Carbonyl Compounds: Chemistry 9701 (Cambridge International AS & A Level)

A technician has two unlabelled colourless liquids, R and S. One is hexanal, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{CHO}$, and the other is hexan-3-one, $\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_2\text{CH}_3$, both share the molecular formula $\text{C}_6\text{H}_{12}\text{O}$.

Both liquids form an orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNPH). When warmed separately with Tollens' reagent, liquid R produces a shiny silver deposit (a silver mirror) on the inside of the test tube, while liquid S shows no visible change.

Which statement correctly identifies liquid R and explains this observation?

Compound J is butanal, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$. Compound K is butan-2-one, $\text{CH}_3\text{COCH}_2\text{CH}_3$. Both are treated separately with sodium tetrahydridoborate(III), $\text{NaBH}_4$.

(a) State the type of reaction that $\text{NaBH}_4$ brings about at a carbonyl group, and identify the species that first attacks the carbonyl carbon. [2]

(b) Give the structural formula and name of the organic product formed when J is reduced by $\text{NaBH}_4$, and state whether it is a primary or secondary alcohol. [2]

(c) Give the structural formula and name of the organic product formed when K is reduced by $\text{NaBH}_4$, and state whether it is a primary or secondary alcohol. [2]

Butanal, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$, reacts with hydrogen cyanide, HCN, in the presence of a trace of potassium cyanide, KCN, to form an addition product.

(a) Name the type of mechanism by which HCN adds across the carbonyl group of butanal, and identify the species that attacks the carbonyl carbon in the first step. [2]

(b) Describe this mechanism, including the movement of electron pairs, the shape and charge of any intermediate formed, and how that intermediate is converted into the final neutral product. [3]

(c) Give the structural formula and name of the organic product, and explain why it contains one more carbon atom than butanal. [3]

A student is given two unlabelled colourless liquids, each with the molecular formula $\text{C}_7\text{H}_{14}\text{O}$: heptanal, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{CHO}$, and heptan-4-one, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COCH}_2\text{CH}_2\text{CH}_3$.

(a) A few drops of each liquid are treated separately with 2,4-dinitrophenylhydrazine (2,4-DNPH). State the observation made with each liquid, and explain why this test alone cannot identify which liquid is which. [2]

(b) Each liquid is then warmed separately with Fehling's reagent. State what is observed with each liquid, and explain each result in terms of oxidation. [3]

(c) Each liquid is instead warmed separately with Tollens' reagent. State what is observed with each liquid, and use your answers to (b) and (c) to identify which liquid is heptanal. [3]

Four colourless liquids, P, Q, R and S, are each warmed separately with alkaline aqueous iodine (the tri-iodomethane test):

• P is butan-1-ol, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$
• Q is butan-2-ol, $\text{CH}_3\text{CH(OH)}\text{CH}_2\text{CH}_3$
• R is butan-2-one, $\text{CH}_3\text{COCH}_2\text{CH}_3$
• S is 2-methylpropan-1-ol, $(\text{CH}_3)_2\text{CHCH}_2\text{OH}$

How many of P, Q, R and S give a pale yellow precipitate of tri-iodomethane, $\text{CHI}_3$, in this test?

Pentan-1-ol, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$, and pentan-2-ol, $\text{CH}_3\text{CH(OH)}\text{CH}_2\text{CH}_2\text{CH}_3$, are each oxidised separately using acidified potassium dichromate(VI), $\text{K}_2\text{Cr}_2\text{O}_7$.

(a) State the colour change observed at the dichromate ion when a successful oxidation of either alcohol occurs. [2]

(b) A sample of pentan-1-ol is heated with a limited amount of acidified potassium dichromate(VI), and the organic product is distilled off from the reaction mixture as soon as it forms. Give the structural formula and name of this organic product, and explain why distilling it off as it forms prevents further oxidation. [3]

(c) A separate sample of pentan-2-ol is heated under reflux with an excess of acidified potassium dichromate(VI). Give the structural formula and name of the organic product formed, and state whether this product can be oxidised further by the dichromate ion, explaining your answer. [3]

Which one of the following four-carbon compounds does not give an orange precipitate when treated with 2,4-dinitrophenylhydrazine (2,4-DNPH)?

Butanone, $\text{CH}_3\text{COCH}_2\text{CH}_3$, is reacted with hydrogen cyanide, HCN, in the presence of a trace of potassium cyanide, KCN, in the same way as an aldehyde would react.

(a) State the type of mechanism involved, and identify the species that attacks the carbonyl carbon in the first step. [2]

(b) Give the structural formula and name of the organic product formed. [2]

(c) Butanone reacts more slowly with HCN than butanal does under the same conditions. Explain this difference in reactivity, referring to the groups attached to the carbonyl carbon in each compound. [3]

Pentan-2-one, $\text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3$, is warmed with an excess of alkaline aqueous iodine (iodine dissolved in sodium hydroxide solution), the tri-iodomethane test.

(a) State the observation made, and give the systematic name of the compound responsible for it. [2]

(b) In this reaction, the three hydrogen atoms of the $\text{CH}_3$ group next to the carbonyl group are first replaced by iodine atoms; hydroxide ions then cleave the carbon-carbon bond between this trihalogenated carbon and the carbonyl carbon. Identify the other organic product of the reaction (the salt formed alongside tri-iodomethane), giving both its name and its formula. [3]

(c) Pentan-3-one, $\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_3$, is treated with the same reagent under the same conditions. State, with a reason, whether a pale yellow precipitate is observed. [2]

An unknown colourless liquid, T, has the molecular formula $\text{C}_4\text{H}_8\text{O}$. When tested, T gives an orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNPH), shows no observable change when warmed with Tollens' reagent, and gives a pale yellow precipitate when warmed with alkaline aqueous iodine.

Which compound is T?