ACS Organic Chemistry Practice Test 6
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: carbonyl chemistry, substitution & elimination, stereochemistry, alkenes & alkynes, aromatics, spectroscopy and more.
🧪 Carbonyl Chemistry
Q1The kinetic lithium enolate of 2-butanone (LDA, THF, −78 °C) is treated with pivaldehyde, (CH3)3C–CHO, followed by aqueous workup. What is the major product?
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Correct: (B)
LDA removes the less hindered C1 proton of 2-butanone, and the kinetic enolate attacks the aldehyde carbonyl; dehydration does not occur at −78 °C.
Q2Heptane-2,6-dione (CH3COCH2CH2CH2COCH3) is heated with dilute NaOH. What is the major organic product?
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Correct: (A)
Intramolecular aldol of the 1,5-diketone closes a six-membered ring; dehydration gives the conjugated cyclohexenone.
Q3Ethyl acetate is treated with sodium ethoxide in ethanol, followed by acid workup. The major product is:
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Correct: (A)
Classic Claisen self-condensation of an ester with α-hydrogens gives ethyl acetoacetate.
Q4Diethyl pimelate (EtOOC–CH2–CH2–CH2–CH2–CH2–COOEt) reacts with NaOEt in EtOH, then H3O+. The major product is:
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Correct: (B)
Intramolecular Dieckmann condensation of the seven-carbon diester closes a six-membered cyclic β-keto ester.
Q5Methyl benzoate is treated with excess phenylmagnesium bromide, then aqueous NH4Cl. The major product is:
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Correct: (B)
Two successive Grignard additions occur: ester → ketone → tertiary alcohol.
Q6Ethyl 4-acetylbenzoate (para-CH3–CO–C6H4–COOEt) is treated with NaBH4 in methanol. The major product is:
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Correct: (A)
NaBH4 reduces the ketone but is too mild to reduce the ester.
Q7Cyclohexanone reacts with methylenetriphenylphosphorane (Ph3P=CH2). The major product is:
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Correct: (D)
The Wittig reaction converts the carbonyl carbon into an exocyclic =CH2 group.
Q8Cyclopentanone is refluxed with pyrrolidine and catalytic p-TsOH with continuous water removal. The major product is:
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Correct: (C)
A ketone plus a secondary amine under dehydrating conditions gives the enamine.
Q94-Methylbenzaldehyde (p-CH3–C6H4–CHO) is treated with ethylene glycol (HOCH2CH2OH) and catalytic TsOH with water removal. The major product is:
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Correct: (B)
Acid-catalyzed reaction of an aldehyde with a 1,2-diol gives the cyclic acetal.
Q10Lithium dimethylcuprate, (CH3)2CuLi, is added to cyclohex-2-en-1-one at −78 °C, then H3O+ workup. The major product is:
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Correct: (D)
Gilman reagents undergo conjugate (1,4) addition to enones; protonation of the enolate gives the saturated ketone.
Q11Benzonitrile (Ph–C≡N) is treated with LiAlH4, then water. The major product is:
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Correct: (A)
LiAlH4 reduces nitriles completely to primary amines.
Q12A chemist wants to add exactly one equivalent of methylmagnesium bromide to the ketone of 5-hydroxy-2-pentanone (HO–CH2CH2CH2–CO–CH3) to make the tertiary alcohol, without interference from the –OH group. The best sequence is:
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Correct: (C)
The acidic –OH must be protected before Grignard addition; protecting the ketone instead (D) blocks the desired reaction site. Option A fails because NaH deprotonation followed by only one equivalent of Grignard leaves the ketone unreacted.
Q13Which compound exists predominantly as its hydrate (gem-diol) in aqueous solution?
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Correct: (B)
The strongly electron-withdrawing CCl3 group destabilizes the carbonyl and drives hydrate formation.
Q14The best reagents to convert cyclohexanone into 2-allylcyclohexanone (mono-allylated at the α-carbon) are:
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Correct: (D)
The kinetic lithium enolate gives clean mono-α-alkylation; NaOH leads to polyalkylation and O-alkylation.
⚙ Substitution & Elimination
Q15The hydrolysis of tert-butyl bromide in aqueous acetone proceeds by which mechanism?
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Correct: (C)
A tertiary substrate with a weak nucleophile in polar protic solvent ionizes by SN1.
Q16For the SN2 reaction CH3I + CN− → CH3CN + I−, the rate law is rate = k[CH3I][CN−]. If [CH3I] is doubled and [CN−] is halved, the rate will:
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Correct: (A)
The rate depends on both concentrations: 2 × 1/2 = 1, so the rate is unchanged.
Q172-Bromo-2-methylbutane is treated with potassium tert-butoxide (a bulky base). The major alkene product is:
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Correct: (D)
The bulky base abstracts the less hindered primary β-hydrogen, giving the Hofmann (least substituted) alkene.
Q18(R)-2-Bromobutane reacts with NaOH in an SN2 reaction. The configuration of the 2-butanol product is:
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Correct: (B)
SN2 proceeds with backside attack and complete inversion of configuration.
Q19(S)-2-Bromobutane undergoes solvolysis in aqueous acetone (SN1 conditions). The 2-butanol obtained is:
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Correct: (C)
The planar carbocation is attacked from either face, giving a racemic mixture.
Q20For E2 dehydrohalogenation, the C–H and C–X bonds that break must be oriented:
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Correct: (A)
E2 requires anti-periplanar alignment for proper orbital overlap in the transition state.
Q21Which alkyl chloride undergoes SN1 solvolysis fastest in aqueous ethanol?
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Correct: (D)
The tertiary chloride forms the most stable carbocation in the rate-determining step.
Q222-Methyl-2-butanol is heated with concentrated H2SO4. The major alkene is:
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Correct: (B)
E1 dehydration gives the most substituted (Zaitsev) alkene.
Q23The SN2 reaction CH3Br + N3− → CH3N3 + Br− is run in methanol. Changing the solvent to DMF will:
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Correct: (C)
Polar aprotic DMF solvates the anion poorly, leaving the nucleophile more reactive.
🔬 Stereochemistry
Q24The C2 stereocenter of 2-butanol is drawn with H on a hashed wedge (behind the plane), OH on a solid wedge pointing to the right, CH3 in the plane to the upper left, and CH2CH3 in the plane to the lower left. What is the absolute configuration?
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Correct: (D)
Priorities: OH (1) > CH2CH3 (2) > CH3 (3); with H in back, 1→2→3 traces clockwise.
Q25The stereochemical relationship between (2R,3R)-butane-2,3-diol and (2R,3S)-butane-2,3-diol is:
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Correct: (B)
The (2R,3S) form is meso and is a diastereomer of the chiral (2R,3R) form.
Q26Which of the following is a meso compound?
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Correct: (A)
It has an internal plane of symmetry, making it achiral despite two stereocenters.
Q27Which Newman projection represents the most stable conformation of butane (viewed along the C2–C3 bond)?
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Correct: (C)
The anti staggered conformation places the two methyl groups farthest apart, minimizing steric strain.
Q28A sample of 2-butanol shows [α]D = +6.75°. Pure (R)-2-butanol has [α]D = +13.5°. The sample's enantiomeric excess and composition are:
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Correct: (D)
ee = 6.75/13.5 = 50%, so the major enantiomer is (100 + 50)/2 = 75% (R).
Q29Which compound is optically inactive?
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Correct: (A)
The meso diol has a plane of symmetry and does not rotate plane-polarized light.
Q30In 1-chloro-1-fluoroethane (CH3–CHClF), the stereocenter is drawn with H on a hashed wedge (back), Cl in the plane to the left, F in the plane to the right, and CH3 on a solid wedge pointing down and toward the viewer. The absolute configuration is:
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Correct: (B)
Priorities: Cl (1) > F (2) > CH3 (3); with H in back and CH3 on the down-pointing wedge, the 1→2→3 sequence (Cl → F → CH3) traces counterclockwise, giving (S).
🧬 Alkenes & Alkynes
Q31Rank the following alkenes by stability, most stable first: I. 1-butene; II. 2-methyl-2-butene; III. 2-methyl-1-butene; IV. trans-2-butene.
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Correct: (C)
Stability increases with substitution (trisubstituted > trans-disubstituted > terminal disubstituted > monosubstituted).
Q322-Methyl-2-butene undergoes ozonolysis (O3; then Zn, H2O). The products are:
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Correct: (A)
Cleavage of (CH3)2C=CHCH3 gives (CH3)2C=O and CH3CHO.
Q33Ozonolysis (O3; Zn, H2O) of an unknown alkene gives only acetone and formaldehyde. The alkene was:
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Correct: (D)
Rejoining the fragments (CH3)2C=O and HCHO gives (CH3)2C=CH2.
Q341-Methylcyclopent-1-ene is treated with 1) BH3·THF; 2) H2O2, NaOH. The major product is:
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Correct: (B)
Hydroboration–oxidation adds OH to the less substituted carbon by syn addition, placing OH trans to the methyl group.
Q35Cyclohexene reacts with Br2 in water. The major product is:
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Correct: (C)
Water opens the bromonium ion anti to bromine, giving the trans halohydrin.
Q361-Butene reacts with HBr in the presence of peroxides (ROOR). The major product is:
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Correct: (A)
The peroxide effect gives anti-Markovnikov addition via a radical mechanism.
Q37Which reagent converts 3-hexyne to trans-3-hexene?
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Correct: (D)
Dissolving-metal reduction of alkynes gives the trans alkene; Lindlar's catalyst gives the cis alkene.
💧 Aromatic Chemistry
Q38Nitration (HNO3/H2SO4) of acetanilide (Ph–NHCOCH3) gives mainly:
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Correct: (B)
The –NHCOCH3 group is ortho/para activating, and para substitution predominates for steric reasons.
Q39Rank the following by rate of electrophilic bromination, fastest first: I. anisole; II. toluene; III. benzene; IV. nitrobenzene.
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Correct: (D)
Strongly activating –OCH3 > weakly activating –CH3 > H > deactivating –NO2.
Q40Benzene is subjected to: 1) HNO3, H2SO4; 2) Sn, HCl; 3) NaNO2, HCl, 0 °C; 4) CuCN. The final product is:
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Correct: (A)
Nitration, reduction to the amine, diazotization, and Sandmeyer reaction with CuCN give Ph–C≡N.
Q412,4-Dinitrochlorobenzene reacts with aqueous NaOH (heat). The major product is:
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Correct: (C)
The ortho and para nitro groups strongly activate the ring toward nucleophilic aromatic substitution (SNAr).
Q42Which is the strongest acid?
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Correct: (B)
The electron-withdrawing nitro group stabilizes the carboxylate anion by resonance and induction.
Q43Toluene undergoes: 1) Br2, FeBr3; 2) HNO3, H2SO4. The major final product is:
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Correct: (D)
Bromination gives mainly p-bromotoluene; the stronger activator –CH3 then directs nitration ortho to itself.
🔍 Spectroscopy
Q44A compound C4H8O2 shows 1H-NMR: δ 1.2 (triplet, 3H), δ 2.0 (singlet, 3H), δ 4.1 (quartet, 2H). It is:
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Correct: (A)
The singlet (acetyl CH3), downfield quartet (O–CH2), and triplet (CH3) fit CH3COOCH2CH3 only.
Q45How many signals appear in the proton-decoupled 13C-NMR spectrum of p-xylene?
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Correct: (C)
Symmetry makes the two ipso carbons, the four ring CH carbons, and the two methyl carbons each equivalent: 3 signals.
Q46A mass spectrum shows a molecular ion at m/z 156 and an M+2 peak at m/z 158 of nearly equal intensity. This pattern indicates:
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Correct: (B)
The ~1:1 ratio matches the natural abundances of 79Br and 81Br.
Q47A compound C7H6O2 shows IR bands: broad 2500–3300 cm−1 and strong 1710 cm−1; 1H-NMR: δ 11.5 (singlet, 1H), δ 7.4–8.1 (multiplet, 5H). It is:
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Correct: (D)
The broad O–H stretch, carbonyl stretch, and far-downfield exchangeable proton are diagnostic of a carboxylic acid.
Q48A compound C4H10O shows 1H-NMR: δ 0.9 (doublet, 6H), δ 1.8 (multiplet, 1H), δ 3.4 (doublet, 2H), plus a broad singlet (1H) that disappears on D2O shake. It is:
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Correct: (A)
The 6H doublet + 1H multiplet + 2H doublet pattern plus an exchangeable proton fits (CH3)2CHCH2OH.
📐 Conformations & Bonding
Q49The most stable conformation of cis-1,3-dimethylcyclohexane has the two methyl groups:
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Correct: (B)
Ring flip interconverts the diaxial and diequatorial chairs; the diequatorial conformer avoids 1,3-diaxial strain.
Q50What is the hybridization of the nitrogen atom in pyridine?
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Correct: (D)
The nitrogen is trigonal planar with its lone pair in an sp2 orbital and one p electron in the aromatic π system.
Q51Which species is aromatic according to Hückel's rule?
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Correct: (C)
It is cyclic, planar, conjugated, and has 6 π electrons (4n + 2, n = 1).
Q52Which compound is antiaromatic?
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Correct: (A)
It is a planar, conjugated ring with 4 π electrons (4n), making it antiaromatic and highly unstable.
🧫 Acids & Bases
Q53Rank the following by acidity, strongest acid first: I. phenol; II. acetic acid; III. ethanol; IV. p-nitrophenol.
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Correct: (D)
pKa values: acetic acid (~4.8) < p-nitrophenol (~7.1) < phenol (~10.0) < ethanol (~16).
Q54Rank the following by basicity in water, strongest base first: I. aniline; II. p-nitroaniline; III. cyclohexylamine; IV. pyridine.
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Correct: (B)
Alkylamines are more basic than pyridine, which is more basic than aniline, which is more basic than p-nitroaniline.
Q55Which of the following is the strongest Brønsted base?
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Correct: (C)
Its conjugate acid NH3 (pKa ~35) is the weakest acid listed, so NH2− is the strongest base.
⚡ Radicals
Q56Rank the following radicals by stability, most stable first: I. methyl; II. tert-butyl; III. allyl; IV. benzyl.
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Correct: (A)
Benzyl (extended resonance) > allyl (resonance) > tertiary (hyperconjugation) > methyl.
Q57Free-radical chlorination of 2-methylbutane (Cl2, hν) gives as the major monochloride:
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Correct: (D)
The tertiary C–H bond is abstracted fastest, giving the tertiary chloride as the major product.
Q58In the free-radical polymerization of styrene, the chain-propagation step consists of:
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Correct: (C)
Propagation is repeated radical addition to monomer; (A) describes termination and (B) describes initiation.
🔗 Polymers
Q59The monomer used to make poly(vinyl chloride), PVC, is:
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Correct: (B)
CH2=CHCl undergoes addition polymerization to give –[CH2–CHCl]n–.
Q60Nylon-6,6 is a condensation polymer prepared from:
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Correct: (A)
The diamine plus the diacid condense to the polyamide; caprolactam gives nylon-6 and (C) gives PET.
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