Free Nucleophilic Addition Reactions MCQs with Answers

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. Methanol is produced by reduction of

• A. Formaldehyde
• B. Acetaldehyde
• C. Propanal
• D. Propanone

Explanation: Reducing an aldehyde adds hydrogen across the carbonyl group and gives a primary alcohol with the same number of carbon atoms, so the one carbon aldehyde formaldehyde gives the one carbon alcohol methanol. Acetaldehyde would give ethanol and propanal would give propan-1-ol. Propanone is a ketone and would give a secondary alcohol, propan-2-ol.