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 Values

Predicting Acid-Base Reactions

Acid-base reactions favor the formation of the weaker acid and base. The pKa value measures acid strength; lower pKa means a stronger acid. A base can deprotonate an acid if its conjugate acid has a higher pKa than the acid being deprotonated.

Rule: A base will deprotonate an acid if pKa (conjugate acid of base) > pKa (acid).
Example: CH3O- (conjugate acid pKa = 15.9) can deprotonate propanoic acid (pKa = 4.88).

Electrophilic Addition to Alkynes and Alkenes

Regioselectivity in Electrophilic Addition

When an electrophile adds to an unsaturated hydrocarbon, the site of attack is determined by the stability of the resulting carbocation intermediate or by electron density.

Carbocation Stability: Tertiary > Secondary > Primary > Methyl.
Markovnikov's Rule: In the addition of HX to an alkene/alkyne, the hydrogen attaches to the carbon with more hydrogens, and the halide to the carbon with fewer hydrogens.

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

Oxidative Cleavage of Alkynes

Determining Alkyne Structure by Oxidative Cleavage

Oxidative cleavage with reagents like ozone (O3) or potassium permanganate (KMnO4) breaks triple bonds, forming carboxylic acids. The products reveal the original position of the triple bond.

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 of Alkenes

Mechanism and Product Prediction

Oxymercuration–demercuration is a two-step hydration of alkenes that adds water across the double bond in a Markovnikov fashion, without carbocation rearrangement.

Step 1: Alkene reacts with mercuric acetate in water to form a mercurinium ion intermediate.
Step 2: Water attacks the more substituted carbon, followed by reduction (usually with NaBH4), replacing mercury with hydrogen.
Product: Alcohol at the more substituted carbon.

Acid-Catalyzed Hydration of Alkenes

Markovnikov Addition and Rearrangement

In the presence of acid (H2SO4) and water, alkenes undergo hydration to form alcohols. The reaction proceeds via carbocation intermediates, which may rearrange to form more stable carbocations.

Markovnikov's Rule: The OH group attaches to the more substituted carbon.
Carbocation Rearrangement: Hydride or alkyl shifts may occur to stabilize the carbocation.

Ring Opening Reactions and Group Effects

Effect of Substituents on Ring Opening Direction

The direction of ring opening in epoxides or related systems depends on the stability of the resulting carbocation and the electronic effects of substituents.

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

Example: Tetraalkylammonium groups stabilize carbocations better than ethoxy groups, influencing the direction of ring opening.

Oxidative Cleavage of Alkenes

Reaction with Potassium Permanganate

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

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 (e.g., Br2) and water to an alkene. The reaction is both regioselective (OH attaches to the more substituted carbon) and stereospecific (anti addition).

Mechanism: Formation of a halonium ion intermediate, followed by nucleophilic attack by water.
Product: Halohydrin with anti stereochemistry.

Hydrogenation of Alkenes and Alkynes

Catalytic Hydrogenation

Hydrogenation is the addition of hydrogen (H2) across double or triple bonds using a metal catalyst (e.g., Pt, Pd, Ni). The reaction reduces alkenes to alkanes and alkynes to alkanes.

Stereochemistry: Syn addition (both hydrogens add to the same face).
Complete Hydrogenation: Excess H2 reduces all multiple bonds.

Alkyl Halide Formation from Alkenes

Hydrohalogenation

Alkenes react with hydrohalic acids (HX) to form alkyl halides. The reaction follows Markovnikov's rule, with the halide attaching to the more substituted carbon.

Regioselectivity: Only one alkene structure will yield a specific alkyl halide upon reaction with HX.

Alkyne Reactions: Reduction and Synthesis

Reduction of Alkynes

Alkynes can be reduced to alkanes using H2 and a metal catalyst (Pd/C). Partial reduction to cis- or trans-alkenes is possible with specific catalysts.

Complete Reduction:

Alkyne Synthesis via Acetylide Ions

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

Example: Synthesis of 3,7-dimethyloct-4-yn-3-ol by reacting an acetylide ion with a carbonyl compound.

Reactions of But-1-yne with Various Reagents

Product Identification

NaNH2: Deprotonates terminal alkynes to form acetylide ions.
HgSO4/H2SO4, H2O: Hydrates alkynes to form ketones (Markovnikov addition).
Sia2BH, H2O2, OH-: Hydroboration-oxidation gives aldehydes (anti-Markovnikov addition).

SN2 Reactions and Competing Acid-Base Reactions

Limitations of SN2 with Alkynides

SN2 reactions require a good leaving group and an aprotic solvent. If the substrate contains an acidic proton (e.g., an alcohol), the strong base (alkynide) will preferentially deprotonate the alcohol rather than undergo substitution.

Key Point: Acid-base reactions outcompete SN2 if the substrate is acidic.

Summary Table: Acid-Base Strengths (pKa Values)

Species	Conjugate Acid	pKa
CH3O-	CH3OH	15.9
NH3	NH4+	9.4
CH3CC-	CH3CCH	25
CH3CH2OH	CH3CH2OH2+	-2.5
H2O	H3O+	-1.7
Br-	HBr	-9

Additional info: This table summarizes the acid-base strengths relevant to Problem 2 and can be used to predict the direction of acid-base reactions.