ACS Organic Chemistry Practice Test 17
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
Q1Propanal is treated with dilute aqueous NaOH at 0 °C, and the reaction is stopped before any dehydration can occur. What is the major organic product?
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Correct: (B)
The enolate of one propanal molecule adds to the carbonyl carbon of a second, giving the beta-hydroxy aldehyde (aldol addition product); dehydration to the enone requires heating.
Q2Benzaldehyde is heated with excess acetone and dilute NaOH. What is the major organic product?
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Correct: (D)
Benzaldehyde has no alpha-hydrogens, so it can only act as the electrophile in a crossed aldol with acetone; dehydration of the initial adduct gives benzalacetone (4-phenylbut-3-en-2-one).
Q3Ethyl acetate is treated with sodium ethoxide, followed by dilute aqueous acid workup. What is the major product?
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Correct: (A)
Self-Claisen condensation of ethyl acetate gives the beta-keto ester ethyl acetoacetate (ethyl 3-oxobutanoate).
Q4Diethyl hexanedioate (EtOOC-CH2-CH2-CH2-CH2-COOEt) undergoes Dieckmann cyclization with NaOEt. What size ring is formed in the major product?
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Correct: (C)
Intramolecular Claisen attack of the C2 enolate on the C6 ester closes a five-membered ring, giving ethyl 2-oxocyclopentanecarboxylate.
Q5Methyl benzoate is treated with excess methylmagnesium bromide, followed by aqueous acid workup. What happens?
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Correct: (B)
Grignard reagents add twice to esters: the first equivalent forms a ketone intermediate that is more reactive than the ester, so a second equivalent adds to give the tertiary alcohol after workup.
Q6A molecule contains both a ketone and an ester functional group. It is treated with one equivalent of NaBH4 in ethanol at 0 °C. What is the outcome?
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Correct: (A)
NaBH4 is a mild hydride donor that reduces aldehydes and ketones but is not reactive enough to reduce esters (LiAlH4 would reduce both).
Q7The non-stabilized ylide prepared from ethyltriphenylphosphonium bromide and n-BuLi reacts with benzaldehyde. What is the major alkene product?
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Correct: (C)
Non-stabilized Wittig ylides react under kinetic control through a cis-oxaphosphetane, giving predominantly the (Z)-alkene.
Q8Cyclohexanone is converted to its pyrrolidine enamine, treated with methyl iodide, then hydrolyzed with dilute aqueous acid. What is the major product?
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Correct: (D)
Stork enamine alkylation occurs at the alpha-carbon of the enamine; acidic hydrolysis regenerates the ketone, giving the alpha-methylated product.
Q9A ketone is protected as its ethylene ketal so that a Grignard reagent can be used elsewhere in the molecule. Which conditions remove the ketal protecting group at the end of the sequence?
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Correct: (B)
Ketals are stable to base, nucleophiles, and hydride reagents but are cleaved by aqueous acid, regenerating the carbonyl compound.
Q10Which of the following aldehydes forms the most stable hydrate in water?
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Correct: (A)
The strongly electron-withdrawing CCl3 group destabilizes the carbonyl pi bond, shifting the hydration equilibrium far toward chloral hydrate; hydrate stability otherwise decreases with increasing alkyl substitution.
Q11Benzaldehyde is treated with concentrated aqueous NaOH. What are the organic products?
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Correct: (C)
Aldehydes with no alpha-hydrogens undergo Cannizzaro disproportionation in concentrated base: one molecule is reduced to the alcohol while another is oxidized to the carboxylate.
Q12Cyclohex-2-en-1-one is treated with NaBH4 and CeCl3 in methanol (Luche conditions). What is the major product?
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Correct: (D)
Luche reduction delivers selective 1,2-reduction of alpha,beta-unsaturated ketones, giving the allylic alcohol while leaving the C=C bond intact.
Q132-Butanone is treated with LDA at -78 °C, followed by methyl iodide. What is the major product?
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Correct: (B)
LDA at -78 °C is a bulky base that irreversibly removes the less hindered C1 proton, giving the kinetic enolate; methylation at C1 gives CH3CH2COCH2CH3 (pentan-3-one).
Q14Ethyl acetoacetate is treated with NaOEt and then ethyl bromide; the alkylated product is hydrolyzed with aqueous NaOH and heated. What is the final organic product?
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Correct: (A)
Alkylation gives CH3COCH(Et)COOEt; hydrolysis followed by heating causes decarboxylation of the beta-keto acid, leaving the methyl ketone CH3COCH2CH2CH3.
⚙ Substitution & Elimination
Q15Which statement about substitution reaction rate laws is correct?
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Correct: (C)
SN2 has a bimolecular rate-determining step (rate = k[RX][Nu-]), while the SN1 rate-determining ionization involves only the substrate (rate = k[RX]).
Q16Rank the following ions in order of decreasing leaving-group ability: F-, Cl-, Br-, I-.
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Correct: (A)
Leaving-group ability parallels the inverse of basicity: the weakest bases (conjugate bases of the strongest acids, HI > HBr > HCl > HF) are the best leaving groups.
Q17For the SN2 reaction CH3I + CN- -> CH3CN + I-, which solvent gives the fastest reaction rate?
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Correct: (D)
Polar aprotic solvents such as DMSO solvate the cation but leave the anionic nucleophile poorly solvated and highly reactive; protic solvents hydrogen-bond to CN- and slow SN2 attack.
Q182-Bromobutane is treated with potassium tert-butoxide (t-BuOK) in tert-butanol. What is the major alkene product?
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Correct: (B)
The bulky tert-butoxide base preferentially abstracts the least hindered beta-hydrogen, giving the Hofmann (less substituted) alkene as the major E2 product.
Q193,3-Dimethylbutan-2-ol is heated with concentrated H2SO4. What is the major alkene product?
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Correct: (C)
Loss of water gives a secondary carbocation that undergoes a methyl shift to the more stable tertiary carbocation; deprotonation then gives the most substituted (Zaitsev) alkene, the tetrasubstituted 2,3-dimethylbut-2-ene.
Q20(R)-2-Bromobutane reacts with NaOH in DMSO. What is the stereochemical outcome at C2?
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Correct: (A)
SN2 proceeds by backside attack of the nucleophile, which inverts the configuration at the stereocenter.
Q21Optically pure (R)-3-chloro-3-methylhexane is solvolyzed in aqueous ethanol. What is the stereochemistry of the alcohol product?
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Correct: (B)
Ionization gives a planar, achiral tertiary carbocation that is attacked by water from either face with equal probability, producing a racemic mixture (SN1).
Q22For an E2 elimination to occur readily, what geometric relationship must the departing C-H and C-X bonds have?
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Correct: (D)
The C-H sigma bond must align anti-periplanar to the C-X bond so its electrons can donate into the C-X sigma* orbital as the double bond forms.
Q23Which substrate/nucleophile/solvent combination reacts predominantly by the SN2 mechanism?
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Correct: (C)
A primary alkyl halide with a strong nucleophile in a polar aprotic solvent is the classic SN2 setup; the other options favor SN1, E2, or SN1/E1 pathways.
🔬 Stereochemistry
Q24Consider a wedge-dash drawing of butan-2-ol in which the C2 hydroxyl group is on a solid wedge, the C2 hydrogen is on a hashed wedge, the methyl group is drawn to the left, and the ethyl group is drawn to the right. What is the absolute configuration at C2?
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Correct: (B)
The lowest-priority H points away on the dashed bond; priorities are OH (1) > ethyl (2) > methyl (3), and 1->2->3 traces a clockwise circle, giving R.
Q25The Fischer projection of butan-2-ol is drawn with CH3 at the top, CH2CH3 at the bottom, OH on the right, and H on the left. Which stereoisomer is shown?
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Correct: (A)
In a Fischer projection the horizontal bonds project forward, so the lowest-priority H is coming out toward the viewer; the apparent clockwise 1->2->3 order (OH->ethyl->methyl) must therefore be inverted, giving S.
Q26Which of the following compounds is a meso compound?
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Correct: (D)
(2R,3S)-butane-2,3-diol has identical ends and opposite configurations at the two stereocenters, giving an internal plane of symmetry that makes it achiral.
Q27One bottle contains (2R,3R)-2,3-dichlorobutane and a second bottle contains meso-(2R,3S)-2,3-dichlorobutane. What is the stereochemical relationship between the contents of the two bottles?
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Correct: (C)
Both have the same connectivity and are stereoisomers, but they are not non-superimposable mirror images, so they are diastereomers.
Q28A sample of 2-butanol shows an observed specific rotation of +6.75°. A reference table gives [α]D = +13.5° for enantiopure (S)-2-butanol. What is the enantiomeric excess of the sample?
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Correct: (A)
ee = (observed rotation / rotation of pure enantiomer) x 100% = (6.75/13.5) x 100% = 50%, and the positive sign matches the (S) enantiomer.
Q29In Newman projections of butane, what is the relationship between the anti conformation and a gauche conformation?
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Correct: (D)
They have the same connectivity but different, non-mirror-image spatial arrangements, interconvertible by rotation about the C2-C3 bond, so they are diastereomeric conformers.
Q30Which of the following molecules is optically active?
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Correct: (B)
trans-1,2-Dimethylcyclopropane is chiral: it has no plane of symmetry and exists as a pair of enantiomers, so it rotates plane-polarized light; each of the other molecules has a plane of symmetry and is achiral.
🧬 Alkenes & Alkynes
Q31Ozonolysis of 2-methyl-2-butene, (CH3)2C=CHCH3, is carried out with (i) Zn, H2O (reductive workup) and separately with (ii) H2O2, H2O (oxidative workup). The products in each case are, respectively:
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Correct: (B)
Cleavage of the trisubstituted double bond gives acetone from the (CH3)2C= carbon in both cases; the =CHCH3 carbon gives acetaldehyde under reductive conditions and is further oxidized to acetic acid under oxidative conditions.
Q32Treatment of 1-methylcyclohexene with Br2 in water gives mainly:
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Correct: (D)
Br2 forms a cyclic bromonium ion; water attacks the more substituted carbon from the back side, giving anti addition with the OH on the more substituted carbon.
Q33Hydroboration-oxidation of 1-methylcyclohexene, (1) BH3/THF then (2) H2O2, NaOH, gives mainly:
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Correct: (A)
Boron adds syn to the less substituted carbon and H to the more substituted carbon; oxidation replaces B with OH with retention, placing the OH trans to the methyl group.
Q341-Butene is treated with (1) Hg(OAc)2, H2O, then (2) NaBH4. The major organic product is:
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Correct: (C)
Oxymercuration-demercuration adds water with Markovnikov regiochemistry (OH on the more substituted carbon) and proceeds without carbocation rearrangement.
Q35The major product of the Diels-Alder reaction between cyclopentadiene and maleic anhydride is:
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Correct: (B)
Secondary orbital interactions between the diene pi system and the anhydride carbonyls stabilize the endo transition state (the endo rule).
Q363-Hexyne is treated with sodium metal in liquid ammonia. The major product is:
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Correct: (D)
Dissolving-metal reduction of an alkyne proceeds by anti addition of two hydrogens, giving the trans (E) alkene.
Q37Propene reacts with HBr in the presence of peroxides to give product X, and with HBr in the absence of peroxides to give product Y. X and Y are, respectively:
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Correct: (A)
Peroxides initiate a radical chain giving anti-Markovnikov addition (Br to the less substituted carbon); without peroxides, ionic Markovnikov addition places Br on the more substituted carbon.
💧 Aromatic Chemistry
Q38Which of the following substituents is a meta director in electrophilic aromatic substitution?
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Correct: (C)
The nitro group is strongly deactivating and directs incoming electrophiles to the meta position; -OH, -NH2, and -OCH3 are activating ortho/para directors.
Q39Rank the following compounds in order of decreasing rate of nitration (fastest first): toluene, benzene, nitrobenzene.
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Correct: (B)
The methyl group is activating and speeds EAS relative to benzene, while the nitro group is strongly deactivating and slows it.
Q40Which compound undergoes nucleophilic aromatic substitution with aqueous NaOH most readily?
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Correct: (D)
SNAr requires strong electron-withdrawing groups ortho and/or para to the leaving group to stabilize the anionic Meisenheimer intermediate.
Q41Which of the following Friedel-Crafts reactions will NOT give the expected substitution product?
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Correct: (A)
Friedel-Crafts reactions fail on strongly deactivated rings such as nitrobenzene, which does not react with the electrophile.
Q42Benzenediazonium chloride is treated with CuCN. The major organic product is:
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Correct: (C)
The Sandmeyer reaction replaces the diazonium group with cyanide, giving benzonitrile with loss of nitrogen gas.
Q43Toluene is heated with KMnO4 and the mixture is then acidified. The major organic product is:
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Correct: (D)
Vigorous oxidation with hot KMnO4 oxidizes any alkyl side chain on the ring completely to a carboxylic acid group.
🔍 Spectroscopy
Q44A compound with molecular formula C4H10O shows a 1H NMR spectrum with a 9H singlet at 1.25 ppm and a broad 1H singlet at 2.1 ppm that disappears on D2O shake. The compound is:
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Correct: (B)
Nine equivalent protons indicate three identical methyl groups on a quaternary carbon, and the D2O-exchangeable proton is the alcohol OH.
Q45How many signals appear in the proton-decoupled 13C NMR spectrum of p-xylene (1,4-dimethylbenzene)?
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Correct: (B)
The D2h symmetry of p-xylene makes the two methyl carbons equivalent, the two ipso ring carbons equivalent, and all four ring CH carbons equivalent, giving only three distinct carbon environments.
Q46An IR spectrum shows a strong absorption near 1715 cm-1 and a very broad absorption centered near 3000 cm-1, with no bands in the 2700-2800 cm-1 region. The compound is most likely:
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Correct: (C)
The strong band is a carbonyl stretch and the broad band is a hydrogen-bonded O-H stretch; the absence of aldehydic C-H bands near 2720 and 2820 cm-1 rules out an aldehyde.
Q47The mass spectrum of 1-bromopropane shows molecular ion peaks at m/z 122 and m/z 124 with approximately equal intensity. This pattern occurs because:
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Correct: (D)
The nearly 1:1 natural abundance of 79Br and 81Br gives the characteristic M and M+2 pair for any bromine-containing ion.
Q48A compound C3H6O shows a strong IR band near 1715 cm-1 with no O-H absorption, and its 1H NMR shows a 3H triplet at 1.1 ppm, a 2H quartet at 2.5 ppm, and a 1H singlet at 9.7 ppm. The compound is:
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Correct: (B)
The 9.7 ppm singlet is an aldehyde proton, and the triplet-quartet pattern is an ethyl group, identifying CH3CH2CHO.
📐 Conformations & Bonding
Q49In the more stable chair conformation of methylcyclohexane, the methyl group occupies the:
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Correct: (C)
The equatorial position avoids 1,3-diaxial steric interactions, making it about 1.7 kcal/mol more stable than the axial conformer.
Q50In allene, CH2=C=CH2, the hybridization of the central carbon atom is:
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Correct: (A)
The central carbon forms two double bonds (two sigma plus two pi bonds), which requires sp hybridization with two unhybridized p orbitals.
Q51Which of the following species is antiaromatic?
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Correct: (D)
Cyclobutadiene is planar and cyclic with 4 pi electrons (a 4n system), making it antiaromatic, while the others are 6 pi-electron aromatic systems.
Q52The nitrogen atom in acetamide, CH3CONH2, is best described as:
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Correct: (B)
Delocalization of the nitrogen lone pair into the carbonyl pi system makes the nitrogen planar with sp2 hybridization.
🧫 Acids & Bases
Q53Rank the following acids from strongest to weakest: acetic acid, chloroacetic acid, trichloroacetic acid.
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Correct: (A)
Each additional electron-withdrawing chlorine stabilizes the carboxylate conjugate base through the inductive effect, lowering the pKa.
Q54Rank the following bases from strongest to weakest: amide ion (NH2-), hydroxide ion (OH-), acetate ion (CH3COO-).
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Correct: (C)
Base strength increases as the conjugate acid gets weaker: NH3 (pKa ~35) > H2O (pKa 15.7) > CH3COOH (pKa 4.8).
Q55Which of the following is the strongest base? (pKa of the conjugate acid is given in parentheses.)
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Correct: (D)
The strongest base has the weakest conjugate acid, i.e., the highest conjugate-acid pKa.
⚡ Radicals
Q56Rank the following carbon radicals from most stable to least stable: tert-butyl radical, benzyl radical, isopropyl radical, methyl radical.
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Correct: (B)
Resonance stabilization of the benzyl radical outweighs hyperconjugation, which then follows the tertiary > secondary > primary order.
Q57In the free-radical halogenation of 2-methylpropane, which statement is correct?
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Correct: (A)
Hydrogen abstraction by Br radical is endothermic with a late transition state, so it discriminates strongly between C-H bond strengths, unlike the highly reactive Cl radical.
Q58In the free-radical chlorination of methane, the step Cl radical + CH4 -> HCl + CH3 radical is an example of:
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Correct: (C)
A radical is consumed and a new radical is generated, so the chain continues; initiation creates radicals and termination destroys them.
🔗 Polymers
Q59A polymer has the repeat unit -[CH2-CH(Cl)]n-. The monomer used to make it is:
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Correct: (D)
Addition polymerization of vinyl chloride (chloroethene, CH2=CHCl) gives poly(vinyl chloride) with this repeat unit.
Q60Nylon-6,6 is formed by:
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Correct: (B)
Nylon-6,6 is a polyamide made by step-growth condensation of hexamethylenediamine and adipic acid, eliminating water at each amide linkage.
ANSWER KEY TEST 17: Q1 B, Q2 D, Q3 A, Q4 C, Q5 B, Q6 A, Q7 C, Q8 D, Q9 B, Q10 A, Q11 C, Q12 D, Q13 B, Q14 A, Q15 C, Q16 A, Q17 D, Q18 B, Q19 C, Q20 A, Q21 B, Q22 D, Q23 C, Q24 B, Q25 A, Q26 D, Q27 C, Q28 A, Q29 D, Q30 B, Q31 B, Q32 D, Q33 A, Q34 C, Q35 B, Q36 D, Q37 A, Q38 C, Q39 B, Q40 D, Q41 A, Q42 C, Q43 D, Q44 B, Q45 B, Q46 C, Q47 D, Q48 B, Q49 C, Q50 A, Q51 D, Q52 B, Q53 A, Q54 C, Q55 D, Q56 B, Q57 A, Q58 C, Q59 D, Q60 B
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