Alkyl Halides & Elimination Reactions

Substitution versus Elimination

Organic reactions involving alkyl halides can proceed via substitution or elimination pathways. These pathways compete, and the outcome depends on the nature of the reactants and reaction conditions.

• Substitution: A nucleophile replaces the leaving group (halide) on the carbon atom.

• Elimination: A base removes a proton (H) from a β-carbon, resulting in the loss of HX and formation of an alkene.

• Competition: Strong bases that are also good nucleophiles may promote both substitution and elimination.

• Key Terms: Electrophile (electron-deficient species, Lewis acid), Nucleophile (electron-rich species, Lewis base).

Example: Hydroxide ion (OH-) can act as both a base (removing H+) and a nucleophile (attacking carbon).

General Features of Elimination

Elimination reactions of alkyl halides with Brønsted-Lowry bases result in the formation of alkenes via dehydrohalogenation (removal of HX).

• Dehydrohalogenation: The process by which a base removes a proton from the β-carbon, and the leaving group (halide) departs from the α-carbon, forming an alkene.

• Steps:

  1. Identify the α-carbon (bonded to the leaving group).

  2. Identify all β-carbons (adjacent to α-carbon) with available hydrogens.

  3. Remove H from β-carbon and X from α-carbon to form the double bond.

• Common Bases Used:

  • Sodium hydroxide (NaOH), pKaH ≈ 15

  • Potassium tert-butoxide (tBuOK), pKaH ≈ 19

  • Sodium amide (NaNH2), pKaH ≈ 38

  • DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), pKaH ≈ 14

Equation:

Mechanisms of Elimination: E2 and E1

Elimination reactions proceed via two main mechanisms: E2 (bimolecular, concerted) and E1 (unimolecular, stepwise).

E2 Mechanism (Bimolecular)

• Occurs in a single concerted step.

• Requires anti-periplanar geometry of H and X.

• Rate depends on both alkyl halide and base concentrations.

Rate Law:

E1 Mechanism (Unimolecular)

• Occurs in two steps: formation of carbocation (slow), then deprotonation (fast).

• Rate depends only on alkyl halide concentration.

Rate Law:

Leaving Group Effects and Product Selectivity

The rate of elimination increases with better leaving groups (I > Br > Cl > F). Elimination can yield one or multiple alkene products, with selectivity for internal vs. terminal alkenes and cis vs. trans stereochemistry.

• Leaving Group Ability: Iodide is the best, followed by bromide, chloride, and fluoride.

• Product Selectivity:

  • Regioselectivity: Internal (more substituted) or terminal (less substituted) alkene.

  • Stereoselectivity: Formation of cis or trans isomers.

Alkenes: Structure and Classification

Alkenes are hydrocarbons containing a carbon-carbon double bond. The double bond consists of a sigma (σ) bond and a pi (π) bond formed by the overlap of sp2 hybrid orbitals and unhybridized p orbitals.

• Bond Framework: σ bond from sp2–sp2 overlap; π bond from p–p overlap.

• Classification by Substitution:

  • Monosubstituted: one R group

  • 1,1-Disubstituted: two R groups on same carbon

  • 1,2-Disubstituted: two R groups on different carbons

  • Trisubstituted: three R groups

  • Tetrasubstituted: four R groups

• (E)/(Z) Diastereomers: For di-, tri-, and tetrasubstituted alkenes, (E) (opposite) and (Z) (together) nomenclature is used based on the Cahn-Ingold-Prelog priority rules.

Example: (E)-2-butene vs. (Z)-2-butene

Alkene Stereochemistry and Rotation

Unlike C–C single bonds, which can freely rotate, C=C double bonds have restricted rotation, leading to cis/trans (or E/Z) isomerism.

• Cis/Trans Isomerism: Cis (same side) and trans (opposite side) isomers are different molecules due to restricted rotation around the double bond.

• Syn/Anti: Used for free rotation (single bonds); cis/trans for restricted rotation (double bonds).

Example: cis-1,2-dichloroethene vs. trans-1,2-dichloroethene

Summary Table: E2 vs. E1 Mechanisms

Key Points for Exam Preparation

• Understand the competition between substitution and elimination.

• Be able to identify α and β carbons and predict possible alkene products.

• Know the difference between E2 and E1 mechanisms, including rate laws and requirements.

• Classify alkenes by substitution and recognize (E)/(Z) stereochemistry.

• Apply Cahn-Ingold-Prelog rules for (E)/(Z) nomenclature.

• Recognize the effect of leaving group ability and base strength on reaction rate and product selectivity.