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Reaction Mechanisms -- Practice Problems

Reaction Mechanisms — Practice Problems

10 MCQ questions covering SN1/SN2, E1/E2, electrophilic aromatic substitution, and carbocation chemistry. Select an option to check your answer and view the explanation.

Worked Examples

Example 1: Predicting SN1 vs SN2, Decision Tree

Problem: For each reaction, predict the mechanism and major product:

(a) 1-bromobutane + NaOEt in EtOH (b) 2-bromo-2-methylpropane + H₂O (c) (R)-2-bromopentane + NaCN in DMSO

Solution:

(a) Primary substrate + strong nucleophile + polar protic solvent → SN2

Product: 1-ethoxybutane (inversion at primary carbon, but primary carbons are not stereogenic)

(b) Tertiary substrate + weak nucleophile + polar protic solvent → SN1

Product: 2-methyl-2-propanol (racemic if stereogenic, but this substrate is achiral)

(c) Secondary substrate + strong nucleophile + polar aprotic solvent → SN2

Product: (S)-2-cyanopentane (inversion of configuration from R to S)

Decision summary:

FactorSN2SN1
Substrate1∘>2∘≫3∘1^\circ > 2^\circ \gg 3^\circ3∘>2∘≫1∘3^\circ > 2^\circ \gg 1^\circ
NucleophileStrong (⁻OH, ⁻CN, ⁻OR)Weak (H₂O, ROH)
SolventPolar aprotic (DMSO, DMF)Polar protic (H₂O, ROH)
StereochemistryInversionRacemization
KineticsSecond-order: k[R-LG][Nu−]k[\text{R-LG}][\text{Nu}^-]First-order: k[R-LG]k[\text{R-LG}]

Example 2: E2 Elimination, Regioselectivity

Problem: Predict the major product of E2 elimination for 2-bromo-2-methylbutane with (a) NaOEt and (b) KOtBu.

Solution:

The substrate has two types of β-hydrogens:

  • C1 hydrogens (3 H’s) → less substituted alkene (Hofmann)
  • C3 hydrogens (2 H’s) → more substituted alkene (Zaitsev)

(a) NaOEt (small base): Favors Zaitsev product (more substituted, more stable)

Major product: 2-methyl-2-butene (trisubstituted alkene)

(b) KOtBu (bulky base): Favors Hofmann product (less substituted, less steric hindrance)

Major product: 2-methyl-1-butene (disubstituted alkene)

Key insight: The base size controls regioselectivity. Small bases approach the more substituted β-hydrogen (Zaitsev). Bulky bases are sterically hindered and abstract the more accessible β-hydrogen (Hofmann).

Example 3: Carbocation Rearrangement

Problem: When 3,3-dimethyl-2-butanol is treated with H₂SO₄, what is the major product?

Solution:

Step 1: Protonation of the hydroxyl group

(CH3)3CCH(OH)CH3+H+→(CH3)3CCH(OH2+)CH3(\text{CH}_3)_3\text{CCH(OH)CH}_3 + \text{H}^+ \rightarrow (\text{CH}_3)_3\text{CCH(OH}_2^+)\text{CH}_3

Step 2: Loss of water → secondary carbocation

(CH3)3CCH+CH3(\text{CH}_3)_3\text{CCH}^+\text{CH}_3

Step 3: 1,2-methyl shift → tertiary carbocation (more stable)

(CH3)2C+CH(CH3)2(\text{CH}_3)_2\text{C}^+\text{CH(CH}_3)_2

Step 4: Elimination → tetrasubstituted alkene (most stable)

Product: (CH3)2C=C(CH3)2 (2,3-dimethyl-2-butene)\text{Product: } (\text{CH}_3)_2\text{C=C(CH}_3)_2 \text{ (2,3-dimethyl-2-butene)}

Key insight: Carbocation rearrangements occur when a more stable carbocation can be formed. The driving force is the stability order: 3∘>2∘>1∘>methyl3^\circ > 2^\circ > 1^\circ > \text{methyl}. Always check for possible rearrangements in SN1 and E1 reactions.

Nucleophilic Substitution and Elimination

Electrophilic Reactions and Rearrangements

Intuition

Reaction mechanisms are the “stories” of how bonds break and form: Every organic reaction has a mechanism, a step-by-step sequence of electron movements shown with curly arrows. Understanding mechanisms lets you predict products of reactions you have never seen before.

Why it matters: Mechanistic reasoning is how pharmaceutical companies design drug syntheses, how chemists troubleshoot failed reactions, and how new catalysts are developed.

The key insight: SN1 vs SN2, E1 vs E2, the competition between substitution and elimination is governed by the same factors: substrate structure, nucleophile strength, solvent polarity, and temperature.

Common Mistakes

Confusing SN1 and SN2 mechanisms: SN2 is a one-step, backside attack with inversion of configuration and second-order kinetics. SN1 is a two-step mechanism with a carbocation intermediate, racemisation, and first-order kinetics. The substrate (primary/secondary/tertiary), nucleophile strength, and solvent polarity determine which pathway dominates. Do not assume one mechanism applies to all substrates.

Ignoring stereochemistry in elimination reactions: E2 elimination requires an anti-periplanar arrangement of the leaving group and the β\beta-hydrogen. This stereoelectronic requirement determines which product forms (Zaitsev vs Hofmann). Failing to check the anti-periplanar geometry leads to predicting the wrong alkene product.

Confusing kinetic and thermodynamic enolates: The kinetic enolates form faster (less substituted, using a strong base at low temperature), while the thermodynamic enolates are more stable (more substituted, using a weaker base at higher temperature). The conditions determine which enolates forms, which in turn determines the alkylation product.

Cross-References