CHM 233 : Fall 2017 Quiz #11 - Answer Key Question 1

Transcription

1 CM 233 : Fall 2017 Quiz #11 - Answer Key Question 1 MC26k Which best describes the stereochemistry of the C=C double bonds 1 and 2 in the following structure? 1 2 Br A) 1 = E, 2 = E B) 1 = E, 2 = Z Br C) 1 = Z, 2 = Z D) 1 = Z, 2 = E priorities at each double bond are indicated highest = 1 and lowest = Br Z 2 E Br 1

2 Question 2 MC26m For which of the following isomers is the following reaction MST exothermic? C C 8 18 A 2,3-dimethyl-(3E)-hexene B 4,5-dimethyl-(2E)-hexene C 2,3-dimethyl-2-hexene D 4,5-dimethyl-1-hexene C all reactions give the same alkane product, all "end" at the same energy, the difference must be where they "start" A 2,3-dimethyl-(3E)-hexene = TRIsubstituted alkene (1 on the C=C bond) B 4,5-dimethyl-(2E)-hexene = DIsubstituted alkene (2 on the C=C bond) C 2,3-dimethyl-2-hexene = TETRAsubstituted alkene (0 on the C=C bond) D 4,5-dimethyl-1-hexene = MNsubstituted alkene (3 on the C=C bond) among these isomeric alkenes, the least substituted alkene will be the highest energy isomer, since D has only NE substituent on the C=C bond, it is a MNsubstituted alkene, it is highest in energy, it thus starts higher in energy, its reaction is thus most exothermic

3 Question 3 MCmxnd For a reaction energy diagram corresponding to the following mechanism, how many energy minima will there be corresponding to INTERMEDIATES (do not include minima associated with the reactants or the products)? A five minima C seven minima B six minima D eight minima Energy reaction coordinate remember, resonance contributors describe the SAME species note, you would not normally be expected to be able to get all of the relative energies of the intermediates correct at this stage = NE cation does NT imply another step in the mechanism

4 Question 4 MC20q Using the homolytic bond dissociation energies provided, which statement best describes the following reaction? C + C Br C Br Br C Br + B.D.E. (kcal/mol) A The reaction is exothermic by 41 kcal/mol B The reaction is exothermic by 5 kcal/mol C The reaction is endothermic by 5 kcal/mol D The reaction is endothermic by 41 kcal/mol Even though the reaction involves heterolytic bond cleavage in Lewis acid/base reactions, we are only comparing the reactants and products The bond dissocaition energies measure the energies of the electrons in bonds, and reactions involve breaking bonds and making new ones All we need to do is to compare the energy cost of breaking the bonds that are broken to the energy gain of the bonds that are made Energy "cost" due to breaking bonds = 86 (C-) + 88 (-Br) Total = 174 kcal/mol Energy "gain" due to making bonds = 68 (C-Br) (-) Total = 179 kcal/mol Electron energy in products lower than in reactants (the bonds are stronger in the products than in the reactants, thus the reaction is EXTERMIC by = 5 kcal/mol

5 Question 5 MC20o ow many transition states are involved in the following complex mechanism. You do not have to understand the details of this mechanism, just how many transition states are involved (hint, there is one transition state for each individual step in the mechanism). 1 no here, resonance again #5 #2 #3 #4 no here, this is just a resonance arrow, not a step in the mechanism A 4 transition states B 5 transition states C 6 transition states D 7 transition states

6 Question 6 MCalkenesIb Which is the major product of the following reaction???? A C B D simple electrophilic addition to a C=C bond, Markovnkov, adds to the most substituted "end" of the C=C bond, reaction proceeds via the most stable cation intermediate 2 substituents at this "end", most substituted end 1 substituent at this "end" ( does not "count") LB/BB LA/BA LB mechanism LA adds to most substituted end (reaction proceds via most stable cation intermediate), thus Markovnikov 3 cation and resonance stabilized (resonance contributors not shown)

7 Question 7 MC20m Which of the following statements is most correct for the reaction described by the following reaction energy diagram? (remember, a reaction with a negative free energy is exergonic, a reaction with a negative enthalpy is exothermic) 3 transition states ( ) ΔG reactants intermediates 1 intermediates 2 exergonic products reaction coordinate A the mechanism involves 2 transition states and the reaction is exergonic B the mechanism has 3 steps and the reaction is endergonic C the mechanism involves 2 intermediates and the reaction is exergonic D the mechanism involves 2 transition states and the reaction is endergonic

8 Question 8 MC26q Which of the following will be the MAJR organic product of the following reaction? 2 S 4 2 A B C D LB BB BB LB BB BA LA BA note the YDRIDE SIFT carbocation rearrangement ERE BA LA