Unit 3: Elimination and Substitution Reactions

Labeling Base/Nucleophile Strength, E2 Reactions, Zaitsev's Rule

Understanding the strength of bases and nucleophiles is essential for predicting the outcome of elimination and substitution reactions. The E2 reaction is a bimolecular elimination process, and Zaitsev's rule helps determine the major product.

• Base/Nucleophile: A base forms a bond with hydrogen, while a nucleophile forms a bond with carbon.

• Strong base/strong nucleophile: Examples include NaH, NH2-, OH-.

• Bulky bases: Examples include (CH3)3CO-.

• Weak base/weak nucleophile: Examples include polar protic solvents such as H2O, ROH, RCO2H.

• E2 Mechanism: A one-step, concerted elimination where a base abstracts a β-hydrogen, and a leaving group departs from the α-carbon, forming a double bond.

• Zaitsev's Rule: The major product of elimination is the more substituted alkene due to increased stability.

Example: Dehydrohalogenation of 2-bromobutane with a strong base yields 2-butene as the major product.

Stereoselectivity of E2, Dehydration of Alcohols, E1 Mechanism

Stereoselectivity in E2 reactions arises from the requirement that the leaving group and the β-hydrogen must be antiperiplanar. Dehydration of alcohols is an E1 process, often acid-catalyzed.

• E2 Stereochemistry: The anti-periplanar arrangement leads to trans-alkene products.

• E1 Mechanism: Involves two steps: loss of the leaving group to form a carbocation, followed by deprotonation to form the alkene.

• Dehydration of Alcohols: Acid-catalyzed (H2SO4 or H3PO4), forms alkenes via E1 mechanism.

Example: Dehydration of 2-butanol yields 2-butene as the major product.

Rates of Elimination: E1, E2, Substrate Effects, Mechanisms

The rate of elimination reactions depends on the substrate structure and the mechanism.

• E1 Rate: Faster when the alkyl halide is more substituted (tertiary > secondary > primary) due to carbocation stability.

• E2 Rate: Faster when the alkene forming in the transition state is more substituted (tertiary > secondary > primary).

• Heat Effects: Elimination reactions are favored at higher temperatures due to increased entropy ().

• Thermodynamics: ; spontaneous reactions have negative .

Example: E2 elimination of 2-bromopropane with a strong base yields propene.

Alkyl Halides: Substitution and Elimination

Primary, Secondary, Tertiary Alkyl Halides: SN1, SN2, E1, E2 Mechanisms

Alkyl halides undergo substitution and elimination reactions depending on their structure and the strength of the base/nucleophile.

• Primary Alkyl Halides: Favor SN2 with strong nucleophiles/bases; E2 with bulky bases.

• Secondary Alkyl Halides: SN2 with strong nucleophiles; E2 with strong bases; SN1/E1 with weak nucleophiles/bases.

• Tertiary Alkyl Halides: Favor SN1/E1 with weak nucleophiles/bases; E2 with strong bases.

Example: 2-bromo-2-methylpropane reacts via SN1 mechanism to form tert-butyl alcohol.

Table: Reaction Mechanisms vs Alkyl Halide Type

Alcohols: Reactions and Mechanisms

Halogenation of Alcohols, Epoxide Opening, Alkylation of Amines

Alcohols can be converted to alkyl halides, ethers, and other derivatives. Epoxides are reactive three-membered cyclic ethers that undergo ring opening. Amines can be alkylated to form higher-order amines and ammonium salts.

• Halogenation: Alcohols react with HX (HCl, HBr, HI) to form alkyl halides via SN1 or SN2 mechanisms.

• Epoxide Opening: Acid-catalyzed opening yields trans-1,2-diols; base-catalyzed opening yields anti products.

• Alkylation of Amines: SN2 reaction with alkyl halides; repeated alkylation forms quaternary ammonium salts.

Example: Reaction of ethanol with HBr yields bromoethane.

Amines: Structure, Naming, and Reactions

Structure and Classification of Amines

Amines are organic compounds containing a nitrogen atom bonded to one or more alkyl or aryl groups. They are classified as primary, secondary, tertiary, or quaternary based on the number of carbon groups attached to nitrogen.

• Primary Amine: Nitrogen attached to one carbon (e.g., methylamine).

• Secondary Amine: Nitrogen attached to two carbons (e.g., dimethylamine).

• Tertiary Amine: Nitrogen attached to three carbons (e.g., trimethylamine).

• Quaternary Ammonium Salt: Nitrogen attached to four carbons, carrying a positive charge (e.g., tetramethylammonium chloride).

Naming Amines and Stereochemistry

Amines are named based on the alkyl groups attached to nitrogen. Stereochemistry is relevant when nitrogen is part of a stereocenter.

• Naming: List alkyl groups alphabetically, add 'amine' as suffix.

• Stereochemistry: Nitrogen with a lone pair is not a stereocenter unless it is part of a quaternary ammonium salt.

Example: N-ethyl-N-methyl-1-propanamine.

Aromaticity, Anti-Aromaticity, and Non-Aromatic Compounds

Definitions and Properties

Aromatic compounds are cyclic, planar molecules with a continuous ring of p orbitals and follow Hückel's rule (4n+2 π electrons). Anti-aromatic compounds are cyclic and planar but have 4n π electrons, leading to instability. Non-aromatic compounds lack a continuous ring of p orbitals.

• Aromatic: Benzene, cyclopentadienyl anion (6 π electrons).

• Anti-aromatic: Cyclobutadiene (4 π electrons).

• Non-aromatic: Cyclohexane (no continuous p orbitals).

Example: Pyrrole is aromatic; cyclobutadiene is anti-aromatic.

Bond Lengths and Molecular Orbital Theory

Aromatic compounds have equalized bond lengths due to delocalization of π electrons. Molecular orbital theory explains the stability of aromatic systems.

• Benzene: All C–C bonds are approximately 1.39 Å.

• Cyclohexane: Alternating single and double bonds, bond lengths differ.

Electrophilic Aromatic Substitution (EAS)

Mechanism and Types

EAS is a reaction where an aromatic ring reacts with an electrophile, substituting a hydrogen atom. Common EAS reactions include nitration, sulfonation, alkylation, and acylation.

• Nitration: Introduction of a nitro group (NO2).

• Sulfonation: Introduction of a sulfonic acid group (SO3H).

• Friedel-Crafts Alkylation/Acylation: Introduction of alkyl or acyl groups using AlCl3 catalyst.

Example: Benzene reacts with HNO3/H2SO4 to form nitrobenzene.

Table: Common EAS Reactions

Summary of Reaction Mechanisms

Key Mechanisms and Steps

• E2: Strong base, anti-periplanar geometry, forms more substituted alkene (Zaitsev product).

• E1: Bulky base, forms more substituted alkene, two-step mechanism.

• SN2: Strong nucleophile, backside attack, inversion of configuration.

• SN1: Weak nucleophile, carbocation intermediate, racemization.

• Alcohol Dehydration: Acid-catalyzed, forms alkene.

• Epoxide Opening: Acid or base-catalyzed, forms trans-1,2-diol.

• Alkylation of Amines: SN2 mechanism, forms quaternary ammonium salts.

• EAS: Electrophilic aromatic substitution, forms substituted aromatic compounds.

Additional info:

• Some mechanistic details and examples have been expanded for clarity and completeness.

• Tables have been inferred and reconstructed for comparison and classification purposes.