All Free Chemistry MCQs with Answers

301. The compound formed when ethanal reacts with a Grignard reagent, followed by acid hydrolysis, is

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

Explanation: The alkyl group of the Grignard reagent adds to the carbonyl carbon, which then carries that new group, the original methyl group, a hydrogen and a hydroxyl, making the product a secondary alcohol. Methanal would give a primary alcohol and a ketone would give a tertiary one. This makes Grignard reagents a general method for building alcohols of any class.

302. Benzaldehyde gives a positive test with Tollens reagent but a negative one with Fehling's solution. This shows that

• A. benzaldehyde is a ketone
• B. benzaldehyde contains no carbonyl group
• C. Fehling's solution reacts only with aromatic compounds
• D. aromatic aldehydes are oxidised less readily than aliphatic aldehydes

Explanation: Fehling's solution is the milder oxidising agent of the two, so it fails with aromatic aldehydes whose carbonyl group is stabilised by conjugation with the ring, while the stronger Tollens reagent still succeeds. Benzaldehyde is definitely an aldehyde, as the silver mirror confirms. This exception is a favourite examination point precisely because it looks like a contradiction.

303. Aldehydes and ketones both react with hydroxylamine to form

• A. oximes
• B. esters
• C. alcohols
• D. amines

Explanation: The nitrogen of hydroxylamine attacks the carbonyl carbon and water is then eliminated, so the product contains a carbon to nitrogen double bond and is called an oxime. Hydrazine and its derivatives react in the same condensation pattern to give hydrazones. These crystalline derivatives were once the standard way to identify a carbonyl compound from its melting point.

304. Propanone is widely used in industry and in the laboratory as

• A. a solvent, because it dissolves both polar and non polar substances and evaporates quickly
• B. an oxidising agent
• C. a fuel for vehicles
• D. a food preservative

Explanation: Its polar carbonyl group and small hydrocarbon portion let propanone dissolve a wide range of materials, and its low boiling point of 56 degrees Celsius means it dries rapidly, which is why it is used as nail polish remover and for cleaning glassware. It is also miscible with water in all proportions. Being highly flammable, it must be kept away from flames.

305. Carboxylic acids are more acidic than alcohols because

• A. they contain more hydrogen atoms
• B. the carboxylate ion formed is stabilised by delocalisation of the negative charge over two oxygen atoms
• C. they are liquids at room temperature
• D. their molecules are larger

Explanation: Losing the acidic hydrogen gives an ion whose charge is shared equally between the two oxygens, so the two carbon to oxygen bonds become identical in length and the ion is far more stable than an alkoxide. That stability makes the loss of the proton much more favourable. This is why an acid liberates carbon dioxide from a carbonate while an alcohol does not.

306. Ethanoic acid reacts with sodium carbonate to give

• A. sodium ethanoate, water and carbon dioxide
• B. sodium ethanoate and hydrogen
• C. ethanol and carbon dioxide
• D. no reaction

Explanation: Being a stronger acid than carbonic acid, ethanoic acid displaces carbon dioxide from a carbonate, and the effervescence produced is the standard test that distinguishes a carboxylic acid from both alcohols and phenols. With sodium metal the acid would give hydrogen instead. Neutralisation with an alkali gives only the salt and water.

307. The presence of chlorine atoms on the carbon next to the carboxyl group, as in chloroethanoic acid, makes the acid stronger because chlorine

• A. donates electrons to the carboxyl group
• B. increases the molar mass
• C. withdraws electron density, spreading and stabilising the negative charge of the anion
• D. makes the molecule non polar

Explanation: The electron withdrawing inductive effect of chlorine pulls charge away from the carboxylate oxygens, so the ion is more stable and the acid ionises more readily, and trichloroethanoic acid is stronger still because three chlorines act together. The effect weakens rapidly with distance along the chain. Electron releasing alkyl groups have the opposite effect, which is why ethanoic acid is weaker than methanoic acid.

308. Carboxylic acids have unusually high boiling points because in the liquid state their molecules

• A. are ionised
• B. form dimers held together by two hydrogen bonds
• C. form covalent bonds with one another
• D. are non polar

Explanation: Two molecules pair up so that the hydroxyl hydrogen of each bonds to the carbonyl oxygen of the other, giving a cyclic dimer with double the effective molar mass, so ethanoic acid boils at 118 degrees Celsius, well above ethanol despite similar size. The same dimerisation occurs in non polar solvents and even in the vapour just above the boiling point. Only in water do the dimers break up, replaced by hydrogen bonds to the solvent.

309. Ethanoic acid can be prepared by the oxidation of

• A. ethanol under reflux with acidified potassium dichromate
• B. ethene with bromine water
• C. propanone with Tollens reagent
• D. ethane with oxygen at room temperature

Explanation: Refluxing keeps the intermediate ethanal in the flask so that oxidation continues to the acid, whereas distilling the product off as it forms would stop the reaction at the aldehyde. The dichromate turns from orange to green as it is reduced. Vinegar is made industrially by the bacterial oxidation of ethanol instead.

310. Hydrolysis of a nitrile such as ethanenitrile with dilute acid gives

• A. an amine
• B. an aldehyde
• C. an alcohol
• D. a carboxylic acid with the same number of carbon atoms as the nitrile

Explanation: The carbon of the nitrile group becomes the carbon of the carboxyl group, so hydrolysing ethanenitrile gives ethanoic acid and ammonia is released as its salt. Since the nitrile itself was made by adding cyanide to an alkyl halide with one fewer carbon, the two steps together lengthen the chain by one. Reduction rather than hydrolysis of a nitrile gives a primary amine.