Final Exam Study Guide: Aromaticity, Resonance, and Alkyl Halide Reactions

Ch. 8 - Delocalized Electrons: Aromaticity, Resonance, and Related Concepts

Resonance and Delocalization

Delocalization of electrons is a key concept in organic chemistry, affecting molecular stability and reactivity. Resonance structures are used to represent molecules where electrons are shared across multiple atoms.

• Resonance Structures: Different Lewis structures for the same molecule, showing delocalization of electrons (e.g., in benzene).

• Criteria for Aromaticity: A molecule is aromatic if it is cyclic, planar, fully conjugated, and follows Hückel's rule (4n+2 π electrons).

• Stability: Aromatic compounds are unusually stable due to electron delocalization.

Example: Benzene has six π electrons, is planar, and all carbons are sp2 hybridized, making it aromatic.

Resonance Contributors and Stability

• Major Contributors: Resonance forms with full octets, minimal charge separation, and negative charges on electronegative atoms are most stable.

• Minor Contributors: Less stable forms, but still contribute to the overall hybrid.

Example: The acetate ion has two equivalent resonance structures, both contributing equally to the hybrid.

Electrophilic Aromatic Substitution (EAS)

Aromatic compounds undergo substitution reactions where an electrophile replaces a hydrogen atom on the ring.

• Common EAS Reactions: Nitration, sulfonation, halogenation, Friedel-Crafts alkylation/acylation.

• Regioselectivity: Substituents can direct new groups to ortho, meta, or para positions.

Example: Toluene undergoes nitration to give ortho and para nitrotoluene.

Conjugated and Nonconjugated Systems

• Conjugated Systems: Alternating single and double bonds allow for delocalization of π electrons.

• Nonconjugated Systems: Double bonds separated by more than one single bond; less stable due to lack of delocalization.

Example: 1,3-butadiene is conjugated; 1,4-pentadiene is nonconjugated.

Multistep Synthesis Involving Aromatic Compounds

• Designing synthetic routes often involves using EAS and resonance stabilization to achieve target molecules.

Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

Alkyl Halides: Structure and Reactivity

Alkyl halides are organic molecules containing a halogen atom bonded to an sp3 carbon. Their reactivity is central to many substitution and elimination reactions.

• Classification: Primary, secondary, and tertiary alkyl halides differ in reactivity due to steric and electronic effects.

Substitution Reactions: SN1 and SN2 Mechanisms

• SN2 Mechanism: One-step, bimolecular process; rate depends on both nucleophile and substrate.

• SN1 Mechanism: Two-step, unimolecular process; rate depends only on substrate; involves carbocation intermediate.

Example: Methyl bromide reacts with hydroxide via SN2; tert-butyl bromide reacts with water via SN1.

Equation:

$\text{Rate}_{\text{S}_N2} = k[\text{Alkyl Halide}][\text{Nucleophile}]$

$\text{Rate}_{\text{S}_N1} = k[\text{Alkyl Halide}]$

Elimination Reactions: E1 and E2 Mechanisms

• E2 Mechanism: One-step, bimolecular elimination; requires anti-coplanar geometry; follows Zaitsev's rule for regioselectivity.

• E1 Mechanism: Two-step, unimolecular elimination; forms carbocation intermediate.

Example: 2-bromopropane with strong base undergoes E2 to form propene.

Equation:

$\text{Rate}_{\text{E}2} = k[\text{Alkyl Halide}][\text{Base}]$

$\text{Rate}_{\text{E}1} = k[\text{Alkyl Halide}]$

Competition Between Substitution and Elimination

• Factors such as substrate structure, base/nucleophile strength, solvent, and temperature determine the preferred pathway.

• Primary Alkyl Halides: Favor SN2 unless bulky base is used (then E2).

• Tertiary Alkyl Halides: Favor SN1/E1 in polar protic solvents; E2 with strong base.

Carbocation Rearrangements

• Carbocations formed during SN1/E1 can rearrange via hydride or alkyl shifts to form more stable carbocations.

Example: 3-bromo-2-methylpentane forms a tertiary carbocation via hydride shift during SN1.

Summary Table: Substitution and Elimination Mechanisms

Mechanism	Order	Intermediate	Favored by
SN2	Second	None	Strong nucleophile, primary substrate
SN1	First	Carbocation	Weak nucleophile, tertiary substrate
E2	Second	None	Strong base, anti-coplanar geometry
E1	First	Carbocation	Weak base, tertiary substrate

Predicting Products and Mechanisms

• Analyze substrate, nucleophile/base, and solvent to predict major products and mechanism.

• Consider rearrangements and regioselectivity (Zaitsev vs. Hofmann product).

Practice Problems and Applications

• Draw mechanisms for SN1, SN2, E1, and E2 reactions.

• Predict major and minor products for given alkyl halide reactions.

• Design synthetic routes using substitution and elimination reactions.

Additional info: This guide omits sections 8.8, 8.10, 8.13, 8.15, 8.21, and 9.14 as per the original document's instructions.