Organic Chemistry: Substitution, Elimination, and Electrophilic Addition Reactions (Ch. 7 & 8)

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

General Principles for Organic Reaction Mechanisms

Key Concepts in Reaction Analysis

Understanding organic reaction mechanisms requires careful attention to reactants, conditions, and the types of reactions involved. The following points summarize essential strategies for analyzing and predicting organic reactions:

Reaction Conditions: Always observe the reactant, solvent, and whether the species involved are electrophiles or nucleophiles, acids or bases.
Reagent Identification: Do not mix up reagents; each has a specific role in a reaction.
Atom Counting: Account for all atoms (carbon, hydrogen, etc.) to ensure mass balance.
Charge Conservation: Make sure charges are correct throughout the mechanism.
Fundamental Concepts: Review key ideas such as nomenclature, acid-base chemistry, structure, and stereochemistry, as these underpin substitution and elimination reactions.
Mechanism Reproduction: Mechanisms that are bolded in your materials must be able to be reproduced in detail.

Application: Use all learned concepts to produce viable synthetic schemes for organic molecules.

Chapter 7: Chemistry of Alkyl Halides – Substitutions and Eliminations

Distinguishing Electrophilic and Nucleophilic Sites

Alkyl halides undergo substitution and elimination reactions, which are central to organic synthesis. Recognizing the nature of the reacting sites is crucial:

Electrophilic Sites: Typically carbon atoms bonded to halogens, susceptible to nucleophilic attack.
Nucleophilic Sites: Atoms or groups with lone pairs or negative charge, capable of donating electrons.

Leaving Groups and Arrow Pushing

Good Leaving Groups: Halides (Cl-, Br-, I-), tosylates, and others that stabilize negative charge.
Arrow Pushing: Use curved arrows to show electron movement during bond formation and breaking.

Types of Substitution and Elimination Mechanisms

SN1 Mechanism: Unimolecular nucleophilic substitution; involves carbocation intermediate.
SN2 Mechanism: Bimolecular nucleophilic substitution; concerted reaction with inversion of configuration.
E1 Mechanism: Unimolecular elimination; forms carbocation intermediate, then loss of a proton.
E2 Mechanism: Bimolecular elimination; concerted removal of a proton and leaving group.

Example: The reaction of 2-bromopropane with hydroxide ion can proceed via SN2 or E2, depending on conditions.

Comparing SN1, SN2, E1, and E2 Mechanisms

Mechanism	Rate Law	Intermediate	Stereochemistry
SN1	Rate = k[Alkyl Halide]	Carbocation	Racemization
SN2	Rate = k[Alkyl Halide][Nucleophile]	None	Inversion
E1	Rate = k[Alkyl Halide]	Carbocation	Mixture
E2	Rate = k[Alkyl Halide][Base]	None	Anti-coplanar elimination

Factors Affecting Mechanism Choice

Substrate Structure: Tertiary substrates favor SN1/E1; primary favor SN2/E2.
Base/Nucleophile Strength: Strong bases favor E2; strong nucleophiles favor SN2.
Solvent Effects: Polar protic solvents favor SN1/E1; polar aprotic favor SN2/E2.

Predicting Products and Mechanisms

Be able to predict the major product and mechanism for a given alkyl halide and reagent.
Draw and explain the stepwise mechanism using curved arrows.

Chapter 8: Electrophilic Addition Reactions

Thermodynamics and Regioselectivity of Addition

Electrophilic addition reactions are fundamental for alkenes and alkynes, involving the addition of electrophiles and nucleophiles across double or triple bonds.

Thermodynamics: Addition reactions are often exothermic, favoring product formation.
Regioselectivity: Markovnikov's rule predicts that the electrophile adds to the carbon with more hydrogens.

Mechanisms of Key Addition Reactions

Markovnikov Hydration: Addition of water across a double bond, following Markovnikov's rule.
Anti-Markovnikov Addition: Occurs in the presence of peroxides (e.g., hydroboration-oxidation).
Acid-Catalyzed Hydration: Uses acid to catalyze the addition of water to an alkene.
Oxymercuration-Demercuration: A two-step process for hydration without carbocation rearrangement.
Hydroboration-Oxidation: Adds water in an anti-Markovnikov fashion.
Halogenation: Addition of halogens (Br2, Cl2) to alkenes, forming vicinal dihalides.
Halohydrin Formation: Addition of halogen and water to form halohydrins.
Hydrogenation: Addition of H2 across double bonds, typically using a metal catalyst.
Ozonolysis: Cleavage of double bonds using ozone, yielding carbonyl compounds.

Mechanism Example: Markovnikov Hydration

Step 1: Protonation of the alkene to form a carbocation.
Step 2: Nucleophilic attack by water.
Step 3: Deprotonation to yield the alcohol.

Equation:

Comparing Markovnikov and Anti-Markovnikov Addition

Type	Reagents	Product Orientation
Markovnikov	Acid, water	Electrophile adds to less substituted carbon
Anti-Markovnikov	BH3, H2O2	Electrophile adds to more substituted carbon

Predicting Products and Drawing Mechanisms

Identify reagents and predict the major product.
Draw the stepwise mechanism, showing electron flow.
Explain the regioselectivity and stereochemistry of the reaction.

Summary Table: Key Addition Reactions

Reaction	Reagents	Product	Mechanism Notes
Hydration	H2O, acid	Alcohol	Markovnikov, carbocation intermediate
Hydroboration-Oxidation	BH3, H2O2	Alcohol	Anti-Markovnikov, concerted mechanism
Halogenation	Br2, Cl2	Dihalide	Anti addition, cyclic halonium ion intermediate
Ozonolysis	O3	Carbonyl compounds	Oxidative cleavage

Additional info:

Students should be able to use synthetic strategies to build complex molecules from simple precursors.
Recognize and use common protecting groups and functional group interconversions.
Apply knowledge of reaction mechanisms to predict outcomes and design syntheses.