All Free Chemistry MCQs with Answers

281. Which reagent distinguishes ethanol from phenol?

• A. Sodium metal
• B. Bromine water
• C. Water
• D. A lighted splint

Explanation: Phenol decolourises bromine water immediately and gives a white precipitate of the tribromo compound, whereas ethanol produces no visible change. Sodium metal is not suitable, since both compounds contain a hydroxyl group and both release hydrogen with it. Neutral iron three chloride is the other reliable distinguishing test.

282. The oxidation of propan-2-ol with acidified potassium dichromate gives

• A. propanal
• B. propanoic acid
• C. propene
• D. propanone

Explanation: Propan-2-ol is a secondary alcohol, so oxidation removes the single hydrogen on the carbinol carbon and leaves a ketone, which resists further oxidation because no hydrogen remains on that carbon. A primary alcohol would give an aldehyde and then an acid. This difference in oxidation product is the standard way of identifying the class of an unknown alcohol.

283. Alcohols with short carbon chains are soluble in water, but solubility falls sharply as the chain lengthens because

• A. the hydroxyl group disappears in longer molecules
• B. long chain alcohols are gases
• C. the growing non polar hydrocarbon chain cannot form hydrogen bonds with water and dominates the molecule
• D. longer alcohols react with water

Explanation: Methanol, ethanol and propan-1-ol mix with water in all proportions because the hydroxyl group hydrogen bonds effectively, but as the chain grows the hydrophobic portion outweighs it and solubility falls until the higher alcohols are essentially insoluble. Branching increases solubility slightly by making the molecule more compact. The same principle explains why soaps need a hydrophilic head on a long tail.

284. Warming a phenol with a carboxylic acid gives an ester only with difficulty, whereas an alcohol reacts readily. This is because

• A. phenols contain no hydroxyl group
• B. the phenolic oxygen donates its lone pair into the ring, making it a poorer nucleophile
• C. phenols are stronger bases than alcohols
• D. esters cannot contain a benzene ring

Explanation: Delocalisation of the oxygen lone pair into the aromatic system, the very effect that makes phenol acidic, leaves that oxygen less available to attack the carbonyl carbon of an acid. Phenyl esters are therefore prepared using more reactive acyl chlorides or anhydrides instead. The same delocalisation also shortens and strengthens the carbon to oxygen bond in phenol.

285. The carbonyl carbon of an aldehyde or ketone is attacked by nucleophiles because

• A. it carries a partial positive charge, oxygen being more electronegative
• B. it carries a partial negative charge
• C. it has a lone pair of electrons
• D. it is bonded to hydrogen

Explanation: The carbon to oxygen double bond is strongly polarised towards oxygen, leaving the carbon electron deficient and open to attack by a species with a lone pair. This is why the characteristic reaction of the carbonyl group is nucleophilic addition, in contrast with the electrophilic addition of alkenes, whose double bond is non polar. The oxygen carries the lone pairs and the partial negative charge.

286. Aldehydes are more reactive than ketones towards nucleophilic addition because in a ketone

• A. the carbonyl group is absent
• B. two alkyl groups release electron density towards the carbonyl carbon and also hinder attack
• C. the carbonyl carbon carries a full positive charge
• D. the carbon to oxygen bond is much longer

Explanation: Alkyl groups push electron density towards the carbonyl carbon, reducing its positive character, and their bulk physically blocks the approaching nucleophile, so two of them make a ketone markedly less reactive than an aldehyde with only one. Methanal, having none at all, is the most reactive carbonyl compound of the series. Both electronic and steric factors point the same way here.

287. Tollens reagent, ammoniacal silver nitrate, gives a silver mirror with

• A. ketones only
• B. both aldehydes and ketones
• C. aldehydes only
• D. neither aldehydes nor ketones

Explanation: An aldehyde still has a hydrogen on the carbonyl carbon, so it is easily oxidised to a carboxylic acid while the silver ions are reduced to metallic silver, which deposits as a mirror on clean glass. A ketone has no such hydrogen, so it resists oxidation and gives no reaction. This is the standard test for distinguishing the two classes.

288. Fehling's solution is reduced to a red precipitate of copper one oxide by

• A. propanone
• B. benzaldehyde
• C. ethanol
• D. ethanal

Explanation: Aliphatic aldehydes such as ethanal reduce the blue copper two complex to a brick red precipitate, whereas ketones do not react at all. Aromatic aldehydes such as benzaldehyde are the notable exception, since they give a negative result with Fehling's solution although they do respond to Tollens reagent. Ethanol must first be oxidised to an aldehyde before it could react.

289. The addition of hydrogen cyanide to ethanal gives

• A. ethanoic acid
• B. 2-hydroxypropanenitrile
• C. ethanol
• D. propanone

Explanation: The cyanide ion attacks the carbonyl carbon and the resulting alkoxide picks up a proton, giving a hydroxynitrile with one more carbon than the starting aldehyde. The reaction is catalysed by a trace of base, which generates the cyanide nucleophile. The product can be hydrolysed to a hydroxy acid, making this a useful chain lengthening step.

290. Reduction of a ketone with sodium borohydride or lithium aluminium hydride gives

• A. a primary alcohol
• B. a carboxylic acid
• C. a secondary alcohol
• D. an alkane

Explanation: The hydride ion adds to the carbonyl carbon, which then carries two alkyl groups and a hydroxyl, so the product is a secondary alcohol. An aldehyde reduced in the same way gives a primary alcohol. Reduction is simply the reverse of the oxidation that produced the carbonyl compound in the first place.