CHE 417 ORGANIC CHEMISTRY
MIDTERM EXAM
Advanced Organic Synthesis Reaction Mechanisms Spectroscopy
Stereochemistry Questions and Answers Verified Solutions 2026 2027
1. Which mechanistic path dominates when a tertiary alkyl halide reacts with a weak
nucleophile like ethanol under neutral thermal conditions?
A. SN2 displacement
B. SN1 solvolysis
C. E2 elimination exclusively
D. Addition-elimination
Rationale: Tertiary substrates form stable carbocation intermediates upon leaving group loss, which is the
rate-limiting step in SN1 solvolysis. Weak nucleophiles like ethanol are insufficiently basic to force
bimolecular E2 or SN2 pathways.
2. What major product forms when 2-methyl-2-butene undergoes hydroboration-
oxidation using BH3-THF followed by basic hydrogen peroxide?
A. 2-methyl-2-butanol
B. 3-methyl-2-butanol
C. 2-methyl-1-butanol
D. 3-methyl-1-butanol
Rationale: Hydroboration-oxidation adds water across double bonds in an anti-Markovnikov fashion with
syn-stereospecificity. Boron attaches to the less substituted alkene carbon, yielding 3-methyl-2-butanol
after peroxide oxidation.
3. Which organometallic reagent adds cleanly to alpha,beta-unsaturated ketones via 1,4-
conjugate addition rather than 1,2-direct carbonyl addition?
A. Methyllithium
B. Methylmagnesium bromide
C. Lithium dimethylcuprate (Gilman reagent)
D. Sodium borohydride
, Rationale: Organocuprates (Gilman reagents) are soft nucleophiles that selectively undergo 1,4-conjugate
addition to enones. Harder nucleophiles like Grignard and organolithium reagents preferentially attack the
carbonyl carbon directly in a 1,2-fashion.
4. What characteristic infrared spectroscopy absorption band confirms the presence of a
terminal alkyne monosubstituted C-H stretch?
A. Sharp peak near 3300 cm⁻¹
B. Broad absorption centered between 2500–3100 cm⁻¹
C. Strong doublet at 1715 cm⁻¹
D. Medium band at 1650 cm⁻¹
Rationale: The sp-hybridized C-H bond of a terminal alkyne is stiff and produces a strong, sharp C-H
stretching absorption around 3300 cm⁻¹. Carboxylic acid O-H stretches appear as broad signals between
2500 and 3100 cm⁻¹.
5. In proton NMR spectroscopy, why does the aldehydic proton appear significantly
downfield between 9.0 and 10.0 ppm?
A. Strong intermolecular hydrogen bonding
B. Diamagnetic anisotropy of the carbonyl pi bond coupled with electron withdrawal
C. Hyperconjugation with adjacent alkyl groups
D. Homolytic radical shielding
Rationale: The carbonyl group induces a local magnetic field through pi-electron circulation that reinforces
the external magnetic field at the aldehyde proton position. This anisotropic deshielding combined with
inductive oxygen electron withdrawal pushes the signal downfield.
6. Which pericyclic transformation is classified as a thermal [3,3]-sigmatropic
rearrangement of an allyl vinyl ether?
A. Cope rearrangement
B. Claisen rearrangement
C. Diels-Alder reaction
D. Ene reaction
Rationale: The Claisen rearrangement is a concerted, thermal [3,3]-sigmatropic shift converting allyl vinyl
ethers into gamma,delta-unsaturated carbonyl compounds. The Cope rearrangement is similar but
involves a 1,5-hexadiene framework without oxygen.
MIDTERM EXAM
Advanced Organic Synthesis Reaction Mechanisms Spectroscopy
Stereochemistry Questions and Answers Verified Solutions 2026 2027
1. Which mechanistic path dominates when a tertiary alkyl halide reacts with a weak
nucleophile like ethanol under neutral thermal conditions?
A. SN2 displacement
B. SN1 solvolysis
C. E2 elimination exclusively
D. Addition-elimination
Rationale: Tertiary substrates form stable carbocation intermediates upon leaving group loss, which is the
rate-limiting step in SN1 solvolysis. Weak nucleophiles like ethanol are insufficiently basic to force
bimolecular E2 or SN2 pathways.
2. What major product forms when 2-methyl-2-butene undergoes hydroboration-
oxidation using BH3-THF followed by basic hydrogen peroxide?
A. 2-methyl-2-butanol
B. 3-methyl-2-butanol
C. 2-methyl-1-butanol
D. 3-methyl-1-butanol
Rationale: Hydroboration-oxidation adds water across double bonds in an anti-Markovnikov fashion with
syn-stereospecificity. Boron attaches to the less substituted alkene carbon, yielding 3-methyl-2-butanol
after peroxide oxidation.
3. Which organometallic reagent adds cleanly to alpha,beta-unsaturated ketones via 1,4-
conjugate addition rather than 1,2-direct carbonyl addition?
A. Methyllithium
B. Methylmagnesium bromide
C. Lithium dimethylcuprate (Gilman reagent)
D. Sodium borohydride
, Rationale: Organocuprates (Gilman reagents) are soft nucleophiles that selectively undergo 1,4-conjugate
addition to enones. Harder nucleophiles like Grignard and organolithium reagents preferentially attack the
carbonyl carbon directly in a 1,2-fashion.
4. What characteristic infrared spectroscopy absorption band confirms the presence of a
terminal alkyne monosubstituted C-H stretch?
A. Sharp peak near 3300 cm⁻¹
B. Broad absorption centered between 2500–3100 cm⁻¹
C. Strong doublet at 1715 cm⁻¹
D. Medium band at 1650 cm⁻¹
Rationale: The sp-hybridized C-H bond of a terminal alkyne is stiff and produces a strong, sharp C-H
stretching absorption around 3300 cm⁻¹. Carboxylic acid O-H stretches appear as broad signals between
2500 and 3100 cm⁻¹.
5. In proton NMR spectroscopy, why does the aldehydic proton appear significantly
downfield between 9.0 and 10.0 ppm?
A. Strong intermolecular hydrogen bonding
B. Diamagnetic anisotropy of the carbonyl pi bond coupled with electron withdrawal
C. Hyperconjugation with adjacent alkyl groups
D. Homolytic radical shielding
Rationale: The carbonyl group induces a local magnetic field through pi-electron circulation that reinforces
the external magnetic field at the aldehyde proton position. This anisotropic deshielding combined with
inductive oxygen electron withdrawal pushes the signal downfield.
6. Which pericyclic transformation is classified as a thermal [3,3]-sigmatropic
rearrangement of an allyl vinyl ether?
A. Cope rearrangement
B. Claisen rearrangement
C. Diels-Alder reaction
D. Ene reaction
Rationale: The Claisen rearrangement is a concerted, thermal [3,3]-sigmatropic shift converting allyl vinyl
ethers into gamma,delta-unsaturated carbonyl compounds. The Cope rearrangement is similar but
involves a 1,5-hexadiene framework without oxygen.
