Advanced General Chemistry: Reaction Mechanisms, Acid-Base Chemistry, and Organic Transformations

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Reaction Coordinate Diagrams and Mechanisms

Understanding Reaction Coordinate Diagrams

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

Transition State: The highest energy point along the reaction path; corresponds to the peak of the curve.
Intermediate: A species that exists temporarily during the reaction, represented by valleys between peaks.
Activation Energy: The energy difference between reactants and the transition state.

Example: In a multi-step reaction, the number of intermediates equals the number of valleys (excluding reactants and products), and the number of transition states equals the number of peaks.

Acid-Base Chemistry and pKa Comparisons

Deprotonation and Base Strength

The ability of a base to deprotonate an acid depends on the relative pKa values. A base can deprotonate an acid if its conjugate acid has a higher pKa (is less acidic) than the acid being deprotonated.

pKa: A measure of acid strength; lower pKa means a stronger acid.
Rule: A base will deprotonate an acid if pKa (conjugate acid of base) > pKa (acid).

Example: To deprotonate propanoic acid (pKa = 4.88), use a base whose conjugate acid has a pKa > 4.88.

Electrophilic Addition to Alkynes and Alkenes

Regioselectivity in Electrophilic Addition

When an electrophile adds to an unsaturated hydrocarbon (alkene or alkyne), the site of addition is determined by the stability of the resulting carbocation intermediate or by electron density.

Markovnikov's Rule: The electrophile adds to the carbon with more hydrogens, forming the most stable carbocation.
Carbocation Stability: Tertiary > Secondary > Primary > Methyl.

Example: In hex-1-en-5-yne, the electrophile will attach to the carbon that leads to the most stable carbocation intermediate.

Oxidative Cleavage of Alkynes

Determining Alkyne Structure by Oxidative Cleavage

Oxidative cleavage of alkynes with reagents like ozone (O3) or potassium permanganate (KMnO4) breaks the triple bond and forms carboxylic acids. The products reveal the position of the triple bond in the original molecule.

Terminal Alkynes: Yield carboxylic acid and CO2.
Internal Alkynes: Yield two carboxylic acids.

Example: An alkyne that gives dodecanedioic acid upon oxidative cleavage must have the triple bond in the center of a 12-carbon chain.

Oxymercuration–Demercuration and Acid-Catalyzed Hydration

Oxymercuration–Demercuration of Alkenes

This reaction adds water across a double bond in a Markovnikov fashion (OH to the more substituted carbon) without carbocation rearrangement.

Step 1: Oxymercuration forms a mercurinium ion intermediate.
Step 2: Water attacks the more substituted carbon.
Step 3: Demercuration replaces mercury with hydrogen.

Example: The product is an alcohol with the OH group on the more substituted carbon.

Acid-Catalyzed Hydration of Alkenes

In the presence of H2SO4 and H2O, alkenes undergo Markovnikov addition of water, forming alcohols. Carbocation rearrangement may occur if a more stable carbocation can be formed.

Ring Opening Reactions and Carbocation Stability

Factors Affecting Ring Opening Direction

The direction of ring opening in epoxides or related systems depends on the stability of the carbocation intermediate and the nature of substituents (electron-donating or withdrawing groups).

Electron-Donating Groups: Stabilize positive charge, favoring carbocation formation adjacent to them.
Electron-Withdrawing Groups: Destabilize carbocations, making ring opening less favorable at those positions.

Example: Tetraalkylammonium groups stabilize carbocations better than ethoxy groups.

Oxidative Cleavage of Alkenes

Potassium Permanganate Oxidation

Hot, concentrated KMnO4 cleaves alkenes to form carboxylic acids or ketones, depending on the substitution pattern of the double bond.

Terminal Alkenes: Yield carboxylic acids and CO2.
Internal Alkenes: Yield two carboxylic acids or ketones.

Halohydrin Formation: Mechanism and Selectivity

Regioselectivity and Stereospecificity

Halohydrin formation involves the addition of a halogen and water to an alkene. The reaction is both regioselective (OH adds to the more substituted carbon) and stereospecific (anti addition).

Mechanism: Formation of a halonium ion intermediate, followed by nucleophilic attack by water.

Hydrogenation of Alkenes and Alkynes

Catalytic Hydrogenation

Hydrogenation adds hydrogen across double or triple bonds using a metal catalyst (e.g., Pt, Pd, Ni). The addition is syn (both hydrogens add to the same face).

Alkenes: Reduced to alkanes.
Alkynes: Reduced to alkanes (with excess H2).

Alkyl Halide Formation and Regioselectivity

Hydrohalogenation of Alkenes

Alkenes react with hydrohalic acids (HX) to form alkyl halides. The product depends on the position of the double bond and Markovnikov's rule.

Alkyne Chemistry: Synthesis and Reactions

Alkyne Synthesis via Acetylide Ions

Acetylide ions (deprotonated alkynes) are strong nucleophiles that react with alkyl halides or carbonyl compounds to form new C–C bonds.

Preparation: Terminal alkynes are deprotonated with strong bases (e.g., NaNH2).
Reactions: Nucleophilic substitution (SN2) or addition to carbonyls.

Hydration and Hydroboration of Alkynes

Acid-Catalyzed Hydration: Forms ketones via enol intermediates (tautomerization).
Hydroboration-Oxidation: Forms aldehydes from terminal alkynes.

Limitations of SN2 with Alkynides

Alkynide ions may fail to react with certain alkyl halides if the substrate contains acidic protons, leading to acid-base reactions instead of substitution.

Summary Table: Acid-Base Strengths and Deprotonation Ability

Base	Conjugate Acid pKa	Can Deprotonate Propanoic Acid?
CH3O-	15.9	Yes
NH3	9.4	Yes
CH3CC-	25	Yes
CH3CH2OH	-2.5	No
H2O	-1.7	No
Br-	-9	No

Key Equations and Concepts

Activation Energy:
Acid-Base Equilibrium:
Markovnikov Addition: Electrophile adds to the carbon with more hydrogens.
Hydrogenation:
Oxidative Cleavage:

Additional info:

Some problems involve advanced organic chemistry concepts (e.g., oxymercuration, hydroboration, and oxidative cleavage), which are often included in the latter part of a General Chemistry course or in Organic Chemistry I.
Mechanistic details (e.g., arrow-pushing) are essential for understanding regioselectivity and stereochemistry in organic reactions.