Question 1Level 2 · ExamSingle correct
The dipole moments of propane (CH3CH2CH3), propene (CH3CH=CH2) and propyne (CH3C≡CH) are all small but different. The correct decreasing order of dipole moment is ______. Exactly one option is correct.
APropane > propene > propyne
BPropene > propyne > propane
CPropyne > propene > propane
DPropyne > propane > propene
Show answer and solution
Answer: C
The molecular dipole arises mainly from the bond between the CH3 carbon (sp3) and the unsaturated carbon. The more s character a carbon orbital has, the more electronegative that carbon is: sp (50% s) > sp2 (33%) > sp3 (25%).
Propyne: CH3–C(sp) bond has the largest electronegativity difference, μ≈0.78 D.
Propene: CH3–C(sp2), smaller difference, μ≈0.37 D.
Propane: only sp3 carbons, almost non-polar, μ≈0.08 D.
Order: propyne > propene > propane.
Answer: (C).
Difficulty: HardQuestion 2Level 3 · AdvancedSingle correct
3,3-Dimethylbutan-2-ol, (CH3)3C–CH(OH)–CH3, is heated with concentrated H2SO4. The major alkene formed is treated with O3 and then with Zn and H2O. The organic product(s) obtained is/are ______. Exactly one option is correct.
A2,2-Dimethylpropanal and methanal
B3-Methylbutan-2-one and methanal
CEthanal and 2-methylpropanal
DPropanone only
Show answer and solution
Answer: D
Step 1 (acid-catalysed dehydration): protonation of OH and loss of water gives the secondary carbocation (CH3)3C–CH+–CH3.
Step 2 (rearrangement): a 1,2-methyl shift from the adjacent quaternary carbon gives the more stable tertiary carbocation (CH3)2C+–CH(CH3)2.
Step 3 (loss of H+, Saytzeff): removal of the H on the neighbouring CH gives the tetrasubstituted alkene 2,3-dimethylbut-2-ene, (CH3)2C=C(CH3)2, as the major product (the less substituted 2,3-dimethylbut-1-ene is minor; unrearranged 3,3-dimethylbut-1-ene is minor).
Step 4 (reductive ozonolysis): (CH3)2C=C(CH3)2 → 2 (CH3)2C=O. Only propanone is formed (2 mol per mol of alkene).
Distractors: (A) would come from unrearranged 3,3-dimethylbut-1-ene; (B) from 2,3-dimethylbut-1-ene; (C) from 4-methylpent-2-ene.
Answer: (D).
Difficulty: Peak challengeQuestion 3Level 3 · AdvancedSingle correct
1-Bromo-1-methylcyclohexane is heated with alcoholic KOH. The major alkene formed is treated with O3 and then with Zn and H2O. The major organic product is ______. Exactly one option is correct.
A6-Oxoheptanal
BCyclohexanone and methanal
CHeptanedial
D2-Methylhexanedial
Show answer and solution
Answer: A
Dehydrohalogenation follows the Saytzeff rule: the more substituted alkene is the major product. Removing an H from a ring CH2 gives 1-methylcyclohexene (trisubstituted C=C); removing an H from CH3 gives methylenecyclohexane (disubstituted, minor).
Reductive ozonolysis of 1-methylcyclohexene cleaves the ring C=C but keeps all seven carbons in one molecule: C1 (bearing CH3) becomes a ketone C=O and C2 (bearing H) becomes CHO:
CH3CO–CH2CH2CH2CH2–CHO = 6-oxoheptanal.
(B) is the product from methylenecyclohexane (the minor alkene). (C) and (D) do not correspond to this skeleton.
Answer: (A).
Difficulty: Very hard