Substitution, Elimination, and Addition Reactions: Study Guide for Organic Chemistry

Chapter 7: Substitution and Elimination Reactions

a) Naming Alkyl Halides

Alkyl halides are organic compounds containing halogen atoms (F, Cl, Br, I) attached to an alkyl group. Proper nomenclature is essential for clear communication in organic chemistry.

• Parent Chain Identification: Select the longest continuous carbon chain containing the halogen.

• Halogen Substituents: Name halogen substituents as prefixes (fluoro-, chloro-, bromo-, iodo-).

• Substituent Location: Number the chain to give the halogen the lowest possible number.

• Example: 2-bromopropane

b) Identifying Alkyl Halide Structure: Primary, Secondary, Tertiary

Classification is based on the carbon atom bonded to the halogen:

• Primary (1°): Halogen attached to a carbon bonded to one other carbon.

• Secondary (2°): Halogen attached to a carbon bonded to two other carbons.

• Tertiary (3°): Halogen attached to a carbon bonded to three other carbons.

• Example: 1-bromobutane (primary), 2-bromobutane (secondary), tert-butyl chloride (tertiary)

c) Replicating Reaction Mechanisms

Understanding and drawing mechanisms is crucial for predicting products and stereochemistry.

• SN2: Bimolecular nucleophilic substitution; single step, backside attack, inversion of configuration.

• SN1: Unimolecular nucleophilic substitution; two steps, carbocation intermediate, racemization.

• E2: Bimolecular elimination; single step, anti-coplanar arrangement required.

• E1: Unimolecular elimination; two steps, carbocation intermediate, possible rearrangements.

• Carbocation Rearrangements: Hydride and methyl shifts can occur in SN1 and E1 mechanisms.

d) Factors Affecting SN2/SN1 Reactions

Several factors influence the rate and outcome of substitution reactions:

• Nucleophile Strength: Strong nucleophiles favor SN2; weak nucleophiles favor SN1.

• Solvent Effects: Polar aprotic solvents favor SN2; polar protic solvents favor SN1.

• Leaving Group Ability: Good leaving groups (e.g., I-, Br-) facilitate both SN1 and SN2.

• Steric Hindrance: SN2 is hindered by bulky substrates; SN1 is favored by tertiary carbons.

• Concentration Effects: SN2 rate depends on both substrate and nucleophile concentrations; SN1 depends only on substrate.

e) Predicting Products and Stereochemical Outcomes (SN2 and SN1)

Understanding stereochemistry is essential for predicting reaction outcomes.

• SN2: Inversion of configuration at the reaction center (Walden inversion).

• SN1: Mixture of stereoisomers due to planar carbocation intermediate.

• R/S Configurations: Assign absolute configuration before and after reaction.

f) Predicting Products of Elimination Reactions (E2 and E1)

Elimination reactions form alkenes; product distribution depends on mechanism and substrate.

• E2: Anti-coplanar arrangement required; stereochemistry can be cis or trans.

• E1: Follows Zaitsev's rule (most substituted alkene favored); possible rearrangements.

• Example: Dehydrohalogenation of 2-bromobutane yields 2-butene (cis and trans).

g) Using Newman Projections for E2 Reactions

Newman projections help visualize the anti-coplanar arrangement required for E2 eliminations.

• Anti-coplanar: Hydrogen and leaving group must be 180° apart for E2 to occur.

• Application: Use Newman projections to identify possible conformations and predict major product.

h) Predicting E2 Products Using Conformations

Conformational analysis is used to determine which hydrogens are available for elimination.

• Use of Tables: Reference tables may be provided to assist in predicting products.

i) Electrophile Reactivity: Leaving Groups

Leaving group ability affects the rate and feasibility of substitution and elimination reactions.

• Common Leaving Groups: I-, Br-, Cl-, OTs (tosylate).

• Note: Alcohols are poor leaving groups unless converted to tosylates.

j) Substrate and Reagent Selection

Choosing the correct combination of substrate and reagent is essential for desired transformations.

• Example: SN2 requires a strong nucleophile and a primary substrate; E2 requires a strong base and anti-coplanar geometry.

Chapter 8: Alkenes and Addition Reactions

a) IUPAC Naming of Alkenes (cis/trans, E/Z)

Alkenes are named based on the longest chain containing the double bond, with proper indication of stereochemistry.

• Cis/Trans: Used for simple alkenes with two different groups on each carbon of the double bond.

• E/Z: Used for more complex alkenes; based on Cahn-Ingold-Prelog priority rules.

• Example: (E)-2-butene, (Z)-2-butene

b) Relative Stabilities of Alkenes

Alkene stability is influenced by substitution and conjugation.

• Trans Alkenes: More stable than cis due to less steric strain.

• More Substituted Alkenes: Increased stability due to hyperconjugation and alkyl group electron donation.

• Example: Tetra-substituted alkenes are more stable than di-substituted.

c) Predicting Products of Addition Reactions

Addition reactions convert alkenes to saturated compounds by adding atoms across the double bond.

• Markovnikov Addition: Hydrogen adds to the carbon with more hydrogens; halide or other group adds to the more substituted carbon.

• Anti-Markovnikov Addition: Hydrogen adds to the less substituted carbon; often occurs with peroxides.

• Carbocation Rearrangements: Possible in reactions involving carbocation intermediates (e.g., hydride or methyl shifts).

• Syn vs. Anti Addition: Syn addition: both groups add to the same face; anti addition: groups add to opposite faces.

• Common Reactions:

  • Hydrohalogenation (HX): Markovnikov addition; rearrangements possible.

  • Hydration (H2O/H+): Markovnikov addition; rearrangements possible.

  • Hydroboration-Oxidation (BH3/H2O2): Anti-Markovnikov, syn addition.

  • Halogenation (Br2, Cl2): Anti addition.

  • Oxymercuration-Demercuration (Hg(OAc)2/NaBH4): Markovnikov, no rearrangement.

  • Ozonolysis (O3): Cleavage of double bond; work backwards if product is given.

d) Mechanism Drawing and Carbocation Rearrangements

Mechanisms must be drawn for key reactions, with attention to possible rearrangements.

• Required Mechanisms: SN2, E2, SN1, E1 (from Chapter 7); HX addition, hydration (from Chapter 8).

• Carbocation Rearrangements: Be aware that hydride and methyl shifts can occur in carbocation intermediates.

e) Multistep Reaction Synthesis

Multistep synthesis involves predicting products and selecting reagents for a sequence of reactions.

• Predicting Products: Given a starting material, determine the outcome of multiple reactions.

• Identifying Intermediates: Recognize common intermediates between steps.

• Reagent Selection: Choose appropriate reagents from a provided list to achieve desired transformations.

• Example: Convert an alkene to an alcohol via hydration, then to an alkyl halide via substitution.

Additional info: These notes expand on the skills and concepts listed in the original document, providing definitions, examples, and academic context for each topic. Mechanism drawing, stereochemistry, and reaction prediction are emphasized as key skills for exam success.