Chp.9-4 E2 Elimination Essentials

Overview

The transcript explains E2 elimination reactions of alkyl halides, including mechanism, terminology, regioselectivity (Zaitsev’s rule), reactivity trends, and the effect of bulky bases on product distribution.

Substitution vs. Elimination Basics

• Substitution replaces a leaving group with another group on an alkane carbon.
• Elimination removes a leaving group and a beta hydrogen from adjacent carbons.
• Elimination forms an alkene by creating a C=C between the alpha and beta carbons.
• The base removes the beta hydrogen; the leaving group departs from the alpha carbon.

E2 Mechanism and Kinetics

• E2 stands for elimination, bimolecular; one-step, concerted process.
• Rate depends linearly on both alkyl halide and base concentrations.
• Transition state includes both alkyl halide and base; bonds break/form simultaneously.
• Base abstracts a beta hydrogen as the leaving group departs; pi bond forms.

Key Terminology

• Alpha carbon: carbon directly bonded to the halogen (leaving group).
• Beta carbon: carbon adjacent to alpha carbon from which the hydrogen is removed.
• Dehydrohalogenation: elimination removing H and halogen from the substrate.
• Beta elimination / 1,2-elimination: removal from adjacent carbons (relative positions).

Reactivity Trends of Alkyl Halides in E2

• Tertiary alkyl halides react faster than secondary; primary are least reactive.
• Reason: tertiary substrates form more substituted, more stable alkenes.
• More substituted alkenes have lower activation energies in E2 transition states.

Regioselectivity and Zaitsev’s Rule

• E2 reactions are regioselective when multiple beta positions exist.
• Major product is the more substituted, more stable alkene (Zaitsev product).
• Example: 2-bromobutane gives more 2-butene than 1-butene.
• Rationale: transition state resembles the alkene; factors stabilizing alkenes stabilize the TS.

Limitations: Bulky Bases and Hofmann Products

• Bulky bases have difficulty accessing more hindered beta hydrogens.
• Bulky bases favor removal of more accessible hydrogens, giving less substituted alkenes.
• This yields the less stable (Hofmann) product as major in sterically hindered cases.
• Steric hindrance raises energy to abstract hindered hydrogens, shifting product ratios.

Product Control by Base Size

• Smaller bases more readily form Zaitsev products (more substituted alkenes).
• Increasing base bulk decreases formation of the more substituted alkene.
• Very bulky bases can make the less substituted alkene the dominant product.

Structured Summary of E2 Concepts

Example: Base Bulk vs. Product Distribution

• Less bulky base (e.g., ethoxide): higher percentage of more stable alkene.
• Increasing bulk: decreases more stable alkene; increases less stable alkene.
• Extremely bulky base: more stable alkene ~8%; less stable alkene ~92%.

Key Terms & Definitions

• Alkyl halide: alkane derivative with a halogen substituent as leaving group.
• Leaving group: atom/ion that departs with electron pair (e.g., bromide).
• Base: species that abstracts a proton (beta hydrogen) in E2.
• Transition state: high-energy state where bonds are partially broken/formed.
• Zaitsev product: most substituted alkene formed preferentially in E2.
• Hofmann product: less substituted alkene favored by bulky bases.

Action Items / Next Steps

• Practice identifying alpha and all possible beta carbons in given substrates.
• Predict E2 major/minor products under small vs. bulky base conditions.
• Apply Zaitsev’s rule and note exceptions with sterically hindered bases.
• Work through the three assigned E2 problems and ask questions if unclear.