Carboxylic Acids and Derivatives: Chemistry 9701 (Cambridge International AS & A Level)

Hexan-1-ol, $\text{CH}_3(\text{CH}_2)_4\text{CH}_2\text{OH}$, is a primary alcohol. A student wants to convert it completely into hexanoic acid, $\text{CH}_3(\text{CH}_2)_4\text{COOH}$, in a single procedure, without isolating the intermediate aldehyde.

Which reagent and set of conditions would achieve this?

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]

Pentanenitrile, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CN}$, is heated under reflux with an excess of dilute hydrochloric acid.

(a) State the type of reaction taking place, name the two products formed, and write an equation for the overall reaction. [3]

(b) The pentanoic acid formed in (a) is treated with $\text{LiAlH}_4$ in dry ether, followed by dilute acid on work-up. State the type of alcohol formed (primary, secondary or tertiary) and give its name. [2]

(c) Explain why $\text{NaBH}_4$ would not bring about the same reduction as in (b). [2]

Propanoic acid, $\text{CH}_3\text{CH}_2\text{COOH}$, is converted into propanoyl chloride, $\text{CH}_3\text{CH}_2\text{COCl}$, which is then used in three further reactions.

(a) Propanoic acid is treated with sulfur dichloride oxide (thionyl chloride), $\text{SOCl}_2$. Write a word equation for this reaction and state one advantage of using $\text{SOCl}_2$ rather than $\text{PCl}_5$ for this preparation. [2]

(b) Propanoyl chloride is added, drop by drop, to water. State what would be observed and name the two products formed. [2]

(c) Propanoyl chloride is added, drop by drop, to excess ethanol. Name the organic product and state one observation made during the reaction. [2]

(d) Propanoyl chloride is added to an excess of methylamine, $\text{CH}_3\text{NH}_2$. Name the main organic product and explain why an excess of methylamine is used. [2]

A few drops of butanoyl chloride, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COCl}$, are added to a test tube of cold water: steamy white fumes are seen immediately and the reaction is complete within seconds. In contrast, butanoic acid, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$, shows no visible reaction at all when added to cold water.

Which statement best explains this difference in behaviour?

Aqueous ethanoic acid and aqueous phenol are each treated separately, in a test tube, with a spatula measure of solid sodium carbonate.

Which statement correctly describes what would be observed with each?

Pentanoyl chloride, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COCl}$, is added dropwise to liquid phenol, $\text{C}_6\text{H}_5\text{OH}$, and the mixture is gently warmed.

(a) Write an equation for the reaction and name the ester formed. [3]

(b) The corresponding reaction between pentanoyl chloride and ethanol proceeds immediately in the cold, with no need for warming. Explain, in terms of the availability of the oxygen atom's lone pair, why the reaction with phenol is slower and often needs warming (or the phenol first converted into sodium phenoxide) to obtain a good yield. [3]

(c) Describe a simple test, using concentrated aqueous ammonia, that could be used to confirm that hydrogen chloride gas is released during the reaction in (a), stating the observation expected. [2]

Hexanoic acid, $\text{CH}_3(\text{CH}_2)_4\text{COOH}$, is converted into hexanoyl chloride, $\text{CH}_3(\text{CH}_2)_4\text{COCl}$, which is then reacted further.

(a) Hexanoic acid is treated with solid phosphorus(V) chloride, $\text{PCl}_5$, at room temperature. Write an equation for the reaction, state one observation, and name the other two products formed alongside hexanoyl chloride. [3]

(b) Hexanoyl chloride is added dropwise to an excess of concentrated aqueous ammonia. Write an equation for the reaction, name the organic product formed, and explain why an excess of ammonia is needed. [3]

(c) Describe, in words, the two-step mechanism by which the reaction in (b) occurs, explaining why the carbonyl carbon of hexanoyl chloride is especially open to attack. [2]

Methyl propanoate, $\text{CH}_3\text{CH}_2\text{COOCH}_3$, can be made either (i) by warming propanoic acid under reflux with methanol and a few drops of concentrated sulfuric acid, or (ii) by adding propanoyl chloride dropwise to excess methanol.

Which statement correctly compares these two routes to the same ester?

Two unlabelled colourless liquids, J and K, are known to be either propanoyl chloride, $\text{CH}_3\text{CH}_2\text{COCl}$, or ethyl propanoate, $\text{CH}_3\text{CH}_2\text{COOCH}_2\text{CH}_3$, but their labels have been lost.

(a) Describe a simple test, using only water, that would allow J and K to be distinguished, stating the observation expected for each liquid. [3]

(b) Write an equation for any reaction that occurs during the test in (a). [1]

(c) The resulting mixtures from (a) are then treated with a few drops of dilute nitric acid followed by aqueous silver nitrate. State and explain the observation expected for the mixture derived from each liquid, giving an ionic equation for any reaction. [4]