CHE 230-001 – Organic Chemistry

Exam 2, March 5, 2008

 

Name ______ANSWER KEY_________________________ Student ID Number ________________

 

Before you begin this exam: First: Note the blank page at the end provided for scrap paper.  It must be returned at the end of the exam.  Second: Place your student identification on your desk.  A proctor will come around to check everyone’s ID.  Third: Read through the entire exam.  Do not waste time on problems you can’t do if there are others that look easy.  Fourth: It is critically important that your answers be written in a clear, unambiguous manner. Answers in which your intentions are unclear will not receive credit.  Fifth:  Read the instructions for each problem.  You will have until 9:50 a.m. to complete this exam.  There will be no extensions, so budget your time carefully.

 

 

                      Problem       Points           Score

 

                      A                   12                 _____

                                   

                      B.                  4                    _____

 

                      C.                  6                    _____

 

                      D                   9                    _____

                     

                      E.                  7                    _____

 

                      F.1                4                    _____

                      F.2                4                    _____

                      F.3                4                    _____

                      F.4                8                    _____

                      F.5                4                    _____

 

                      G.1                4                    _____

                      G.2                4                    _____

                      G.3                4                    _____

                      G.4                4                    _____

                      G.5                4                    _____

 

                      H.1                8                    _____

                      H.2.               3                    _____

                      H.3                3                    _____

                      H.4                3                    _____

                      H.5                3                    _____

                     

                     

                      Total             102               _______

A. (12)     Draw all the stereoisomers of 2-chloro-3-methylpentane, CH3-CHCl-CH(CH3)-CH2CH3 by filling in the rest of the structure on the templates below.  (There are more than you need, so cross out [large X] the one(s) you don’t need.)

 

 

        

 

B. (4)  Using wedged and hashed bonds, draw the lowest energy (i.e. most stable) stereoisomer of 1, 2, 3-trimethylcyclohexane.  You should draw in the wedged and hashed bonds on the structure below.  (No partial credit)

       either answer

                        

 

C. (6)  Draw (2S, 3S)-2,3-pentanediol, CH3- CH(OH)-CH(OH)-CH2-CH3, on the template below.  (No partial credit if you don’t draw 2,3-pentandiol.)

       Other drawings with the same configurations are also possible.

 

                                   

 

D. (9)  On each line below write one of the following terms to describe the relationship between the two compounds in each pair: identical, constitutional isomers, enantiomers, diastereomers, meso compounds.

       1.                                                                                           

                               _____identical_____________

 

       2.

                                _____diastereomers________

 

 

       3.    

                             ____constitutional isomers__________

 

E. (7)       Draw the mechanism of the following reaction.  It should have two steps.  (NaH supplies hydride ion [H:‾ ], which is a strong base.  Use the conventional electron-flow arrows.

 

                     

Primary carbocations are not formed.  If you have a mechanism that involves a primary carbocation (except for special cases, like benzyl), it must be wrong.

      

 

 

 

F. (24)  Draw the structures of the missing organic products or missing organic reactants in the reactions below.

 

       1.     SN2 with negative nucleophile

              

 

       2.  SN1 with neutral nucleophile

              

 

      

       3.     Indicate the stereochemistry at every stereocenter with a single wedged (up), hashed (down), or squiggly (a mixture of up and down) bond.

                      SN2 with inversion at reaction site only; methyl group affected.

 

              

 

 

 

 

 

       4.     Leaving groups attached to sp2 carbons don’t leave (usually), so aryl halides are unreactive

              

       5.     Indicate the stereochemistry at every stereocenter with a single wedged (up), hashed (down), or squiggly (a mixture of up and down) bond.

 

                      SN1 with racemization at reaction site only; methyl stereochem. unaffected.

              

      

G. (20)  Circle the best answer for each question.

 

       1.     Which one of the following molecules is chiral? (If it is achiral in any conformation it is not chiral.)  only (e) lacks a plane of symmetry

 

       a)                                         b)                                  c)                   d)                                  e)

             

 

       2.     Which is the best nucleophile in acetone?  answer: (a) .

 

                 In polar aprotic solvents like acetone, nucleophilicity follows basicity, and CH3NH2 is the strongest base.

 

                      a)  CH3NH2                    b)     CH3SH                   c)  CH3OH                     d)  CH3OH2+

 

      

       3.     Which of the following reacts fastest by the SN1 mechanism?  answer: (a) .  tert-halide à tert. carbocation = most stable carbocation

 

                      a)                                                                                            b)

                                                             

 

                      c)                                                                                            d)

                                                         

 

 

 

       4.     Which is the most stable conformation for cis-1-isopropyl-3-methylcyclohexane?  answer: (b).  largest group (i-Pr) is equatorial; Me is cis

 

              

 

 

       5.     The C-H bond energy in methane is 104 kcal/mol. For which reaction below does ΔHo = +104 kcal/mol ?  answer: (b) 

                      This is the definition of bond energy = hemolytic cleavage of the bond in question.

 

               a) CH3• + H• à CH4                 b) CH4 à CH3• + H•             c) CH4 à CH3-  +  H+             d) CH3-  +  H+ à CH4

 

H (20)  Consider the following mechanism (some arrows missing).

 

      

 

Using the letters A – D or Roman numerals I-IV, answer the following questions. each correct label in #1 worth 2 points; parts #2-5 worth 3 points each

 

1.     On the energy diagram below, write I, II, III or IV in four of the empty circles corresponding to the structures above.

      

2.     Which distance (A, B, C or D) on the diagram corresponds to the rate-determining step?    ___A______

 

3.     Which distance (A, B, C or D) on the diagram is related to Keq = [IV] / [I] for the reaction?  __D________

 

4.     Why is III more stable than II?  __tertiary carbocations are more stable than secondary carbocations_____

 

5.     Locate the transition state for the reaction, I à II, by writing TS 1 in the appropriate circle on the diagram.