Learn & Review: Organic Chemistry 2: Every Reaction You Need to Know!

Organic Chemistry II Reactions Summary

This summary covers key reactions and concepts typically found in an Organic Chemistry II course, organized by functional group and reaction type.

I. Dienes and Electrophilic Addition

• Formation of Conjugated Dienes:
  • Generated via elimination reactions from single halides or dihalides using a strong base like potassium tert-butoxide.
• Reaction with HBr:
  • Produces a mixture of 1,2-adduct (kinetic product) and 1,4-adduct (thermodynamic product).
  • Kinetic Product: Favored at low temperatures (e.g., 0°C) and short reaction times.
  • Thermodynamic Product: Favored at higher temperatures and longer reaction times.
• Reaction with Br₂:
  • Similar to HBr addition, yielding both 1,2- and 1,4-dibromo adducts.

II. Diels-Alder Reactions

• Mechanism: A [4+2] cycloaddition reaction between a conjugated diene and a dienophile.
• Reactants:
  • Diene: Must be in the s-cis conformation. Cyclic systems that lock the diene into s-cis conformation accelerate the reaction.
  • Dienophile: Typically contains a pi bond, especially those with electron-withdrawing groups (e.g., esters, nitro, cyano groups) which activate it.
• Product: A cyclohexene ring.
• Stereochemistry:
  • The relative stereochemistry of substituents on the dienophile is preserved in the product. Cis substituents remain cis, and trans substituents remain trans.
  • New stereocenters are formed, requiring wedge/dash notation.
• Endo Rule: Electron-withdrawing groups on the dienophile tend to point towards the diene (endo position) in the transition state, leading to the endo product.

III. Electrocyclic Reactions

• Definition: Reactions involving the movement of pi electrons to form or break cyclic systems.
• Conditions: Can be initiated by heat or light.
• Stereochemistry: The conditions (heat vs. light) determine whether the reaction proceeds conrotatory or disrotatory, influencing the stereochemistry of the product.
• Systems: Applicable to 4π, 6π, and 8π electron systems.
• Equilibrium: Favors products in 6π systems but reactants in 4π systems due to ring strain.

IV. Sigmatropic Rearrangements

• Definition: Reactions where a sigma bond migrates across a pi system, involving a concerted rearrangement of sigma and pi bonds.
• Types:
  • Cope Rearrangement: Involves the rearrangement of carbon-carbon sigma bonds.
  • Claisen Rearrangement: Involves the rearrangement of carbon-oxygen sigma bonds.
• Classification: Often described as [3,3]-sigmatropic rearrangements, indicating the number of atoms involved in the bond migration on each fragment.
• Conditions: Typically require heat.

V. Reactions at Benzylic Positions

• Reactivity: The carbon atom adjacent to a benzene ring (benzylic position) is highly reactive.
• Oxidation: Can be oxidized to a carboxylic acid using strong oxidizing agents (e.g., KMnO₄), regardless of the length of the carbon chain attached.
• Halogenation: Free radical bromination (e.g., using NBS) can occur at the benzylic position.
• Substitution Reactions:
  • SN1: Possible at a benzylic bromide if the carbon is quaternary.
  • SN2: Possible at a benzylic bromide if the carbon is primary or secondary.
• Elimination Reactions: Alcohols at the benzylic position can be eliminated to form alkenes using strong acids (e.g., H₂SO₄).

VI. Reduction of Benzene

• Stability: Benzene is highly stable due to its aromaticity.
• Conditions: Full reduction to cyclohexane requires harsh conditions: hydrogen gas, a metal catalyst (e.g., Ni), high temperature, and high pressure.
• Substituted Benzenes:
  • Electron-Donating Groups (EDG): Reduction occurs at carbons not attached to the EDG.
  • Electron-Withdrawing Groups (EWG): Reduction occurs at the carbon attached to the EWG and adjacent carbons.

VII. Electrophilic Aromatic Substitution (EAS) Reactions

• General Principle: Substitution of a hydrogen atom on the benzene ring with an electrophile.
• Common Reactions:
  • Halogenation: Bromination (Br₂/FeBr₃) or Chlorination (Cl₂/AlCl₃).
  • Nitration: HNO₃/H₂SO₄ to introduce a nitro group (-NO₂). The nitro group can be reduced to an amine (-NH₂) using Fe or Zn/HCl followed by a base.
  • Sulfonation: Fuming H₂SO₄ to introduce a sulfonic acid group (-SO₃H). This reaction is reversible with dilute H₂SO₄.
  • Friedel-Crafts Alkylation: Introduction of an alkyl group using an alkyl halide and AlCl₃. Prone to carbocation rearrangements.
  • Friedel-Crafts Acylation: Introduction of an acyl group (R-C=O) using an acyl halide and AlCl₃. Not prone to rearrangement. The resulting ketone can be reduced to an alkyl group (e.g., using Zn(Hg)/HCl).
• Directing Effects: Substituents already on the ring influence the position of further substitution (ortho/para directors vs. meta directors).

VIII. Reactions of Ketones and Aldehydes

• Nucleophilic Addition: The carbonyl carbon is electrophilic and susceptible to nucleophilic attack.
  • Hydrate Formation: Addition of water.
  • Acetal Formation: Reaction with two equivalents of alcohol (or one equivalent of a diol for cyclic acetals). Acetals are useful protecting groups.
  • Imine Formation: Reaction with a primary amine (forms C=N).
  • Enamine Formation: Reaction with a secondary amine (forms C=C-N).
  • Hydrazone Formation: Reaction with hydrazine (forms C=N-NH₂). Can be reduced via Wolff-Kishner reduction.
  • Grignard Reagents: Form new C-C bonds and alcohols after acidic workup.
  • Cyanohydrin Formation: Addition of cyanide (CN⁻).
  • Wittig Reaction: Reaction with a phosphorus ylide to form an alkene, replacing the carbonyl oxygen.
  • Ester Formation: Reaction with hydric acid (less common).

IX. Reactions of Carboxylic Acids

• Preparation:
  • Oxidation of benzylic C-H bonds.
  • Substitution of alkyl bromides with cyanide, followed by hydrolysis of the nitrile.
  • Reaction of Grignard reagents with CO₂, followed by acidic workup.
• Reduction: Can be reduced to primary alcohols using strong reducing agents like LiAlH₄ or borane esters.

X. Acid Chlorides

• Reactivity: Most reactive carboxylic acid derivatives.
• Conversions:
  • To Carboxylic Acids: Reaction with water.
  • To Esters: Reaction with alcohols (ROH) in the presence of pyridine.
  • To Amides: Reaction with excess amines (ammonia, primary, or secondary amines).
  • To Ketones: Reaction with organometallic reagents like Gilman reagents (R₂CuLi).
  • To Alcohols: Reaction with excess Grignard reagents (RMgBr) followed by acidic workup.
  • To Primary Alcohols: Reaction with LiAlH₄.
  • To Aldehydes: Reaction with specific lithium aluminum reagents (e.g., LiAlH(OR)₃).

XI. Anhydrides

• Reactivity: Similar to acid chlorides, but generally less reactive.
• Conversions:
  • To Carboxylic Acids: Reaction with water.
  • To Esters: Reaction with alcohols.
  • To Amides: Reaction with excess amines.
  • To Ketones: Reaction with Gilman reagents (R₂CuLi).
  • To Alcohols: Reaction with excess Grignard reagents and acid.
  • To Primary Alcohols: Reaction with excess LiAlH₄.
  • To Aldehydes: Reaction with lithium aluminum ethers.

XII. Esters

• Preparation:
  • Deprotonation of carboxylic acid followed by SN2 reaction.
  • Fischer Esterification: Reaction of a carboxylic acid with an alcohol in the presence of a catalytic acid. This is a common industrial method.
• Mechanism of Fischer Esterification: Involves protonation of the carbonyl oxygen, nucleophilic attack by the alcohol, proton transfers, and elimination of water.

XIII. Nitriles

• Preparation:
  • Substitution of alkyl halides with sodium cyanide (SN2).
  • Dehydration of amides using thionyl chloride (SOCl₂).
• Reactions:
  • Reduction to Primary Amines: Using LiAlH₄ followed by water.
  • Hydrolysis to Carboxylic Acids: Using acid and heat.

XIV. Enol and Enolate Reactions

• Alpha Carbon: The carbon adjacent to the carbonyl group. Alpha hydrogens are acidic.
• Enol/Enolate Formation: Can be formed under acidic or basic conditions.
• Alpha Halogenation: Introduction of a halogen at the alpha position.
  • Acidic Conditions: Halogenation of ketones and carboxylic acids.
  • Basic Conditions: Can lead to polyhalogenation.
• Haloform Reaction: A two-step process (bromination followed by hydrolysis) that converts a methyl ketone into a carboxylic acid and a haloform (e.g., CHBr₃).
• Aldol Reaction:
  • Aldol Addition: Formation of a β-hydroxy ketone or aldehyde via C-C bond formation between two carbonyl compounds.
  • Aldol Condensation: Dehydration of the β-hydroxy carbonyl compound (often with heat) to form an α,β-unsaturated carbonyl compound.
  • Crossed Aldol Reactions: Involve two different carbonyl compounds.
  • Intramolecular Aldol Condensation: Occurs within the same molecule, leading to ring formation.
• Claisen Condensation:
  • Similar to the aldol reaction but involves esters. Forms a β-keto ester.
  • Crossed Claisen Condensation: Involves an ester and another carbonyl compound (usually an ester without acidic α-hydrogens).
  • Intramolecular Claisen Condensation: Leads to ring formation.

XV. Alkylation of Enolates

• Process: Enolates act as nucleophiles and can be alkylated at the alpha carbon using alkyl halides.
• Base Choice: The choice of base (e.g., NaH vs. LDA) can determine regioselectivity (thermodynamic vs. kinetic enolate formation).
  • LDA (Lithium Diisopropylamide): Strong, bulky base that favors the formation of the kinetic enolate (less substituted alpha carbon).
  • NaH (Sodium Hydride): Smaller base that can favor the thermodynamic enolate (more substituted alpha carbon).
• Michael Addition: Addition of stabilized carbon nucleophiles (like enolates or Gilman reagents) to α,β-unsaturated carbonyl compounds (Michael acceptors).

XVI. Preparation of Amines

• General Strategy: Transform a functional group into a nitrogen-containing precursor, then reduce it to an amine.
• Methods:
  • From Alkyl Halides:
    • SN2 reaction with sodium cyanide (CN⁻) followed by reduction (e.g., LiAlH₄) to extend the carbon chain and form a primary amine.
    • SN2 reaction with sodium azide (N₃⁻) followed by reduction (e.g., H₂/Pt or LiAlH₄) to form a primary amine without carbon chain extension.
  • From Carboxylic Acids: Convert to acid chloride (SOCl₂), then to an amide, followed by reduction (LiAlH₄) to a primary amine.
  • Gabriel Synthesis: Uses phthalimide as a nitrogen source, followed by alkylation and liberation of the primary amine with hydrazine.
  • Reductive Amination: Proceeds through an imine intermediate formed from a carbonyl compound and an amine, followed by reduction (e.g., NaBH₃CN).
    • Ammonia + Carbonyl → Primary Amine
    • Primary Amine + Carbonyl → Secondary Amine
    • Secondary Amine + Carbonyl → Tertiary Amine

XVII. Reactions of Aryldiazonium Salts

• Formation: Typically formed from primary aromatic amines via reaction with nitrous acid (NaNO₂/HCl).
• Sandmeyer Reactions: Replacement of the diazonium group (-N₂⁺) with:
  • Halides (Cl, Br, I) using copper(I) salts (CuCl, CuBr, CuI).
  • Cyano group (-CN) using copper(I) cyanide (CuCN).
• Other Transformations:
  • Fluorination: Replacement with fluoride (F) using HBF₄ (Schiemann reaction).
  • Reduction: Replacement with hydrogen (-H) using H₃PO₂. Useful for removing directing groups after they have served their purpose.
  • Hydrolysis: Replacement with a hydroxyl group (-OH) using H₂SO₄/H₂O and heat.

XVIII. Organometallic Compounds

• Definition: Compounds containing a metal-carbon bond.
• Preparation of Reagents:
  • Organolithium Reagents (RLi): Alkyl halide + 2 Li.
  • Grignard Reagents (RMgX): Alkyl halide + Mg in ether.
  • Gilman Reagents (R₂CuLi): Organolithium reagent + Copper(I) salt.
• Reactions:
  • Suzuki Coupling: Coupling of a vinyl or aryl boronate ester with an alkyl or aryl halide using a palladium catalyst to form a new C-C bond.
  • Formation of C-C Bonds: Grignard and organolithium reagents react with carbonyl compounds, epoxides, etc., to form new C-C bonds.
  • Gilman Reagents: React with acid chlorides to form ketones.
  • Michael Addition: Can act as soft nucleophiles in Michael additions.