Chem 210 Exam 3 questions and answers

what factors explain the outcomes of reactions? hybridization, delocalization, eN, atom density what is a leaving group? - group of atom(s) that can be stable on its own - eN atoms that are good LG also make strong acids what is the α carbon? C directly attached to leaving group what are β Cs? β Hs? βC: C(s) attached to α carbon βH: H(s) attached to βC what is a LB? is it a nucleophile or electrophile? lewis base, nucleophile what is nucleophilic substitution? substitution of LG by a nucleophile, like LB is the C-LG complex a nucleophile electrophile? electrophile what happens in an elimination reaction (in general)? - LG & beta H break off - LB bonds to beta H - 2 sigma bonds -> 1 pi bond btwn alpha and beta C in the nucleophile O(neg)-CH3, what is the nucleophilic atom? oxygen what are the types of substitution products? - substitution w/ retention of configuration - substitution w/ inversion of configuration what does allylic mean? benzylic? propargylic? - allylic: (attached to) a carbon that is attached to C double bond C - benzylic (attached to) a C that is attached to benzene - propargylic: (attached to) a C that is attached to C triple bond C what is degree of substitution? # of Cs attached to Carbon atom in question what does "heteroatom substituted" mean? substituted by a heteroatom (atom/group of atoms that replace >= 1 H atoms from hydrocarbon) what are the reactants of a substitution reaction? LB, electrophile w/ leaving group what are the reactants of an elimination reaction? LG, beta H, LB what are the mechanistic pathways for a substitution reaction? - SN through biomolecular collision (SN2) - SN involving unimolecular loss of LG (SN1) what is SN through biomolecular collision? TS? - forms sigma btwn LB-C as sigma breaks btwn LG-C - TS: LB-C-LG: 180 degrees - 1 possible outcome: inversion of configuration (flipping umbrella inside out) - 1 step Mechanism, 1st Step RDS what is SN involving unimolecular loss of LG? - forms sigma btwn LB-C AFTER sigma breaks btwn LG-C - TS: LB-C-LG: 180 degrees - 2 possible outcomes: inversion or retention of configuration - 2-Step Mechanism, 1st Step RDS what are the mechanistic pathways for an elimination reaction? - elimination through biomolecular collision (E2) - elimination involving unimolecular loss of LG (E1) - elimination involving deprotonation first (E1cb) what is elimination through biomolecular collision? - sigma bond forms btwn LB-H as e- from betaH-betaC bond are used to from alphaC-betaC pi bond. LG leaves, taking e- with it - ends up with C=C, LB-H, & LG - fastest E2 elimination when alignment of betaH & LG: anti - 2nd fastest: eclipsed - 1-Step Mechanism, 1st Step RDS what is elimination involving unimolecular loss of LG? - LG leaves first, forms carbocation. LB removes H, e= of beta H & beta C interact with alpha C p orbitals to form pi bond - 2-Step Mechanism, 1st Step RDS what is elimination involving deprotonation first - LB-H first, e- of betaH-betaC go to beta C, forming carboanion. LG leaves as e- from carboanion form pi bond w/ alpha C - least common of 3 mechanisms when inversion occurs, does R change to S and vice versa? - most of the time, but not always - absolute configuration always changes which are preferred, biomolecular or unimolecular reactions? why? biomolecular, because they take place in a single step and there are no charged intermediates which is easier to bond with, unhindered C or hindered? which one forms SN2 and which one forms E2? - unhindered C is easier to bond with easiest substitution), so it forms SN2 - hindered C forms E2, because in E2 the LB only needs to bond to the beta H (small atom), not the hindered C (hardest substitution) what are the 4 types of lewis bases/nucleophiles? which are capable of SN2/E2? 1. good e- donors, weak bronsted bases (capable of SN2/E2) 2. good e- donors, moderate bronsted bases (capable of SN2/E2) 3. good e- donors, strong bronsted bases (capable of SN2/E2) 4. poor e- donors, weak bronsted bases (not capable of SN2/E2) what are the pkas of the anions/conjugate acids of the types of lewis bases/nucleophiles? 1. conj acid pka <15 (good e- donors, weak bronsted bases) 2. conj acid pka 15-30 (good e- donors, moderate bronsted bases) 3. conj acid pka >30 (good e- donors, strong bronsted bases) 4. (poor e- donors, weak bronsted bases) what are some examples of good e- donors/weak bronsted bases that are NOT anions - uncharged sp3 N, S, P - examples: NH3, H2S, PH3 what are some examples of good e- donors/ strong bronsted bases that are NOT anions - hindered bases - examples: -O-C(CH3)3, DBU, DIPEA what are the steps to take when trying to predict the kind of SN or E reaction? 1. SN/E possible? Needs to have sp3C AND lewis base/nucleophile 2. classify/characterize lewis base & electrophile. for lewis base: is it a good e- donor? what is the basicity? for electrophile: what is the degree of substitution? are there beta Hs? 3. predict (big chart) a. good e- donor & weak base 1°: SN2. 2°: SN2. 3°: No SN2 no E2 b. good e- donor & moderate base 1°: SN2. 2°: E2 but if no beta H then SN2. 3°: E2 c. good e- donor & strong base 1°: E2 but if no beta H then SN2. 2°: E2. 3°: E2. d. poor e- donor & weak base No SN2 or E2 for all substitutions 4. if no SN2/E2 possible, then check: is reasonable C+ formed (resonance-stabilized or 3°)? is there a polar solvent? what reaction will occur when for a good e- donor & weak base when there is 1° of substitution? SN2 what reaction will occur when for a good e- donor & weak base when there is 2° of substitution? SN2 what reaction will occur when for a good e- donor & weak base when there is 3° of substitution? No SN2 or E2 what reaction will occur when for a good e- donor & moderate base when there is 1° of substitution? SN2 what reaction will occur when for a good e- donor & moderate base when there is 2° of substitution? E2 but if no beta H then SN2 what reaction will occur when for a good e- donor & moderate base when there is 3° of substitution? E2 what reaction will occur when for a good e- donor & strong base when there is 1° of substitution? E2 but if no beta H then SN2 what reaction will occur when for a good e- donor & strong base when there is 2° of substitution? E2 what reaction will occur when for a good e- donor & strong base when there is 3° of substitution? E2