tailieunhanh - Lecture Organic chemistry - Chapter 7: Unimolecular substitution and elimination

Lecture Organic chemistry - Chapter 7: Unimolecular substitution and elimination. The main contents of this chapter include all of the following: Some observations, a new mechanism, what is the intermediate? The mechanism explains the data, the strong effect of polar solvents on the SN1 reaction,.and other contents. | Relative “rates” of the four arrows differ and depend on substrate, reagent, and conditions C H C L B Nu : - : - Chapter 7: Unimolecular Substitution And Elimination E1, E2 SN2, SN1, E1, E2 E1, E2 SN2, SN1 (CH3)2CH Br (CH3)3C Br H OCH3 + H OH (CH3)3C OCH3 H Br + Hydrolysis Generally: “Solvolysis” Some Observations Recall from Chapter 6: Indeed: Tertiary! H Br + (CH3)2CH OH + Methanolysis Secondary Lousy Nu Lousy Nu Moderate rate Fast! 8/9/2015 © Univesity of California 2 1. Rate = k [R-L], 1st order unimolecular, only R-L in rate-determining TS: “bottleneck”. 2. Stereochemistry: not stereospecific, . enantiomerically pure starting material leads to (extensively) racemic products; pure cis (or trans) gives cis/trans mixtures, etc Both observations inconsistent with SN2 mechanism A New Mechanism k1 Intermediate 3. Accelerates with polar (best with protic, in contrast to SN2) solvents: Hexane > CH3OH . . . . (CH3)3OCH3 . . (CH3)3CN3 wins 8/9/2015 © Univesity of California 4 What Is The Intermediate? Mechanism 1. Electron deficient! 2. 3. Unimolecular nucleophilic substitution: 1st Order rate law Racemization Acceleration in polar solvents Acceleration with better L Product determining step occurs after rate determining step SN1 SN1 The Mechanism Explains The Data SN1PotE Bottleneck: SN2 Versus SN1 “Bottleneck” Racemization In practice: Often, slight inversion is observed, due to “ion pairing”, R+····Br-. SN1Racem The Strong Effect Of Polar Solvents On The SN1 Reaction Increasing solvent polarity speeds reaction Increasing solvent polarity retards reaction As Expected: Good Leaving Groups Accelerate The SN1 Reaction The Nucleophile Has No Effect On The Rate Of The SN1 Reaction All reactions take place at the same rate k1. But in .