Organic Reaction Mechanisms and Concepts

Reaction Coordinate Diagrams

Reaction coordinate diagrams are graphical representations of the energy changes that occur during a chemical reaction. They help visualize intermediates, transition states, and the relative stability of reactants and products.

• Intermediates: Species formed during the reaction that are neither reactants nor final products. They appear as valleys between peaks in the diagram.

• Transition States: High-energy states corresponding to the peaks in the diagram. These are not isolable and represent the point of highest energy along the reaction path.

• Stability: The lower the energy of a species, the more stable it is.

• Fastest Step: The step with the lowest activation energy (smallest energy difference between reactant/intermediate and transition state).

Example: In a multi-step reaction, if the diagram shows three valleys (intermediates) and four peaks (transition states), then there are three intermediates and four transition states.

Acid-Base Reactions and pKa Values

Acid-base reactions depend on the relative strengths of acids and bases, which are quantified by their pKa values. A base can deprotonate an acid if its conjugate acid has a higher pKa than the acid being deprotonated.

• pKa: The negative logarithm of the acid dissociation constant; lower pKa means stronger acid.

• Deprotonation: Occurs when the base is strong enough (i.e., its conjugate acid has a higher pKa than the acid).

Electrophilic Addition to Alkynes and Alkenes

Electrophiles tend to attack positions in molecules where the resulting carbocation intermediate is most stable. Stability is increased by resonance, hyperconjugation, and inductive effects.

• Carbocation Stability: Tertiary > Secondary > Primary > Methyl

• Steric Hindrance: Less hindered positions are more accessible to electrophiles.

Example: In hex-1-en-5-yne, an electrophile will attach at the carbon that leads to the most stable carbocation, often at the terminal position.

Oxidative Cleavage of Alkynes

Oxidative cleavage is a reaction where triple bonds in alkynes are broken to form carboxylic acids. The position of the triple bond determines the products.

• Reagents: OsO4 (osmium tetroxide), H2O

• Product: Dicarboxylic acids are formed at the positions where the triple bond was present.

Example: An unknown alkyne giving dodecanedioic acid upon oxidative cleavage must have the triple bond positioned such that both ends yield carboxylic acids.

Oxymercuration-Demercuration of Alkenes

This reaction hydrates alkenes in a Markovnikov fashion without carbocation rearrangement. It proceeds via a mercurinium ion intermediate.

• Markovnikov Addition: The OH group attaches to the more substituted carbon.

• Regioselectivity: No rearrangement occurs, unlike acid-catalyzed hydration.

Equation:

Acid-Catalyzed Hydration of Alkenes

Alkenes react with water in the presence of acid to form alcohols, typically following Markovnikov's rule.

• Mechanism: Protonation of alkene, formation of carbocation, nucleophilic attack by water, deprotonation.

• Product: Alcohol at the more substituted carbon.

Equation:

Ring Opening Reactions

Ring opening reactions can proceed in different directions depending on the substituents present, which affect carbocation stability and electron donation/withdrawal.

• Electron Donating Groups: Stabilize positive charge, favoring carbocation formation nearby.

• Electron Withdrawing Groups: Destabilize carbocations, directing ring opening away from them.

Oxidation of Alkenes with Potassium Permanganate

Hot, concentrated KMnO4 cleaves double bonds, oxidizing them to carboxylic acids or ketones.

• Products: Carboxylic acids (if the double bond is terminal), ketones (if internal).

Equation:

Halohydrin Formation Mechanism

Halohydrins are formed by the addition of halogens and water to alkenes. The reaction is regioselective and stereospecific.

• Mechanism: Formation of bromonium ion, nucleophilic attack by water, deprotonation to yield halohydrin.

Equation:

Hydrogenation of Alkenes and Alkynes

Hydrogenation is the addition of hydrogen to double or triple bonds, converting them to single bonds. Catalysts like Pt or Pd are used.

• Alkene Hydrogenation: Converts alkenes to alkanes.

• Alkyne Hydrogenation: Converts alkynes to alkanes (with excess H2).

• Stereochemistry: Syn addition (both hydrogens add to the same side).

Equation:

Regioselectivity in Alkyl Halide Formation

Alkyl halides are formed by the addition of hydrohalic acids to alkenes. The position of the halide depends on the stability of the carbocation intermediate.

• Markovnikov's Rule: Halide attaches to the more substituted carbon.

Acetylide Ion Reactions

Acetylide ions are strong nucleophiles that react with carbonyl compounds to form alcohols with new carbon-carbon bonds.

• Mechanism: Nucleophilic addition to carbonyl, followed by protonation.

Equation:

Alkyne Reactions with Various Reagents

Alkynes undergo different reactions depending on the reagents used, such as reduction, hydration, and hydroboration.

• NaNH2: Deprotonates terminal alkynes to form acetylide ions.

• HgSO4/H2SO4, H2O: Hydrates alkynes to form ketones (Markovnikov addition).

• Sia2BH, H2O2: Hydroboration-oxidation yields aldehydes (anti-Markovnikov addition).

SN2 Reaction Limitations

SN2 reactions are bimolecular nucleophilic substitutions that require a good leaving group and an aprotic solvent. If the substrate contains an acidic proton, an acid-base reaction may occur instead.

• Leaving Group: Must be able to depart easily.

• Solvent: Aprotic solvents favor SN2 reactions.

• Competing Reactions: Acidic protons can lead to acid-base reactions rather than substitution.

Bond Dissociation Energy in Halogenated Carbons

Bond dissociation energy (BDE) is the energy required to break a bond homolytically. The BDE of C–Br bonds varies with the stability of the resulting radicals.

• Stability: More stable radicals (e.g., tertiary) have lower BDEs.

• Comparison: Tertiary C–Br < BDE than primary C–Br.

Additional info: These topics are foundational for understanding organic reaction mechanisms, which are essential for General Biology students who need to grasp biochemical transformations and molecular interactions.