanoic acid still shows the standard reactions of an acid:

Litmus: turns blue litmus red.
Reactive metals (sodium, magnesium, zinc) liberate hydrogen gas and form the corresponding ethanoate salt:

2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2\uparrow

Metal carbonates/bicarbonates liberate carbon dioxide with brisk effervescence, and the gas turns limewater milky:

2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2\uparrow

Bases undergo ordinary neutralisation to give a salt and water (for example, with NaOH, sodium ethanoate and water).
Esterification is the reaction that sets carboxylic acids apart from mineral acids: warmed with an alcohol in the presence of a little concentrated sulphuric acid, ethanoic acid forms a sweet-smelling ester:

CH_3COOH + C_2H_5OH \xrightarrow{\text{conc. } H_2SO_4} CH_3COOC_2H_5 + H_2O

The svg figure below lines up these five reaction "faces" of ethanoic acid around the central –COOH group so the pattern is easy to hold in memory.

Figure

Ethanoic acid structure at the centre with five arrows to litmus turning red, metal giving hydrogen gas, carbonate giving carbon dioxide gas, base giving salt and water, and alcohol giving a sweet smelling ester

Derivation Amass of ethanoic acid from the oxidation of ethanol

Because the oxidation of ethanol to ethanoic acid is a balanced 1:1 mole reaction with fixed molar masses on each side, the mass of ethanoic acid obtainable from a known mass of ethanol can be calculated directly from the equation without any experimental data, purely by conserving moles across the reaction and converting each side to mass using molar mass.

Step 1. Start from the balanced equation for the oxidation:

C_2H_5OH + 2[O] \rightarrow CH_3COOH + H_2O

The equation shows that 1 mole of ethanol produces 1 mole of ethanoic acid — the mole ratio of ethanol to ethanoic acid is exactly 1:1, since one $-CH_2OH$ -type carbon is oxidised straight to one $-COOH$ carbon with no carbon lost.

Step 2. Convert the given mass of ethanol into moles using its molar mass. Ethanol, $C_2H_5OH$ ( $C_2H_6O$ ), has molar mass

M(C_2H_5OH) = 2(12) + 6(1) + 16 = 46\ \text{g/mol}

so a mass $m_{\text{ethanol}}$ of ethanol corresponds to $n = m_{\text{ethanol}}/46$ moles.

Step 3. Since the mole ratio is 1:1 (Step 1), the moles of ethanoic acid produced equal the moles of ethanol consumed: $n_{\text{acid}} = n_{\text{ethanol}}$ .

Step 4. Convert moles of ethanoic acid back into mass using its molar mass. Ethanoic acid, $CH_3COOH$ ( $C_2H_4O_2$ ), has molar mass

M(CH_3COOH) = 2(12) + 4(1) + 2(16) = 60\ \text{g/mol}

so the theoretical mass of ethanoic acid formed is

m_{\text{acid}} = n_{\text{ethanol}} \times 60 = \frac{m_{\text{ethanol}}}{46} \times 60

where: $m_{\text{ethanol}}$ is the mass of ethanol oxidised (g), $46$ and $60$ g/mol are the molar masses of ethanol and ethanoic acid, and $m_{\text{acid}}$ is the theoretical mass of ethanoic acid formed (g). This single ratio, $60/46$ , is the conversion factor for any mass of ethanol oxidised completely to ethanoic acid.

Worked example

Question: 4.6 g of ethanol is completely oxidised to ethanoic acid in the laboratory. Calculate the theoretical mass of ethanoic acid produced.

Step 1. Find the moles of ethanol supplied, using $M(C_2H_5OH) = 46\ \text{g/mol}$ :

n_{\text{ethanol}} = \frac{4.6}{46} = 0.1\ \text{mol}

Step 2. Apply the 1:1 mole ratio from the balanced equation $C_2H_5OH + 2[O] \rightarrow CH_3COOH + H_2O$ :

n_{\text{acid}} = n_{\text{ethanol}} = 0.1\ \text{mol}

Step 3. Convert moles of ethanoic acid to mass using $M(CH_3COOH) = 60\ \text{g/mol}$ :

m_{\text{acid}} = 0.1 \times 60 = 6\ \text{g}

So 4.6 g of ethanol, oxidised completely, gives a theoretical 6 g of ethanoic acid.

Competitive stretch

If this 6 g of ethanoic acid is then fully esterified with excess ethanol (using conc. $H_2SO_4$ ), how many moles of ethyl ethanoate, $CH_3COOC_2H_5$ , form? Since esterification is also 1:1 ( $CH_3COOH + C_2H_5OH \rightarrow CH_3COOC_2H_5 + H_2O$ ), the 0.1 mol of ethanoic acid gives 0.1 mol of ethyl ethanoate (theoretical, ignoring any loss of yield).

Common traps

Calling ethanoic acid a strong acid — it is a weak acid, ionising only partially in water, unlike mineral acids such as HCl or $H_2SO_4$ .
Treating "glacial acetic acid" as a separate chemical rather than the pure, water-free form of ethanoic acid that happens to freeze near room temperature.
Forgetting the co-product of esterification: writing the ester without the water molecule, or omitting the dual role (catalyst and dehydrating agent) of concentrated sulphuric acid.
In stoichiometry, using the wrong molar mass — mixing up ethanol's 46 g/mol with ethanoic acid's 60 g/mol, or forgetting that the mole ratio from the equation (not the mass ratio) is what carries over between reactant and product.

Ask the tutor: Want to practise the ethanol-to-ethanoic-acid mass calculation with different starting masses, or work through more of ethanoic acid's reaction equations step by step? Ask the AI tutor for guided examples.

Why is ethanoic acid called a weak acid if it still reacts vigorously with metals and carbonates

Strength and reactivity are different ideas. "Weak" refers to the extent of ionisation — in water, only a small fraction of $CH_3COOH$ molecules split into $CH_3COO^-$ and $H^+$ ions, unlike HCl which ionises almost completely. But the ions it does form still react readily with metals, carbonates and bases, which is why you still see brisk fizzing and hydrogen gas — the reactions just proceed at a somewhat gentler pace and lower ion concentration than a strong acid of the same molarity would give.

Why does esterification need concentrated sulphuric acid instead of just warming the acid and alcohol together

Without a catalyst, the direct reaction between ethanoic acid and ethanol is very slow and also reversible, so very little ester forms. Concentrated sulphuric acid speeds up the forward reaction (catalyst) and, being a strong dehydrating agent, also soaks up the water produced. Removing water pushes the reaction further toward the ester side, so both roles work together to give a usable yield of ethyl ethanoate in a reasonable time.

Is vinegar the same thing as glacial acetic acid

No. Vinegar is a dilute aqueous solution of ethanoic acid, typically only about 5 to 8 percent acid by volume, with the rest water — safe to use in food. Glacial acetic acid is the pure, essentially water-free compound, corrosive and far more concentrated, which freezes into ice-like crystals just below room temperature. The two share the same acid but differ hugely in concentration and how they can be handled.

Carboxylic acids

The core idea

Worked example

Competitive stretch

Common traps

Before this

Where this leads