All Free Chemistry MCQs with Answers

291. Aldehydes and ketones both react with 2,4-dinitrophenylhydrazine to give

• A. a silver mirror
• B. a colourless solution
• C. carbon dioxide
• D. an orange crystalline precipitate

Explanation: This reagent, often called Brady's reagent, condenses with any carbonyl group to give a bright orange or yellow solid, so it detects the presence of the group without distinguishing aldehyde from ketone. The precipitate has a sharp melting point characteristic of the parent compound, which allows identification. Tollens or Fehling's is then used to decide which of the two classes it is.

292. Ethanal can be prepared industrially by the oxidation of

• A. ethanol
• B. ethanoic acid
• C. ethene with hydrogen
• D. propanone

Explanation: Oxidising the primary alcohol ethanol with acidified dichromate, distilling off the product as it forms, gives ethanal, and the same conversion is achieved industrially by passing ethanol vapour over hot copper. Oxidation of ethanoic acid is not possible, since it is already at a higher oxidation level. The general rule is that primary alcohols give aldehydes and secondary alcohols give ketones.

293. Propanone is prepared by the oxidation of

• A. propan-1-ol
• B. propane
• C. propan-2-ol
• D. propanoic acid

Explanation: Propan-2-ol is a secondary alcohol, so removing the hydrogen from the carbinol carbon leaves a ketone, and the reaction stops there because no further hydrogen remains on that carbon. Propan-1-ol, being primary, would give propanal and then propanoic acid instead. Most propanone is actually obtained as the by product of the cumene process for phenol.

294. Which compound gives a positive iodoform test?

• A. Propanal
• B. Propanone
• C. Benzaldehyde
• D. Methanal

Explanation: The test requires a methyl group attached directly to the carbonyl carbon, and propanone is the simplest methyl ketone, giving the pale yellow precipitate of triiodomethane with iodine in alkali. Ethanal is the only aldehyde that responds, since it too has that methyl group. Propanal, methanal and benzaldehyde all lack the required structure.

295. Aldehydes and ketones have lower boiling points than alcohols of similar molar mass because

• A. they have smaller molecules
• B. they are non polar
• C. they are ionic
• D. they cannot hydrogen bond to one another, having no hydrogen attached to oxygen

Explanation: Carbonyl compounds are polar and attract one another by dipole to dipole forces, which lifts their boiling points above those of alkanes, but with no O-H bond they cannot form hydrogen bonds between their own molecules and so boil below the corresponding alcohols. They can still accept hydrogen bonds from water, which is why the smaller members are water soluble. Propanone dissolving in water while boiling at only 56 degrees illustrates both points.

296. The reaction of a carbonyl compound with sodium hydrogen sulphite gives a crystalline adduct that is useful for

• A. measuring the pH of the compound
• B. separating and purifying the carbonyl compound, since the adduct can be decomposed to regenerate it
• C. converting the compound into an alkane
• D. detecting the presence of a benzene ring

Explanation: The bisulphite adduct crystallises out and can be filtered off, and treating it with dilute acid or alkali releases the original aldehyde or ketone in a pure state. Only aldehydes, methyl ketones and a few cyclic ketones form the adduct, because larger groups hinder the approach of the bulky nucleophile. It is therefore a purification method and, incidentally, a structural test.

297. In nucleophilic addition to a carbonyl group, the carbon atom changes hybridisation from

• A. sp3 to sp2
• B. sp to sp2
• C. sp2 to sp3
• D. sp3 to sp

Explanation: The carbonyl carbon starts as trigonal planar sp2 with a bond angle of 120 degrees, and when the nucleophile adds it acquires a fourth group and becomes tetrahedral sp3 with angles near 109.5 degrees. This geometrical change is why bulky groups around the carbonyl slow the reaction down so much. Elimination reactions run the same change in reverse.

298. Methanal, HCHO, is used in a 40 per cent aqueous solution known as formalin, mainly for

• A. preserving biological specimens
• B. flavouring food
• C. treating drinking water
• D. use as a fuel

Explanation: Methanal cross links the proteins of tissue, hardening it and killing microorganisms, which is why formalin preserves specimens and is also used to make thermosetting resins such as bakelite. It is a gas at room temperature, so the aqueous solution is the practical form. The compound is toxic and is now treated as a carcinogen, so exposure is controlled.

299. Which of the following compounds is a ketone?

• A. CH3CHO
• B. CH3COCH3
• C. CH3COOH
• D. CH3CH2OH

Explanation: In propanone the carbonyl group lies between two carbon atoms, which is the defining feature of a ketone, while CH3CHO is the aldehyde ethanal with the carbonyl at the end of the chain. CH3COOH is a carboxylic acid, whose carbonyl carbon also carries a hydroxyl group, and CH3CH2OH is an alcohol. Reading which atoms flank the carbonyl carbon is the whole test.

300. Oxidation of an aldehyde with acidified potassium permanganate gives

• A. a primary alcohol
• B. a ketone
• C. a carboxylic acid with the same number of carbon atoms
• D. carbon dioxide only

Explanation: The hydrogen on the carbonyl carbon is replaced by a hydroxyl group, so ethanal becomes ethanoic acid with no change in the length of the chain. This ready oxidation is why aldehydes act as reducing agents and give positive results with Tollens and Fehling's reagents. Ketones cannot be oxidised in this way without breaking the carbon skeleton.