1 Chemistry 301 Fall, 2018 Final Examination Start: 1:30 pm January 22, 2019 Duration: 3.0 hr preceded by a 20-min reading opportunity Please do not do any writing during the reading period Name________________________________________________ (official name, as on Blackboard) This is an “open book” examination; you may use anything that is not alive nor connected to the internet. No computers, cell phones, nor calculators are allowed to be used in the examination room. Note: if you do not know the complete or specific answer, give a partial or general answer— We love to give partial credit. If there seems to be more than one good answer, explain your thinking. If you invoke resonance delocalization as part of your answer, draw the relevant resonance structures. If you draw a chair cyclohexane, be sure to orient the bonds carefully. If you do not know a structure and need to write a mechanism, write a general mechanism for partial credit. USE THE ARROW FORMALISM CAREFULLY FOR ALL MECHANISMS BE SURE TO INCLUDE ALL FORMAL CHARGES p2.______/13 p3.______/15 p4.______/27 p5._______/19 p6.______/21 p7.______/18 p8.______/22 p9._______/13 p10._______/16 p11.______/22 p12._____/14 Total: ___________/200 Lab-related problem: ____/26 There are 14 pages in this exam; please check now to be sure you have a complete set. There is a separate Data Sheet handout. Pledge:_____________________________________________________________________________2 I. (28 pts). Consider substitution reactions on the structurally related compounds, U and V. A. (07 pts). Write the most likely mechanism for the formation of Z from V, including a representation of the transition state for the rate-determining step, showing all partial charges and partial bonds. Be sure to distinguish between transition states and intermediates (if any). On the chart below, draw a reaction coordinate diagram representing the progress of the reaction. Show clearly the activation energy (DG‡) for the rate-determining step. B. (06 pts). Use your mechanism and transition state picture from part (A) above to explain why U and V both react much faster than ethyl bromide under these conditions. Continued… BrOBrNa SMeMeOHNa SMeMeOHSMeOSMeYZUVErxn progress3 C. (08 pts). Write the most likely mechanism for the formation of W from U, show all steps, and label the rate-determining step. Show all intermediates clearly. On the chart below, draw a reaction coordinate diagram representing the progress of the reaction. Show clearly the activation energy (DG‡) for the rate-determining step. D. (07 pts). Specify whether the formation of X or W is slower, and explain carefully referring to the rate-determining step. Erxn progress OMeOOMeAgMeOHBrOBrWXUVAgMeOH4 II. (27 pts). Consider the reaction of X in the presence of HBr and NaBr, leading to a mixture of Y and Z. A. (12 pts). Write the best mechanisms for the formation of Y and Z from X. Label the rate-determining step in each case. B. (05 pts). Suppose the concentration of HBr is increased 10-fold. Would the rate of disappearance of X: increase decrease stay the same (circle best answer) Explain your choice briefly in terms of your mechanism: C. (05 pts). Suppose the concentration of NaBr is increased 10-fold. Would the rate of disappearance of X: increase decrease stay the same (circle best answer) Explain your choice briefly in terms of your mechanism: D. (05 pts). When the concentration of NaBr is increased 10-fold, the ratio of Z to Y increases. Explain briefly in terms of your mechanism: CH3BrCH3CH3OHDMSOXYZHBrNaBr+5 III. (08 pts). Note the reaction of S with cyanogen bromide (Br-CN) to give the single regioisomer T. Write the best mechanism to account for the selective formation of T. Draw the structure of T carefully, making clear the stereochemistry. Draw and label the other (not formed) regioisomer of T. Explain the basis for the regioselectivity. IV. (11 pts). Consider the conversion of Q to R upon heating in acid. Write the best mechanism for this process, showing all intermediates along the way. Br-CNBrCNSTOQHORH+6 V. (21 pts). A. (07 pts). The reaction of bromine in water with Z-2-butene produces a racemic mixture of bromohydrins (A and its enantiomer). The enantiomers can be separated. Write a careful mechanism for the formation of A, using the arrow formalism and showing all intermediates. You need not show transition states. Draw the enantiomer of A here and label clearly each stereogenic carbon as (R) or (S). B. (04 pts). One of the enantiomers, A, reacts with HBr (now in dichloromethane solvent), to give dibromides C and D, a racemic mixture. No trace of the meso isomer (E) of the dibromide was detected. Draw the meso isomer (E) of the dibromide in the box and label clearly each stereogenic carbon as (R) or (S). C. (10 pts). Write a careful mechanism for the formation of C + D from A using the arrow formalism and showing all intermediates. You need not show transition states. Make clear why two enantiomers (C, D) form, but not the meso isomer, E. Me MeH HZ-2-buteneBr2H2OMeMeHHHOBrA+ enantiomer enantiomer of AMeMeHHHOBrAHBrCH2Cl2MeMeHHBrBrC + D[racemic] E (meso)7 VI. (18 pts). It is often possible for a molecule to react in two or more different ways, and sometimes we can predict which process will be favored. In the following examples, the reaction is selective for one of the isomeric products.. Select the favored product, draw the mechanism and explain why it is preferred. A. (09 pts). B. (09 pts). HOOHBrOOH+ NaH orOOH-H2-NaBrorOOHOHOMeMeMeMeH+or orOMeMeOMeHMe8 VII. (22 pts). Consider the following reaction where stereoselectivity is a paramount issue. Only one isomer of the product, P is formed. A. (05 pts). Draw the two chair representations of the starting bromide, Q. Calculate the relative torsional strain for each chair and circle the more