Organic Reaction Mechanisms

Oxidative Cleavage of Alkenes

Oxidative cleavage is a reaction where alkenes are split into carbonyl-containing fragments using reagents such as ozone (O3) or potassium permanganate (KMnO4).

• Ozonolysis: Alkenes react with ozone followed by reductive workup (e.g., Zn/acetic acid) to yield aldehydes or ketones.

• Equation:

• Application: Used to determine the position of double bonds in unknown alkenes.

Hydrogenation of Alkynes

Alkynes can be reduced to alkenes or alkanes using different catalysts.

• Lindlar's Catalyst: Converts alkynes to cis-alkenes via syn addition of hydrogen.

• Na/NH3: Converts alkynes to trans-alkenes via anti addition.

• Equation: (complete reduction to alkane)

Alkyne Alkylation

Terminal alkynes can be deprotonated with a strong base (e.g., NaNH2) and then alkylated with alkyl halides.

• Equation:

• Application: Useful for chain elongation in synthesis.

Substitution and Elimination Reactions

SN1 and SN2 Mechanisms

Substitution reactions are classified as SN1 (unimolecular) or SN2 (bimolecular) based on their mechanisms.

• SN2: One-step, concerted mechanism; nucleophile attacks as leaving group departs. Stereochemistry is inverted.

• SN1: Two-step mechanism; formation of carbocation intermediate. Racemization occurs.

• Equation (SN2):

• Key Terms: Nucleophile (attacks), Electrophile (accepts electrons), Leaving group (departs).

E2 and E1 Elimination

Elimination reactions remove atoms/groups to form double bonds.

• E2: One-step, requires anti-periplanar geometry; strong base favors E2.

• E1: Two-step, carbocation intermediate; weak base and polar solvent favor E1.

• Zaitsev Product: More substituted alkene (major product in most E2 reactions).

• Hofmann Product: Less substituted alkene (favored by bulky bases).

Stereochemistry and Stability

Ranking Stability of Alkenes and Carbocations

Alkene and carbocation stability increases with substitution due to hyperconjugation and inductive effects.

• Alkene Stability Order: Tetra > Tri > Di > Mono substituted.

• Carbocation Stability Order: Tertiary > Secondary > Primary > Methyl.

Newman Projections and Chair Conformations

Newman projections help visualize stereochemistry in reactions, especially for E2 eliminations. Chair conformations are used for cyclohexane derivatives to determine axial/equatorial positions and reactivity.

• Axial vs. Equatorial: E2 elimination requires the leaving group and hydrogen to be axial and anti-periplanar.

• Example: Only one product forms if only one anti-periplanar arrangement is possible.

Alkene and Alkyne Addition Reactions

Markovnikov and Anti-Markovnikov Addition

Addition reactions to alkenes and alkynes can follow Markovnikov or anti-Markovnikov rules.

• Markovnikov Addition: Electrophile adds to the less substituted carbon; nucleophile to the more substituted.

• Anti-Markovnikov Addition: Occurs with hydroboration-oxidation; nucleophile adds to less substituted carbon.

• Equation (Hydroboration-Oxidation):

Hydration of Alkynes

Alkynes can be hydrated to form ketones or aldehydes using acid-catalyzed or hydroboration-oxidation methods.

• Acid-Catalyzed Hydration: Forms ketones via Markovnikov addition.

• Hydroboration-Oxidation: Forms aldehydes via anti-Markovnikov addition.

Functional Group Transformations

Alcohols, Alkyl Halides, and Ethers

Alcohols can be formed by substitution reactions, alkyl halides by addition/elimination, and ethers by Williamson synthesis.

• Williamson Ether Synthesis:

• Alcohol Formation:

Tables

Comparison of SN1 vs. SN2 Mechanisms

Ranking Reactivity in SN1 Reactions

Key Terms and Definitions

• Nucleophile: Species that donates an electron pair to form a new bond.

• Electrophile: Species that accepts an electron pair.

• Leaving Group: Atom or group that departs with an electron pair in substitution/elimination reactions.

• Markovnikov Rule: In addition reactions, the hydrogen adds to the carbon with more hydrogens.

• Anti-Markovnikov Rule: Hydrogen adds to the less substituted carbon.

• Zaitsev Product: Most substituted alkene formed in elimination.

• Hofmann Product: Least substituted alkene formed in elimination.

Examples and Applications

• Example (E2 Elimination): Cyclohexyl bromide reacts with methoxide to give only one product due to chair conformation constraints.

• Example (Alkyne Addition): Hydroboration-oxidation of a terminal alkyne yields an aldehyde.

• Example (SN2 Mechanism): 2-chlorobutane reacts with NaOH to give 2-butanol via backside attack and inversion of configuration.

Additional info: This study guide covers topics from chapters on alkenes, alkynes, substitution and elimination reactions, stereochemistry, and reaction mechanisms, as reflected in the provided questions. It is suitable for exam preparation in a college-level Organic Chemistry course.