University:
Arizona State University
Course:
CHM 101 | Introductory Chemistry
Academic year:

2020
Views:

197

Pages:

1
Author:

Hallie Randolph

Summary Substrate Rate Nucleophile (Nu) Solvent Organic Chemistry Nucleophilic Substitution and Elimination Reactions (SN1, SN2, Ε1, Ε2) S1 Stepwise reaction in which one substituent replaces another. 1. Formation of carbocation 2. Attack by nucleophile to form substituted product 3°>2°>1° Also on benzyllic or aryllic First-order R = kI substrate] Weak Nu e.g.. H₂O, CHOH, and CH3CH2OH Nu unimportant Polar protic (If reactant is not charged) eg: alcohols (CH,OH), water Requires good leaving group ie. a weak base Leaving Group TsOMSO, Br, Cl, H₂O, NH Stereochemistry Racemic mixture (50:50 R & S) Rearrangement possible (Carbocation is trigonal planar with empty p-orbital attack can happen from top or bottom) S2 Single, concerted step in which one substituent replaces another. Nu-C partially formed, C-X partially broken (transition state theory) 1°>2°>3° Methyl = always Second-order R = k Nu] [substrate) Strong Nu (Lewis bases) e.g.: R-SH, NaOCH, LICH, NaOH, NaCN, KCN, NaNH, LiBr, KI, NaSMe Polar, aprotic e.g.: DMF, DMSO, acetone, CHCI, ethers (R-O-R') Requires good leaving group ie a wenk base TsOMSO,, Br, Cl, H₂O, NH Inversion Rearrangement rare R becomes S and S becomes R E1 Stepwise: two bonds are cleaved from adjacent positions to form a r-bond. 1. Formation of carbocation 2. Removal of adjacent hydrogen to form double bond 3° preferred 3 ^ * >2^ * 1^ * First-order R = k substrate] Weak base e.g.. H₂O, CH3OH, and CH₂CH₂CH High temperature Polar, protic (If reactant is not charged) eg: alcohols (CH₂OH), water B-Elimination Eliminates 1 equivalent acid to form alkene Bulkiest groups on opposite sides Rearrangement likely E2 Single, concerted step in which substituents are eliminated & double bond is formed simultaneously. Hydrogen is removed and double bond is formed Highly substituted alkene formed** 3 ^ * >2^ * >1^ * Second-order R = k [base] [substrate] Strong base for 2 & 3" e.g.: OH & RO Strong, bulky base for 1" eg: 1-BuOK Polar, aprotic e.g.: DMF, DMSO, acetone, CH:Cl. ethers (R-O-R') B-Elimination Eliminates 1 equivalent acid to form alkene Syn or Anti E, Z Rearrangement rare Mechanism Na R RC-GR RH R R-CR RH B R RCR RH B R R R R RC R R R There is competition between Sut and E1 as well as S2 and E2. As a result, there will often be a major and minor product.