Key Organic Chemistry Mechanisms and Concepts: Aromatics, Organometallics, and Alcohol/Ether Chemistry

Mechanisms of Aromatic Compounds (Ch. 13)

Nitration of Benzene

The nitration of benzene introduces a nitro group (-NO2) onto the aromatic ring via electrophilic aromatic substitution.

• Reagents: Concentrated HNO3 and H2SO4

• Mechanism: Generation of the nitronium ion (NO2+), followed by its attack on benzene and subsequent deprotonation.

• Equation: $\mathrm{C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2 + H_2O}$

Sulfonation of Benzene

Sulfonation introduces a sulfonic acid group (-SO3H) onto benzene.

• Reagents: Fuming H2SO4 (contains SO3)

• Mechanism: Electrophilic attack by SO3 on benzene, followed by proton transfer.

• Equation: $\mathrm{C_6H_6 + SO_3 \xrightarrow{H_2SO_4} C_6H_5SO_3H}$

Halogenation of Benzene

Halogenation introduces a halogen atom (Cl or Br) onto the aromatic ring via electrophilic aromatic substitution.

• Reagents: Cl2 or Br2 with FeCl3 or FeBr3 catalyst

• Mechanism: Formation of the electrophilic halogen species, attack on benzene, and deprotonation.

• Equation: $\mathrm{C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl}$

Friedel-Crafts Alkylation

Alkylation introduces an alkyl group onto benzene using an alkyl halide and a Lewis acid catalyst.

• Reagents: R–Cl and AlCl3

• Mechanism: Generation of a carbocation (or equivalent), electrophilic attack on benzene, and deprotonation.

• Limitations: Carbocation rearrangements and polyalkylation can occur.

• Equation: $\mathrm{C_6H_6 + RCl \xrightarrow{AlCl_3} C_6H_5R + HCl}$

Friedel-Crafts Acylation

Acylation introduces an acyl group onto benzene, forming a ketone.

• Reagents: RCOCl and AlCl3

• Mechanism: Generation of acylium ion, electrophilic attack, and deprotonation.

• Equation: $\mathrm{C_6H_6 + RCOCl \xrightarrow{AlCl_3} C_6H_5COR + HCl}$

Thionyl Chloride Conversion of Carboxylic Acids to Acyl Chlorides

Carboxylic acids are converted to acyl chlorides using thionyl chloride (SOCl2).

• Equation: $\mathrm{RCOOH + SOCl_2 \rightarrow RCOCl + SO_2 + HCl}$

Nucleophilic Aromatic Substitution (SNAr)

SNAr occurs on aromatic rings bearing electron-withdrawing groups (e.g., nitro) ortho/para to the leaving group.

• Mechanism: Addition-elimination via a Meisenheimer complex.

• Example: Conversion of 2,4-dinitrochlorobenzene to 2,4-dinitrophenol with NaOH.

Mechanisms and Concepts in Organometallic Chemistry (Ch. 15)

Preparation of Organolithium and Grignard Reagents

• Organolithium: $\mathrm{R-X + 2Li \rightarrow R-Li + LiX}$

• Grignard: $\mathrm{R-X + Mg \xrightarrow{ether} R-MgX}$

• Key Point: Both are strong nucleophiles and bases, used for C–C bond formation.

Preparation of Sodium Alkynides from Terminal Alkynes

• Equation: $\mathrm{RC \equiv CH + NaNH_2 \rightarrow RC \equiv CNa + NH_3}$

• Application: Useful for alkylation reactions.

Organometallic Addition to Aldehydes and Ketones

• Mechanism: Nucleophilic addition to the carbonyl, followed by acidic workup to yield alcohols.

• Equation: $\mathrm{R-MgX + R'CHO \rightarrow R'CH(OH)R}$ (after H3O+ workup)

Simmons-Smith Reaction

• Purpose: Cyclopropanation of alkenes using CH2I2 and Zn(Cu).

• Equation: $\mathrm{Alkene + CH_2I_2 + Zn(Cu) \rightarrow Cyclopropane}$

Gilman Reagent Preparation

• Equation: $\mathrm{2R-Li + CuI \rightarrow R_2CuLi + LiI}$

• Application: Used for coupling with alkyl halides, especially sp2 carbons.

Hydrogenation Catalytic Cycle

• Mechanism: Involves adsorption of H2 and substrate on metal catalyst, followed by hydrogen transfer.

• Equation (general): $\mathrm{RCH=CH_2 + H_2 \xrightarrow{Pd/C} RCH_2CH_3}$

Free Radical Polymerization

• Initiation: Formation of radicals (e.g., by peroxides).

• Propagation: Radical adds to monomer, generating a new radical.

• Termination: Combination or disproportionation of radicals.

Ziegler-Natta Catalyst

• Purpose: Stereospecific polymerization of alkenes (e.g., polyethylene, polypropylene).

• Components: TiCl4 and Al(C2H5)3

Additional Organometallic Concepts

• Organometallic Nomenclature: Naming based on the organic group and metal (e.g., methylmagnesium bromide).

• Bond Polarity: Determined by electronegativity difference; C–Li and C–Mg bonds are highly polar (carbon is nucleophilic).

• Reactivity with Protic Solvents: Organometallics react violently with water or alcohols, producing hydrocarbons and metal hydroxides.

• Gilman Reagent: Lithium dialkylcuprate (R2CuLi) is especially useful for coupling with sp2 carbons.

• Wilkinson’s Catalyst: [RhCl(PPh3)3] used for homogeneous hydrogenation of alkenes.

• Grubbs’ Catalyst: Ruthenium-based catalyst for olefin metathesis.

Mechanisms of Alcohols, Ethers, and Related Compounds (Ch. 16)

Hydrogenation Catalytic Cycle with Carbonyls

• Mechanism: Similar to alkene hydrogenation; carbonyl is reduced to alcohol.

• Equation: $\mathrm{R_2C=O + H_2 \xrightarrow{Pd/C} R_2CHOH}$

Reduction of Aldehydes and Ketones

• NaBH4 Reduction: Mild, selective for aldehydes and ketones.

• Equation: $\mathrm{R_2C=O + NaBH_4 + H_2O \rightarrow R_2CHOH + NaOH + BH_3}$

• LiAlH4 Reduction: Stronger, reduces aldehydes, ketones, carboxylic acids, and esters.

• Equation: $\mathrm{RCOOH + LiAlH_4 \rightarrow RCH_2OH}$

Organometallics with Carbonyls

• Mechanism: Nucleophilic addition to carbonyl, followed by acidic workup to yield alcohols (see above).

Intramolecular Ether Formation

• Mechanism: Williamson ether synthesis can occur intramolecularly to form cyclic ethers.

• Example: Formation of tetrahydrofuran from 1,4-dibromobutane and base.

Fischer Esterification (I, II, III)

• Mechanism: Acid-catalyzed reaction of carboxylic acid and alcohol to form ester and water.

• Equation: $\mathrm{RCOOH + R'OH \xrightarrow{H^+} RCOOR' + H_2O}$

Oxidation of Alcohols

• Chromic Acid (H2CrO4): Oxidizes primary alcohols to carboxylic acids, secondary to ketones.

• PCC (Pyridinium Chlorochromate): Oxidizes primary alcohols to aldehydes, secondary to ketones.

• PDC (Pyridinium Dichromate): Similar to PCC, but can be used in different solvents.

• Periodic Acid (HIO4): Cleaves vicinal diols to aldehydes or ketones.

Summary Table: Key Reagents and Their Functions

Reagent	Main Function	Example Reaction
HNO3/H2SO4	Nitration of benzene	Benzene → Nitrobenzene
SOCl2	Convert carboxylic acid to acyl chloride	RCOOH → RCOCl
R–MgX	Grignard reagent, nucleophilic addition	R–MgX + R'CHO → Alcohol
NaBH4	Reduce aldehydes/ketones	R2C=O → R2CHOH
LiAlH4	Reduce carboxylic acids/esters	RCOOH → RCH2OH
PCC/PDC	Oxidize alcohols to aldehydes/ketones	RCH2OH → RCHO
HIO4	Cleavage of vicinal diols	Glycol → 2 Aldehydes/Ketones
AlCl3	Lewis acid catalyst (Friedel-Crafts)	Benzene + RCl → Alkylbenzene

Additional info: Where mechanisms were only listed by name, standard textbook mechanisms and equations have been provided for completeness. For Fischer Esterification, I, II, and III refer to the same general acid-catalyzed esterification process, possibly with different substrates or conditions.