Organic chemistry questions

10 Organic Chemistry MCQs (Click "Show Answer" for explanation)

Q1. Which compound is aromatic?

(A) Cyclohexane (B) 1,3-Cyclopentadiene (C) Cyclopentadienyl anion (C₅H₅⁻) (D) Cyclohexadiene

Answer: (C)

Cyclopentadienyl anion has 6 π electrons (4n+2, n=1) and is planar & fully conjugated — aromatic. Others are non-aromatic or not fulfilling Huckel rule.

Q2. Which reagent will convert a primary alcohol to an aldehyde without further oxidation to carboxylic acid?

(A) KMnO₄ (acidic) (B) PCC (Pyridinium chlorochromate) (C) HNO₃ (D) Jones reagent (CrO₃/H₂SO₄)

Answer: (B)

PCC oxidizes primary alcohols to aldehydes without overoxidation to carboxylic acids in non-aqueous conditions.

Q3. In an SN2 reaction, which of the following statements is true?

(A) Reaction rate depends on concentration of nucleophile only (B) Reaction shows inversion of configuration at stereocenter (C) Occurs through a carbocation intermediate (D) Favored by polar protic solvents

Answer: (B)

SN2 is bimolecular (rate depends on nucleophile and substrate), proceeds via backside attack causing inversion, does not involve carbocation, and is favored by polar aprotic solvents.

Q4. Which of these is the correct IUPAC name for CH₃-CH=CH-CH₂-OH?

(A) 1-Butanol (B) 2-Butanol (C) 3-Buten-1-ol (D) 3-Buten-2-ol

Answer: (C)

Numbering gives the alcohol lowest possible locant in presence of double bond: chain = but-3-en-1-ol (commonly written 3-buten-1-ol).

Q5. Which mechanism explains formation of tert-butyl carbocation from tert-butyl chloride in presence of solvent?

(A) SN2 (B) E2 (C) SN1 (D) Radical substitution

Answer: (C)

Tert-butyl chloride forms a stable tertiary carbocation by ionization; SN1 (unimolecular) mechanism proceeds via carbocation intermediate.

Q6. Which of the following molecules has a chiral center?

(A) 2-Butanol (CH₃-CH(OH)-CH₂-CH₃) (B) 1-Butanol (C) 2-Butanone (D) 1-Butene

Answer: (A)

2-Butanol has a carbon bonded to -OH, -CH₃, -CH₂CH₃, and H — four different groups → chiral center.

Q7. Which reagent is commonly used for selective hydrogenation of an alkyne to cis-alkene?

(A) H₂/Pd-C (B) H₂/Lindlar's catalyst (C) Na/NH₃ (liquid) (D) KMnO₄/heat

Answer: (B)

Lindlar's catalyst hydrogenates alkynes to cis (Z) alkenes selectively. Na/NH₃ gives trans alkenes.

Q8. Which structure best describes benzyl carbocation stability?

(A) More stable than tertiary carbocation (B) Less stable than methyl carbocation (C) Stabilized by resonance with aromatic ring (D) Cannot exist

Answer: (C)

Benzyl carbocation (Ph-CH₂⁺) is resonance-stabilized by the aromatic ring, making it very stable (often comparable to tertiary carbocations).

Q9. Which functional group has the highest priority in IUPAC parent naming?

(A) Alcohol (-OH) (B) Aldehyde (-CHO) (C) Carboxylic acid (-COOH) (D) Amine (-NH₂)

Answer: (C)

Carboxylic acids have higher suffix priority for parent naming than aldehydes, alcohols, and amines.

Q10. Which statement about resonance is correct?

(A) Resonance structures are real separate species in solution (B) The true structure is a hybrid of resonance contributors (C) Resonance increases the formal charge magnitude always (D) Resonance applies only to aromatic compounds

Answer: (B)

Resonance structures are contributors; the actual molecule is a resonance hybrid — a weighted average that usually increases stability.

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