Radical and Anion Stability; SN1 Reaction Rates

Radical Stability

Organic radicals are species with an unpaired electron. Their stability is influenced by resonance, hyperconjugation, and the nature of the carbon atom bearing the radical.

• Allylic and benzylic radicals are stabilized by resonance.

• Tertiary radicals are stabilized by hyperconjugation from adjacent alkyl groups.

• Primary and methyl radicals are least stable due to lack of stabilization.

• Example: The benzyl radical is more stable than a cyclohexyl radical.

SN1 Reaction Rate

The SN1 (unimolecular nucleophilic substitution) reaction rate depends on the stability of the carbocation intermediate formed after the leaving group departs.

• Tertiary alkyl halides react fastest due to stable carbocations.

• Allylic and benzylic halides are also fast due to resonance stabilization.

• Primary and methyl halides react slowest.

• Example: Benzyl bromide reacts faster than ethyl bromide in SN1 conditions.

Anion Stability

Anion stability is influenced by resonance, inductive effects, and aromaticity.

• Resonance-stabilized anions (e.g., phenoxide) are more stable.

• Electron-withdrawing groups increase anion stability.

• Example: A fluorinated aromatic anion is more stable than a non-fluorinated one.

Free Radical Halogenation and Photolysis

Monobromination Products

Halogenation under photolytic conditions (e.g., Br2, hv) produces monobrominated products via a free radical mechanism.

• Br2, hv selectively brominates the most stable radical position.

• Example: Bromination of cyclohexane yields bromocyclohexane.

Reactivity Trends

• Fastest Reacting Compound: The one forming the most stable radical intermediate.

• Slowest Reacting Compound: The one forming the least stable radical.

Monochlorination Products

Chlorination (Cl2, hv) is less selective than bromination and may yield multiple products.

• Example: Chlorination of cyclohexane can yield several monochlorinated isomers.

Nucleophilic Substitution: SN1 and SN2 Mechanisms

Substitution Products and Reaction Conditions

Substitution reactions can proceed via SN1 or SN2 mechanisms, depending on substrate, nucleophile, and solvent.

• SN1: Favored by tertiary substrates, weak nucleophiles, and polar protic solvents.

• SN2: Favored by primary substrates, strong nucleophiles, and polar aprotic solvents.

• Example: NaCN in ethanol can yield different products depending on concentration and substrate.

Effect of Nucleophile Concentration

• Increasing nucleophile concentration increases SN2 rate but does not affect SN1 rate.

• Equation:

• Equation:

Effect of Leaving Group

• Better leaving groups (e.g., Br- vs. Cl-) increase both SN1 and SN2 rates.

Predicting Products of Substitution and Elimination Reactions

Common Reactions

Organic halides can undergo substitution (SN1/SN2) or elimination (E1/E2) reactions depending on conditions.

• Strong base + alkyl halide: E2 elimination or SN2 substitution.

• Weak base + tertiary halide: SN1 or E1 mechanism.

• Example: Cyclohexyl bromide with NaOH yields cyclohexanol (substitution).

E2 Elimination: Zaitsev vs. Hofmann Products

Zaitsev and Hofmann Rules

E2 elimination produces alkenes. The major product is determined by the base and substrate structure.

• Zaitsev product: Most substituted alkene (favored with small bases).

• Hofmann product: Least substituted alkene (favored with bulky bases).

• Example: Elimination of methyl chloride with sodium ethoxide yields the Zaitsev product.

Reactivity Comparison

• Neomenthyl chloride reacts faster than menthyl chloride in E2 due to less steric hindrance.

Multi-Step Synthesis

Designing Synthetic Routes

Organic synthesis often requires multiple steps to convert starting materials to desired products.

• Identify functional group transformations needed.

• Choose appropriate reagents for each step.

• Example: Converting a methylthio group to a phenylthio group may require nucleophilic substitution.

Alkene Stereochemistry: E/Z Configuration

Assigning E/Z Configuration

Alkene stereochemistry is assigned using the Cahn-Ingold-Prelog priority rules.

• E (entgegen): Higher priority groups on opposite sides of the double bond.

• Z (zusammen): Higher priority groups on the same side of the double bond.

• Example: 2-butene: methyl groups on opposite sides = E; on same side = Z.

Additional info: For each alkene, assign priorities based on atomic number and connectivity, then determine E/Z configuration.