ACS Organic Chemistry Practice MCQs
60 original exam-style multiple-choice questions with answers & explanations — covering every major topic tested on the ACS Organic Chemistry exam.
Note: These are original practice questions written in the ACS exam style — not reproduced exam questions. Use them to test your readiness for the real ACS exam.
📱 How to use: Try each question first, then click “Show Answer” to check. Covers: structure & bonding, stereochemistry, reactions, mechanisms, carbonyl chemistry, aromatics, spectroscopy and more.
🧬 Group A – Foundations & Stereochemistry
Q1What is the hybridization of the five carbon atoms in CH3–C≡C–CH=CH2, listed from left to right?
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Correct: (B)
The CH3 carbon is sp3 (four single bonds), each triply bonded carbon is sp (linear), and each doubly bonded carbon is sp2 (trigonal planar).
Q2In pyridine (C5H5N), what is the hybridization of the nitrogen atom, and in which type of orbital does its lone pair reside?
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Correct: (B)
The nitrogen is trigonal planar (sp2); its lone pair occupies an sp2 orbital in the ring plane, while its p orbital contributes one electron to the aromatic π system.
Q3What is the hybridization of the nitrogen atom in acetonitrile (CH3–C≡N), and in which orbital is its lone pair?
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Correct: (C)
The C≡N unit is linear, so nitrogen is sp hybridized; the lone pair occupies the remaining sp orbital directed away from the triple bond.
Q4Two staggered Newman projections of 2,3-dichlorobutane are viewed along the C2–C3 bond. Projection I has (2S,3S) configuration; Projection II has (2R,3R) configuration. What is the isomeric relationship between I and II?
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Correct: (B)
They are non-superimposable mirror images, hence enantiomers.
Q5A staggered Newman projection shows (2R,3R)-2,3-dichlorobutane with the two CH3 groups anti (180° apart). After rotation about the C2–C3 bond, the CH3 groups are gauche (60° apart). What is the relationship between the two projections?
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Correct: (C)
Both have the (2R,3R) configuration and differ only by rotation about a single bond; they are conformers of the same compound.
Q6Consider 2-chlorobutane drawn with the C2–H bond dashed (pointing away), the Cl atom on the right, the ethyl group pointing up, and the methyl group on the left. What is the absolute configuration at C2?
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Correct: (B)
Priorities are Cl (1) > ethyl (2) > methyl (3) > H (4); with H in the back, the 1→2→3 sequence traces counterclockwise, giving S.
Q7How many stereoisomers of 2,3-dichlorobutane exist?
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Correct: (B)
Two stereocenters allow four max, but (2R,3S) is identical to (2S,3R) — a meso compound — leaving the enantiomeric pair plus meso = 3.
Q8A mixture of (R)- and (S)-2-butanol shows an observed specific rotation of +6.75°. Pure (R)-2-butanol has [α]D = +13.5°. What is the enantiomeric excess and composition?
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Correct: (A)
ee = (6.75/13.5) × 100% = 50%; major enantiomer = (100+50)/2 = 75%.
Q9Which of the following compounds will rotate plane-polarized light?
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Correct: (B)
Only (2R,3R)-2,3-butanediol is chiral; the others are achiral.
Q10Rank the following alkenes from least to most stable: I. CH2=CH2; II. CH2=CHCH2CH3; III. trans-CH3CH=CHCH3; IV. (CH3)2C=CHCH3.
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Correct: (A)
Stability increases with alkyl substitution: unsubstituted < monosubstituted < disubstituted < trisubstituted.
Q11Which of the following alkenes is the most stable?
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Correct: (A)
1-Methylcyclohexene has a trisubstituted double bond; the others are disubstituted.
Q12Rank the following carbon radicals from least to most stable: I. CH3CH2CH2•; II. (CH3)2CH•; III. (CH3)3C•; IV. CH2=CH–CH2•.
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Correct: (A)
Primary < secondary < tertiary < allyl (resonance-stabilized).
Q13Which compound is most soluble in water?
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Correct: (C)
Two hydroxyl groups allow extensive hydrogen bonding with water.
Q14In the most stable conformation of cis-1,3-dimethylcyclohexane, the two methyl groups are:
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Correct: (B)
Ring-flip gives the diequatorial chair, avoiding 1,3-diaxial interactions.
Q15Which disubstituted cyclohexane CANNOT adopt a conformation with both substituents equatorial?
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Correct: (D)
trans-1,3 is always one-axial/one-equatorial.
⚙️ Group B – Reactions & Mechanisms
Q16Ozonolysis of an unknown alkene followed by reductive workup (Zn, H3O⁺) gives propanal (CH3CH2CHO) and acetone ((CH3)2CO). What was the starting alkene?
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Correct: (A)
Rejoining fragments: (CH3)2C=CHCH2CH3 = 2-methyl-2-pentene.
Q17Ozonolysis of an unknown alkene gives formaldehyde (HCHO) and 2-butanone (CH3COCH2CH3). What was the starting alkene?
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Correct: (A)
Terminal =CH2 + =C(CH3)(CH2CH3) gives CH2=C(CH3)CH2CH3.
Q18What is the major stereochemical outcome of hydroboration–oxidation (BH3, then H2O2/NaOH) of 1-methylcyclohexene?
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Correct: (A)
Syn addition, anti-Markovnikov; attack from either face gives racemic trans alcohol.
Q19What is the major product of the Diels–Alder reaction between cyclopentadiene and maleic anhydride?
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Correct: (A)
Normal Diels–Alder favors endo via secondary orbital interactions.
Q20Which diene/dienophile pair gives cis-4-cyclohexene-1,2-dicarboxylic acid via Diels–Alder?
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Correct: (A)
Cis-dienophile (maleic acid) gives cis product; fumaric (trans) would give trans.
Q21What mechanism best describes the reaction of tert-butyl bromide with methanol as solvent?
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Correct: (A)
Tertiary substrate + weak nucleophile + polar protic solvent = SN1.
Q22What is the major product of 1-bromobutane with NaCN in DMSO?
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Correct: (A)
Primary halide + strong nucleophile in polar aprotic = SN2 nitrile.
Q23What is the major alkene from E2 of 2-bromobutane with potassium tert-butoxide?
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Correct: (A)
Bulky base gives Hofmann (least substituted) alkene.
Q24What is the major product of 3,3-dimethyl-1-butene with HBr/peroxides?
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Correct: (A)
Radical addition: Br• adds to terminal carbon (anti-Markovnikov), no rearrangement.
Q25What is the major product of cyclohexene with Br2/H2O?
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Correct: (A)
Water opens bromonium ion anti to Br; racemic trans bromohydrin.
Q26What is the major alkene from E2 of 2-bromo-2-methylbutane with NaOEt?
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Correct: (A)
Small base gives Zaitsev (most substituted) alkene.
Q27What is the major product of methylmagnesium bromide + formaldehyde, then H3O⁺?
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Correct: (A)
Grignard + formaldehyde → primary alcohol (one carbon added).
Q28What is the major product of phenylmagnesium bromide + cyclohexanone, then H3O⁺?
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Correct: (A)
Grignard adds to ketone → tertiary alcohol after workup.
Q29What is the major product of the Wittig reaction of cyclohexanone with Ph3P=CH2?
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Correct: (A)
Wittig converts C=O to C=CH2 (exocyclic methylene).
Q30What is the major product of acid-catalyzed ring opening of 1,2-epoxypropane with CH3OH/H⁺?
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Correct: (A)
Acidic conditions: nucleophile attacks more substituted carbon.
🧪 Group C – Carbonyl & Aromatic Chemistry
Q31Benzaldehyde + acetaldehyde in dilute NaOH, then heat. Major product?
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Correct: (B)
Crossed aldol; benzaldehyde (no α-H) is electrophile only; dehydration gives cinnamaldehyde.
Q32Hexane-2,5-dione (CH3COCH2CH2COCH3) + NaOH, heat. Major product?
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Correct: (A)
Intramolecular aldol of 1,4-diketone → 5-membered ring enone.
Q33Diethyl adipate + NaOEt, then H3O⁺. Major product?
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Correct: (B)
Dieckmann condensation → 5-membered cyclic β-keto ester.
Q34Rank by increasing reactivity toward nucleophilic acyl substitution: I. CH3COCl II. (CH3CO)2O III. CH3COOCH3 IV. CH3CONH2
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Correct: (B)
Amide < ester < anhydride < acid chloride.
Q35Ethyl 4-oxopentanoate + 1 equiv NaBH4. Major product?
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Correct: (B)
NaBH4 reduces ketone only; ester survives.
Q364-Formylbenzoic acid + 1 equiv NaBH4. Major product?
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Correct: (A)
Only aldehyde reduced; carboxylic acid untouched.
Q37Which exists to greatest extent as hydrate in water?
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Correct: (C)
Formaldehyde ~99.9% hydrated; minimal sterics, highly electrophilic.
Q38Cyclohexanone + pyrrolidine, TsOH, –H2O. Major product?
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Correct: (B)
Secondary amine + ketone → enamine.
Q39Major product of anisole nitration (HNO3/H2SO4)?
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Correct: (C)
–OCH3 is strong o/p activator; para dominates (less steric hindrance).
Q40Acetanilide + Br2 in acetic acid. Major product?
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Correct: (C)
Acetamido is moderate o/p director; para dominates.
Q41Rank by increasing nitration rate: I. nitrobenzene II. benzene III. toluene IV. anisole
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Correct: (A)
Strongly deactivating < unsubstituted < weakly activating < strongly activating.
Q421-chloro-2,4-dinitrobenzene + NaOH, heat. Major product?
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Correct: (A)
o,p-nitro groups enable SNAr; Cl replaced by OH.
Q43Rank by increasing acidity: I. benzoic acid II. p-nitrobenzoic acid III. p-methylbenzoic acid IV. p-methoxybenzoic acid
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Correct: (B)
–NO2 strengthens; –CH3 and –OCH3 weaken (OMe most via resonance).
Q44Rank by increasing basicity: I. aniline II. cyclohexylamine III. p-nitroaniline
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Correct: (A)
p-NO2 weakest (electron withdrawal); cyclohexylamine strongest (no delocalization).
Q45Benzonitrile + LiAlH4, then H2O. Major product?
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Correct: (B)
LiAlH4 reduces nitrile fully to primary amine.
🔬 Group D – Spectroscopy, Polymers & More
Q46Which compound displays a septet in its ¹H NMR?
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Correct: (C)
Isopropyl CH couples to 6 equivalent H (n+1=7).
Q47C4H8O2, ¹H NMR: δ 8.05 (s, 1H), 4.15 (t, 2H), 1.65 (sextet, 2H), 0.95 (t, 3H). Compound?
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Correct: (C)
δ 8 singlet = formate H; rest = n-propyl on oxygen.
Q48¹H NMR of 1,3-dimethylbenzene (m-xylene) shows:
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Correct: (B)
Equivalent methyls (6H singlet); three distinct ring-H environments.
Q49C6H12O, four ¹³C signals including δ 210. Compound?
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Correct: (D)
δ 210 = ketone; only pinacolone has symmetry for 4 signals.
Q50M⁺ at m/z 120, base peak at m/z 105. Structure?
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Correct: (C)
Acetophenone (M=120) loses CH3• → C6H5CO⁺ at 105.
Q51Base peak in mass spectrum of 2-butanol?
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Correct: (C)
α-cleavage loses ethyl → CH3CH=OH⁺ at m/z 45.
Q52C4H8O, IR 1715 cm⁻¹, ¹H NMR δ 2.4 (q, 2H), 2.1 (s, 3H), 1.0 (t, 3H). Compound?
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Correct: (B)
Ketone C=O (no aldehyde H); pattern = CH3CH2COCH3.
Q53Which species is aromatic?
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Correct: (B)
Cyclic, planar, conjugated, 6 π electrons (Hückel).
Q54Which is antiaromatic?
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Correct: (C)
Planar, cyclic, 4 π electrons (4n).
Q55In normal Diels–Alder of butadiene + maleic anhydride, key orbital interaction?
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Correct: (B)
Normal electron-demand: diene HOMO → dienophile LUMO.
Q56Which monomer gives polymer –[CH2–CH(CN)]–ₙ?
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Correct: (A)
CH2=CHCN polymerizes head-to-tail.
Q57Polyamide hydrolysis gives adipic acid + 1,6-hexanediamine. Polyamide?
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Correct: (B)
6-carbon diacid + 6-carbon diamine = nylon-6,6.
Q58Major monobromide from free-radical bromination of 2-methylbutane?
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Correct: (B)
Br• highly selective for tertiary C–H.
Q59Which reagent converts 2-butyne to cis-2-butene?
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Correct: (B)
Lindlar’s gives syn addition → Z-alkene.
Q60Strongest Brønsted base?
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Correct: (D)
Conjugate base of weakest acid (acetylene, pKa ~25).
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