AS Chemistry 9701: Reactions of Butanoic Acid with Metal, Alkali, Carbonate and Alcohol

Butanoic acid, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$ , is used in four separate experiments.

(a) Excess magnesium ribbon is added to a solution of butanoic acid. State what would be observed, name the gas produced and write a word equation for the reaction. [2]

(b) Aqueous sodium hydroxide is added to a solution of butanoic acid until the acid is exactly neutralised. Name the salt formed and state one observation that shows this is a neutralisation rather than a redox reaction. [2]

(c) Solid sodium carbonate is added to a solution of butanoic acid. State what would be observed and describe a simple chemical test, with its positive result, that would confirm the identity of the gas produced. [2]

(d) Butanoic acid is warmed under reflux with propan-1-ol in the presence of a few drops of concentrated sulfuric acid. Name the type of reaction taking place, name the organic product formed, and state the role of the concentrated sulfuric acid. [2]

Part (a): Reaction with a reactive metal

Carboxylic acids behave as typical Brønsted acids towards reactive metals, releasing hydrogen gas and forming a salt:

$\text{2CH}_3\text{CH}_2\text{CH}_2\text{COOH(aq)} + \text{Mg(s)} \rightarrow (\text{CH}_3\text{CH}_2\text{CH}_2\text{COO})_2\text{Mg(aq)} + \text{H}_2\text{(g)}$

Observation: effervescence (bubbles of colourless gas) at the surface of the magnesium ribbon, and the ribbon gradually dissolves as it is consumed. Gas: hydrogen, $\text{H}_2$ . Word equation: butanoic acid + magnesium $\rightarrow$ magnesium butanoate + hydrogen.

Part (b): Reaction with an alkali

$\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{COONa(aq)} + \text{H}_2\text{O(l)}$

This is a standard acid-base neutralisation, giving the salt sodium butanoate and water. There is no gas evolved and the mixture becomes noticeably warmer, which distinguishes this exothermic neutralisation from a redox process (which would involve a change in oxidation state and, with a metal, a gas).

Part (c): Reaction with a carbonate

$\text{2CH}_3\text{CH}_2\text{CH}_2\text{COOH(aq)} + \text{Na}_2\text{CO}_3\text{(s)} \rightarrow \text{2CH}_3\text{CH}_2\text{CH}_2\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

Observation: effervescence as the solid sodium carbonate dissolves and reacts, with a colourless gas being released. To confirm the gas is carbon dioxide, bubble it through limewater (aqueous calcium hydroxide): a positive result is the limewater turning milky/cloudy as insoluble calcium carbonate forms.

Part (d): Esterification with an alcohol

Warming a carboxylic acid under reflux with an alcohol, using a few drops of concentrated sulfuric acid, is the standard esterification (condensation) reaction:

$\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} + \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{CH}_2\text{CH}_2\text{COOCH}_2\text{CH}_2\text{CH}_3 + \text{H}_2\text{O}$

The organic product is the ester propyl butanoate (from the propan-1-ol and butanoic acid), released alongside water. The concentrated sulfuric acid is a catalyst: it speeds up the (reversible, equilibrium) reaction and helps drive it towards the ester side, but it is not itself used up in the overall reaction. Reflux is used so that the mixture can be heated to increase the rate without losing the volatile reactants or product as vapour.

Final answers

(a) Effervescence of hydrogen gas, $\text{H}_2$ , as the magnesium dissolves; butanoic acid + magnesium $\rightarrow$ magnesium butanoate + hydrogen.
(b) Sodium butanoate is formed; no gas is evolved (heat is released instead), showing simple neutralisation.
(c) Effervescence of $\text{CO}_2$ ; the gas turns limewater milky/cloudy.
(d) Esterification; product is propyl butanoate; concentrated sulfuric acid acts as a catalyst.