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10 sample questions
Organic Chemistry MCQ Practice Test
Carbon-based compounds and processes
Q1. What are the products of the ozonolysis of 2-methyl-2-butene followed by reductive workup?
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A. Acetone and formaldehyde
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B. Acetone and acetaldehyde ✓
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C. 2-butanone and formaldehyde
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D. 2-butanone and acetaldehyde
Explanation: Ozonolysis of 2-methyl-2-butene followed by reductive workup (e.g., with Zn/H2O) yields acetone and acetaldehyde. The double bond is cleaved, and the resulting fragments are oxidized to ketones and aldehydes. The correct answer is acetone and acetaldehyde.
Q2. Which of the following statements accurately describes the mechanism of the Diels-Alder reaction?
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A. The diene undergoes a concerted, single-step pericyclic reaction with the dienophile to form the cycloaddition product. ✓
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B. The diene first undergoes a 1,4-cycloaddition with the dienophile, followed by a ring-closure reaction to form the cycloaddition product.
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C. The dienophile undergoes a radical addition to the diene, resulting in the formation of a diradical intermediate.
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D. The diene undergoes a stepwise, multi-step reaction with the dienophile to form the cycloaddition product.
Explanation: The Diels-Alder reaction is a concerted, single-step pericyclic reaction that involves the simultaneous formation of two new sigma bonds between the diene and dienophile.
Q3. The π-orbitals of a planar, conjugated pentadienyl cation are described by which of the following molecular orbital configurations?
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A. The π-orbitals are all non-bonding and localized on individual p-orbitals.
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B. The π-orbitals form a π-system with alternating bond orders of 1 and 0. ✓
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C. The π-orbitals form a π-system with alternating bond orders of 0 and 1.
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D. The π-orbitals form a π-system with bond orders of 1 throughout.
Explanation: In a planar, conjugated pentadienyl cation, the π-orbitals form a π-system due to the planarity and conjugation of the system. This system exhibits alternating bond orders of 1 and 0, resulting from the resonance structures with alternating double bonds.
Q4. 2-Methylcyclohexanone is treated with methylmagnesium bromide (MeMgBr) followed by acidic workup. What is the major product?
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A. 1,2-dimethylcyclohexan-1-ol (a tertiary alcohol) ✓
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B. 2-methylcyclohexanol (a secondary alcohol)
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C. 1-phenylethanol
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D. 2-methylcyclohexene
Explanation: A Grignard reagent adds to the ketone carbonyl, and after protonation yields the tertiary alcohol 1,2-dimethylcyclohexan-1-ol, bearing the original ring methyl plus the new methyl on the former carbonyl carbon. The phenylethanol products are impossible without a phenyl reagent, and no dehydration step is specified.
Q5. Which of the following organometallic compounds is a ±-allyl complex?
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A. A complex with an allyl ligand, e.g., [Fe(C3H5)2Cl]. ✓
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B. A ferrocene complex, e.g., Fe(C₅H₅)2.
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C. A complex with a cyclopentadienyl ligand, e.g., C₅H₅Fe(CO)2.
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D. A complex with a diene ligand, e.g., C₄H₆Fe(CO)3.
Explanation: A ±-allyl complex contains an allyl ligand (C3H5) bonded to a metal center. The allyl ligand is a ±,3-dienyl group. Option A provides an example of such a complex.
Q6. The enantiomeric excess of a mixture of R- and S-2-butanol is given by ee = ([R] − [S]) / ([R] + [S]). Calculate the enantiomeric excess of a solution containing 0.50 M R-2-butanol and 0.25 M S-2-butanol.
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A. 0.17
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B. 0.25
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C. 0.33 ✓
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D. 0.50
Explanation: ee = (0.50 − 0.25) / (0.50 + 0.25) = 0.25 / 0.75 = 0.33, i.e. a 33% enantiomeric excess of the R enantiomer.
Q7. In organometallic chemistry, 'transmetalation' is best described as:
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A. the transfer of a ligand or organic group from one metal center to another ✓
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B. the oxidative addition of H2 to a metal center
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C. the reductive elimination of two ligands from one metal
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D. the substitution of a carbonyl ligand by a phosphine
Explanation: Transmetalation is a ligand-exchange step between two metal centers — the key step, for example, in Suzuki and Stille couplings, where an organic group moves from boron or tin onto palladium. Oxidative addition, reductive elimination, and simple CO substitution are distinct elementary reactions.
Q8. Which of the following organic compounds can act as a dienophile in a Diels-Alder reaction with 1,3-butadiene to form a cyclohexene derivative?
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A. trans-2-butene ✓
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B. cis-2-butene
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C. 2-methyl-1,3-butadiene
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D. 1,3-butadiene
Explanation: The Diels-Alder reaction is a [4+2] cycloaddition reaction between a diene (like 1,3-butadiene) and a dienophile. A dienophile is an alkene or alkyne that reacts with the diene. In this case, trans-2-butene can act as the dienophile, reacting with 1,3-butadiene to form a cyclohexene derivative.
Q9. Which of the following is a classic example of a π-allyl metal complex?
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A. The dimeric (η³-allyl)palladium chloride complex [(η³-C3H5)PdCl]2 ✓
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B. AlCl3
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C. TiCl4
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D. Ni(CO)4
Explanation: Allylpalladium chloride dimer is the textbook π-allyl complex: the allyl ligand binds through all three carbons (η³) to palladium, and it is central to Tsuji–Trost allylation chemistry. AlCl3 and TiCl4 are simple Lewis acids with no allyl ligand, and Ni(CO)4 contains only carbonyl ligands.
Q10. What is the major product of the reaction between 2-methyl-1-propanol and acetic anhydride in the presence of a strong acid catalyst?
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A. 2-methyl-2-acetoxypropane
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B. 2-methyl-1-acetoxypropane ✓
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C. 2-methyl-1-propanoyloxyacetate
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D. 2-methyl-2-propanoyloxyacetate
Explanation: The reaction is an example of an acetylation reaction, where the hydroxyl group of the alcohol is replaced by an acetoxy group. The strong acid catalyst protonates the hydroxyl group, making it a good leaving group. The acyl group of the acetic anhydride then attacks the protonated hydroxyl group, forming the acetoxy group and releasing a proton. The resulting product is 2-methyl-1-acetoxypropane.
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