CHE 411 ORGANIC REACTION
MECHANISMS MIDTERM EXAM
Midterm Examination Questions and Answers with Verified Solutions
2026/2027
1. What kinetic rate law is observed for a classic SN2 substitution reaction involving a
primary alkyl halide and a hydroxide nucleophile?
A. Rate = k[Alkyl Halide]
B. Rate = k[Alkyl Halide][Nucleophile]
C. Rate = k[Nucleophile]
D. Rate = k[Alkyl Halide]^2
Rationale: SN2 reactions follow second-order kinetics where both the substrate and the nucleophile
participate in the single rate-determining transition state.
2. What stereochemical outcome occurs when a chiral secondary alkyl halide undergoes
an SN2 substitution reaction?
A. Complete inversion of configuration (Walden inversion)
B. Complete retention of configuration
C. Racemization resulting in a 50:50 mixture of enantiomers
D. Formation of an achiral meso intermediate
Rationale: Backside attack by the incoming nucleophile forces the three remaining substituents to invert
configuration, flipping the chiral center like an umbrella in high wind.
3. Which intermediate is formed during a classic SN1 substitution pathway?
A. Pentacoordinate transition state
B. Free radical intermediate
C. Planar carbocation intermediate
D. Carbanion intermediate
Rationale: Heterolytic cleavage of the carbon-leaving group bond generates an sp2-hybridized, trigonal
planar carbocation intermediate.
, 4. Why do tertiary carbocations exhibit greater thermodynamic stability than primary
carbocations?
A. Stronger hydrogen bonding with polar solvents
B. Greater hyperconjugation and inductive electron donation from adjacent alkyl
groups
C. Increased steric hindrance preventing nucleophilic attack
D. Resonant delocalization across aromatic ring systems exclusively
Rationale: Hyperconjugation (overlap of adjacent sigma C-H or C-C bonds with the empty p-orbital) and
inductive donation stabilize positive charges on carbon centers.
5. Which solvent choice best accelerates the rate of an SN2 nucleophilic substitution
reaction?
A. Protic solvents like water or ethanol
B. Polar aprotic solvents like DMF, DMSO, or acetone
C. Nonpolar solvents like hexane or benzene
D. Acidic protic solvents like acetic acid
Rationale: Polar aprotic solvents solvate cations well while leaving anionic nucleophiles bare and highly
reactive, lowering the activation energy barrier for SN2.
6. What geometric requirement governs the transition state of a concerted E2 elimination
reaction?
A. Anti-periplanar alignment of the beta-proton and the leaving group (dihedral angle
~180°)
B. Syn-coplanar alignment of the leaving group and incoming base
C. Perpendicular orientation of the developing p-orbitals
D. Completely unconstrained random geometry
Rationale: Anti-periplanar geometry permits optimal orbital overlap between the breaking C-H sigma
bond, developing pi bond, and breaking C-LG sigma bond.
7. What product selectivity rule predicts the most substituted, thermodynamically stable
alkene as the major product in E1 or non-hindered E2 eliminations?
A. Hofmann's Rule
MECHANISMS MIDTERM EXAM
Midterm Examination Questions and Answers with Verified Solutions
2026/2027
1. What kinetic rate law is observed for a classic SN2 substitution reaction involving a
primary alkyl halide and a hydroxide nucleophile?
A. Rate = k[Alkyl Halide]
B. Rate = k[Alkyl Halide][Nucleophile]
C. Rate = k[Nucleophile]
D. Rate = k[Alkyl Halide]^2
Rationale: SN2 reactions follow second-order kinetics where both the substrate and the nucleophile
participate in the single rate-determining transition state.
2. What stereochemical outcome occurs when a chiral secondary alkyl halide undergoes
an SN2 substitution reaction?
A. Complete inversion of configuration (Walden inversion)
B. Complete retention of configuration
C. Racemization resulting in a 50:50 mixture of enantiomers
D. Formation of an achiral meso intermediate
Rationale: Backside attack by the incoming nucleophile forces the three remaining substituents to invert
configuration, flipping the chiral center like an umbrella in high wind.
3. Which intermediate is formed during a classic SN1 substitution pathway?
A. Pentacoordinate transition state
B. Free radical intermediate
C. Planar carbocation intermediate
D. Carbanion intermediate
Rationale: Heterolytic cleavage of the carbon-leaving group bond generates an sp2-hybridized, trigonal
planar carbocation intermediate.
, 4. Why do tertiary carbocations exhibit greater thermodynamic stability than primary
carbocations?
A. Stronger hydrogen bonding with polar solvents
B. Greater hyperconjugation and inductive electron donation from adjacent alkyl
groups
C. Increased steric hindrance preventing nucleophilic attack
D. Resonant delocalization across aromatic ring systems exclusively
Rationale: Hyperconjugation (overlap of adjacent sigma C-H or C-C bonds with the empty p-orbital) and
inductive donation stabilize positive charges on carbon centers.
5. Which solvent choice best accelerates the rate of an SN2 nucleophilic substitution
reaction?
A. Protic solvents like water or ethanol
B. Polar aprotic solvents like DMF, DMSO, or acetone
C. Nonpolar solvents like hexane or benzene
D. Acidic protic solvents like acetic acid
Rationale: Polar aprotic solvents solvate cations well while leaving anionic nucleophiles bare and highly
reactive, lowering the activation energy barrier for SN2.
6. What geometric requirement governs the transition state of a concerted E2 elimination
reaction?
A. Anti-periplanar alignment of the beta-proton and the leaving group (dihedral angle
~180°)
B. Syn-coplanar alignment of the leaving group and incoming base
C. Perpendicular orientation of the developing p-orbitals
D. Completely unconstrained random geometry
Rationale: Anti-periplanar geometry permits optimal orbital overlap between the breaking C-H sigma
bond, developing pi bond, and breaking C-LG sigma bond.
7. What product selectivity rule predicts the most substituted, thermodynamically stable
alkene as the major product in E1 or non-hindered E2 eliminations?
A. Hofmann's Rule
