Overview
This lecture covers the thermodynamic and kinetic conditions that favor addition (adhesion) and elimination reactions, explains regioselectivity in hydrohalogenation (addition of HX to alkenes), outlines Markovnikov’s and anti-Markovnikov’s rules, and discusses the mechanisms and stereochemistry involved.
Thermodynamics and Spontaneity of Reactions
- Addition and elimination reactions are reversible; addition is forward, elimination is backward.
- Spontaneity of a reaction depends on Gibbs free energy (ΔG), entropy (ΔS), and enthalpy (ΔH).
- Reaction is spontaneous if ΔG is negative, ΔH is negative, and ΔS is positive.
- ΔG = ΔH − TΔS combines enthalpy and entropy effects.
- At low temperatures, enthalpy dominates (favoring addition); at high temperatures, entropy dominates (favoring elimination).
- Addition reactions are generally favored at low temperatures; elimination at high temperatures.
Entropy and Enthalpy Considerations
- If number of product molecules > reactants, ΔS is positive (favorable).
- If combining two reactants into one product (as in addition), ΔS is negative (unfavorable).
- Enthalpy (ΔH) is often negative for addition reactions, making them enthalpy-driven.
Hydrohalogenation of Alkenes (Addition of HX)
- Hydrohalogenation involves adding HX (HCl, HBr, HI) across a carbon-carbon double bond.
- Reaction type: Electrophilic addition (alkenes act as electron-rich species attracting electrophiles).
- For symmetrical alkenes, product identity does not depend on which carbon gets H or X.
- For unsymmetrical alkenes, regioselectivity (preference for one direction of addition) becomes important.
Regioselectivity and Markovnikov's Rule
- Regioselectivity: Preference for adding the electrophile (H⁺) to one carbon over another in an unsymmetrical alkene.
- Markovnikov's Rule: In electrophilic addition, H⁺ adds to the carbon with more hydrogens; X⁻ adds to the carbon with fewer hydrogens.
- Anti-Markovnikov addition occurs when peroxides are present; H⁺ adds to the carbon with fewer hydrogens.
Mechanism of Electrophilic Addition
- Proceeds via a carbocation intermediate; the more stable carbocation intermediate determines the major product.
- Tertiary carbocations are more stable than secondary, which are more stable than primary.
- Stability is explained by resonance (benzylic/allylic) and hyperconjugation (tertiary > secondary > primary).
Stereochemistry of Addition Reactions
- Addition via a carbocation intermediate forms a planar (sp²) center.
- Nucleophile (e.g., Cl⁻) can attack from either side, leading to racemic mixtures (equal amounts of enantiomers).
Key Terms & Definitions
- Addition Reaction — Process of combining two molecules into one, often across a double bond.
- Elimination Reaction — Process of forming a double bond by removing atoms from a molecule.
- ΔG (Gibbs Free Energy) — Determines spontaneity of a reaction.
- ΔH (Enthalpy) — Heat change of a reaction.
- ΔS (Entropy) — Disorder or randomness change in a reaction.
- Regioselectivity — Selectivity of a chemical reaction toward one structural isomer over another.
- Markovnikov's Rule — Rule that directs where electrophiles add in alkene additions.
- Anti-Markovnikov Addition — Opposite orientation of electrophilic addition, favored by peroxides.
- Carbocation — Carbon atom with a positive charge, an intermediate in many reactions.
- Racemic Mixture — 50:50 mixture of two enantiomers (mirror-image isomers).
Action Items / Next Steps
- Practice drawing products for hydrohalogenation of both symmetrical and unsymmetrical alkenes.
- Understand and apply Markovnikov’s and anti-Markovnikov’s rules to addition reactions.
- Review the mechanism of carbocation formation, stability, and resulting stereochemistry.
- Prepare for additional examples and questions in the next class.