University:
The University Of Western Ontario
Course:
Biology 2290F | Scientific Method in Biology
Academic year:

2024
Views:

257

Pages:

2
Author:

Kyuwon J.

Rearrangements and Exceptions in Hydrohalogenation 1. Carbocation Rearrangements • Definition: Carbocation rearrangements occur when a less stable carbocation undergoes a structural shift to form a more stable carbocation. • These rearrangements follow the general stability order: Tertiary > Secondary > Primary > Methyl • Types of rearrangements: – Hydride shift – A hydrogen with its bonding electrons moves to an adjacent carbon to stabilize the carbocation. – Methyl shift – A CH3 group shifts to an adjacent carbon to form a more stable carbocation. – Ring expansion – In cyclic compounds, the ring may expand to form a more stable carbocation. 2. Example of Carbocation Rearrangement • Consider the reaction of 3-methylpent-2-ene with HCl: CH3 CH CH CH2 CH3 + HCl • Step-by-step mechanism: 1. Protonation of the double bond forms a secondary carbocation. 2. A hydride shift from an adjacent tertiary carbon stabilizes the carbocation. 3. Chloride ion (Cl− ) then attacks the more stable carbocation. • Major product: 2-Chloro-3-methylpentane. 3. Factors Affecting Hydrohalogenation • Nature of the Alkene: – More substituted alkenes react faster due to greater carbocation stability. – Conjugated alkenes show different reactivity due to resonance stabilization. • Reaction Conditions: – Low temperatures favor addition (hydrohalogenation). – High temperatures favor elimination (E1 reaction). 1 4. Effect of Solvent on Hydrohalogenation • The solvent affects the reaction rate and regioselectivity. • Polar protic solvents (e.g., H2 O, alcohols): – Stabilize carbocations, increasing rearrangement likelihood. – Promote side reactions like alcohol formation. • Polar aprotic solvents (e.g., DMSO, acetone): – Do not stabilize carbocations, leading to faster direct addition. – Used for selective halogenation. 5. Competition Between Addition and Elimination • Some HX additions compete with E1 elimination, especially in tertiary alkenes. • Factors that favor elimination over addition: – High temperature – Strong base present – Poor nucleophile instead of a halide ion • Example: 3C 3 CH CH CH2 + HBr – Low temp: Addition to form 2-Bromo-2-methylbutane. – High temp: Elimination to form 2-Methylpent-2-ene. 6. Role of Peroxides in Anti-Markovnikov Addition • Peroxides (ROOR) can change the regioselectivity of hydrohalogenation. • Mechanism: 1. Peroxides break into radicals. 2. Radicals initiate a chain reaction, leading to Anti-Markovnikov addition. • Only works with HBr, not HCl or HI. • Major product: 1-Bromopropane from propene + HBr. 2