JEE Chemistry Hydrocarbons MCQs Set 42

Answer: (c) 2,3-Dimethylbutan-2-ol

Explanation:
1. Protonation gives the secondary carbocation (CH₃)₃C–CH⁺–CH₃.
2. A 1,2-methyl shift converts it into the more stable tertiary cation (CH₃)₂C⁺–CH(CH₃)₂.
3. Water attacks this cation, giving 2,3-dimethylbutan-2-ol.
4. Answer: (c)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.

Answer: (b) Buta-1,2-diene (CH₂=C=CH–CH₃)

Explanation:
1. In an allene the central carbon is joined to two carbons by double bonds, and both bonds are cleaved.
2. The central carbon is left with two C=O bonds, which is CO₂.
3. The terminal carbons give HCHO and CH₃CHO.
4. Answer: (b)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.

Answer: (a) CH₃CH₂CH=C(CH₂CH₃)₂

Explanation:
1. Cleaving C=C in option (a) gives CH₃CH₂CHO from one end and (CH₃CH₂)₂C=O from the other.
2. The other options give ethanal, propanone or other carbonyl compounds.
3. Answer: (a)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.

Answer: (d) Catalytic hydrogenation of acetylene

Explanation:
1. Soda lime on sodium benzoate, zinc dust on phenol and H₃PO₂ on a diazonium salt all give benzene.
2. Benzene arises from acetylene only by cyclic trimerisation through a red-hot tube.
3. Hydrogenation of acetylene gives ethene or ethane.
4. Answer: (d)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.

Answer: (c) Mesitylene

Explanation:
1. Any alkylbenzene with a benzylic hydrogen is oxidised to benzoic acid.
2. Mesitylene has three methyl groups, so all three are oxidised to give benzene-1,3,5-tricarboxylic acid.
3. Answer: (c)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.

Answer: (b) Acetophenone

Explanation:
1. This is Friedel–Crafts acylation, with the acylium ion CH₃CO⁺ as the electrophile.
2. It gives C₆H₅COCH₃, acetophenone.
3. Answer: (b)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.

Answer: (b) Salicylic acid

Explanation:
1. Phenylacetic acid gives toluene, o-toluic acid gives toluene and benzoic acid gives benzene.
2. Salicylic acid gives phenol, which is an aromatic alcohol and not a hydrocarbon.
3. Answer: (b)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.

Answer: (b) A mixture of diastereomers (cis- and trans-1,2-dimethylcyclohexane)

Explanation:
1. The stereocentre at C-2 is untouched.
2. Hydrogen adds to C-1 from either face, which creates a new stereocentre and gives cis and trans-1,2-dimethylcyclohexane.
3. These differ in configuration at only one centre, so they are diastereomers and form in unequal amounts.
4. Answer: (b)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.

Answer: (d) Any of these

Explanation:
1. Wolff–Kishner (NH₂NH₂/KOH), Clemmensen (Zn–Hg/HCl) and red P + HI all reduce C=O to CH₂.
2. None of them touches a saturated hydrocarbon skeleton, so any one can be used.
3. Answer: (d)

Teacher's Note:
1. For alkenes and arenes, decide first which bond or position reacts, then write the product.