Substitution - Organic Chemistry - Lecture Slides, Slides of Organic Chemistry

Download Substitution - Organic Chemistry - Lecture Slides and more Slides Organic Chemistry in PDF only on Docsity! Polar Reaction Mechanisms Substitutions docsity.com Substitution Reaction Mechanisms • Bimolecular (SN2) – Two reactants involved in rate determining step • Unimolecular (SN1) – One reactant involved in rate determining step C Br I- I C Br C Br I- I C Br C slow docsity.com Factors Influencing Substitution Rates • SN2 – Temperature – Electrophilicity of electrophilic atom – Steric hindrance around electrophilic atom – Nucleophile strength – Leaving group ability • SN1 – Temperature – Electrophilicity of electrophilic atom – Stability of carbocation intermediate – Leaving group ability docsity.com Problem • Why doesn’t nucleophile strength influence the rate of SN1 reactions? docsity.com Steric Hindrance Example • In the following series of alkyl iodides (ethyl, isopropyl, t-butyl) less and less of the electrophilic carbon is accessible docsity.com Stereochemical Considerations • SN2 – Nucleophile approaches electrophilic carbon from the back side – No achiral intermediates are generated if the starting electrophile is chiral – A preferred stereoisomer will be formed from a chiral starting electrophile with stereochemical inversion • SN1 – The intermediate carbocation is achiral (planar) – No preference will be shown if the starting electrophile is chiral (racemic mixture forms) docsity.com Problem • Identify all nucleophilic and electrophilic atoms in the starting materials below • Identify the strongest nucleophile and electrophile • Show the mechanism for the reaction of those two • Repeat steps 1-3 until you think the reaction is done HO H + HBr docsity.com Problem • Identify all nucleophilic and electrophilic atoms in the starting materials below • Identify the strongest nucleophile and electrophile • Show the mechanism for the reaction of those two • Repeat steps 1-3 until you think the reaction is done HO H + PBr3 docsity.com