Organic Chemistry Exam #2 Study Guide: Substitution, Elimination, and Alkene Reactions

Substitution and Elimination Reactions

Ranking Nucleophiles and Substrates in SN2 and SN1 Reactions

Understanding the factors that affect the rates of nucleophilic substitution reactions is essential in organic chemistry. The SN2 and SN1 mechanisms differ in their dependence on nucleophile strength, substrate structure, and solvent effects.

• SN2 Reaction: Bimolecular nucleophilic substitution; rate depends on both nucleophile and substrate.

• SN1 Reaction: Unimolecular nucleophilic substitution; rate depends only on substrate (formation of carbocation intermediate).

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

• Substrate Structure: Methyl and primary substrates favor SN2; tertiary substrates favor SN1 due to carbocation stability.

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

Example: Ranking nucleophiles for SN2 reaction with iodomethane: $OH^-$ > $NH_2^-$ > $CH_3COO^-$ > $H_2O$ > $CH_3COOH$ (from fastest to slowest).

Alkene Stability

Alkene stability is influenced by substitution and conjugation. More substituted and conjugated alkenes are generally more stable due to hyperconjugation and electron delocalization.

• Degree of Substitution: Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted.

• Conjugation: Conjugated alkenes are more stable than isolated or cumulated alkenes.

Example: 2,3-dimethyl-2-butene (tetrasubstituted) is more stable than 1-butene (monosubstituted).

Ranking SN1 Reaction Rates

The SN1 reaction rate is determined by the stability of the carbocation intermediate formed after the leaving group departs.

• Carbocation Stability: Tertiary > Secondary > Primary > Methyl.

• Resonance Stabilization: Allylic and benzylic carbocations are stabilized by resonance.

Example: Benzyl bromide reacts faster than cyclohexyl bromide in SN1 due to resonance stabilization.

Radical Halogenation and Photolysis

Monochlorination and Bromination of Alkanes

Halogenation of alkanes via photolysis (light-induced reaction) produces alkyl halides. The selectivity of bromination and chlorination differs due to the reactivity of the halogen radicals.

• Bromination: Highly selective; prefers formation of the most stable (usually tertiary) radical.

• Chlorination: Less selective; can produce a mixture of products.

• Major Product: The product formed via the most stable radical intermediate.

Example: Bromination of isobutane yields tert-butyl bromide as the major product.

Assigning E/Z Configurations to Alkenes

The E/Z system is used to describe the stereochemistry of double bonds based on the Cahn-Ingold-Prelog priority rules.

• E (Entgegen): Higher priority groups on opposite sides of the double bond.

• Z (Zusammen): Higher priority groups on the same side of the double bond.

Example: In 2-butene, if the two methyl groups are on the same side, it is Z; if on opposite sides, it is E.

Predicting Products of Organic Reactions

Common Reaction Types

Organic reactions often involve nucleophilic substitution (SN2, SN1), elimination (E2, E1), and addition reactions. Predicting products requires understanding the mechanism and the nature of the reactants.

• SN2: Inversion of configuration at the reaction center.

• SN1: Racemization possible due to planar carbocation intermediate.

• E2: Anti-coplanar elimination; forms alkenes.

• E1: Carbocation intermediate; forms alkenes.

Example: Reaction of 2-bromopropane with $CH_3O^-$ yields propene via E2 elimination.

Solvent and Nucleophile Effects on Reaction Mechanism

Solvent Effects

Solvents can dramatically affect the rate and outcome of substitution and elimination reactions.

• Polar Aprotic Solvents: Increase SN2 rate by stabilizing cations but not anions (e.g., DMF, DMSO).

• Polar Protic Solvents: Favor SN1 by stabilizing carbocations and nucleophiles (e.g., water, alcohols).

Example: SN2 reaction rate increases in DMF compared to water.

Nucleophile Structure Effects

The structure and basicity of the nucleophile/base can shift the reaction mechanism between SN2, E2, and SN1/E1.

• Bulky Bases: Favor E2 elimination over SN2 substitution (e.g., $t$-butoxide).

• Strong Nucleophiles: Favor SN2 substitution.

• Weak Nucleophiles: Favor SN1/E1 mechanisms.

Example: Sodium methoxide ($CH_3ONa$) favors SN2, while sodium $t$-butoxide favors E2.

Tables and Comparative Data

Exam Points Distribution Table

The following table summarizes the points assigned to each question on the exam, indicating the relative importance of each topic.

Question	Points Possible
1	45
2	38
3	15
4	84
5	30
6	14
7	15
8	9
Total	250

Key Equations and Mechanisms

• SN2 Rate Law: $\text{Rate} = k[\text{Nucleophile}][\text{Electrophile}]$

• SN1 Rate Law: $\text{Rate} = k[\text{Electrophile}]$

• E2 Elimination: $\text{Base} + \text{Alkyl Halide} \rightarrow \text{Alkene} + \text{Halide Ion} + \text{Conjugate Acid}$

• Radical Halogenation: $\text{RH} + X_2 \xrightarrow{hv} \text{RX} + HX$

Summary Table: SN2 vs SN1 vs E2 vs E1

Mechanism	Substrate	Nucleophile/Base	Solvent	Product
SN2	Methyl, 1°	Strong, small	Aprotic	Substitution (inversion)
SN1	3°, Allylic, Benzylic	Weak	Protic	Substitution (racemization)
E2	1°, 2°, 3°	Strong, bulky	Aprotic	Alkene (anti-coplanar)
E1	3°, Allylic, Benzylic	Weak	Protic	Alkene (carbocation)

Additional info:

• Some product structures and mechanistic details were inferred based on standard organic chemistry curriculum.

• Tables and rankings were reconstructed for clarity and completeness.