EXAM 1

CHEMISTRY 220

Friday, September 24, 2010

 

NAME (print): ________________________________________________________

 

TA:_________________ Sect. Day:_________________ Sect. Time:_____________

 

 

Take a few moments to look over the exam.  Answer each question on the exam paper.

 

No calculators or electronic devices.  You may use molecular models.  Important clues and structures are in bold. There is a Periodic Table on page 7.

 

 Do all preliminary drawing or computations on the work sheets at the end of the exam. The work sheets will not be graded. You may detach the work sheets from the exam.

 

The exam is 55 minutes.

 

STOP writing and hand in your exam when you are asked to do so.

 

Remember: Neatness is to your advantage.

 

 

1. (20 pts) Conformation I                  ______

 

 

2. (20 pts) Conformation II                 ______

 

 

3. (25 pts.) Potpourri                           ______

                                               

 

4. (20 pts) Thermochemistry               ______

 

 

5. (15 pts) Bonding/Orbitals               ______

 

______________________________________

 

Total (100 pts) 


1.            Conformation I:  (20 pts) For the eclipsed and staggered conformations of 2,2-dimethybutane viewed along the C2-C3 sigma bond, draw a Newman projection of the most stable eclipsed and staggered conformations.  Place the energies for each interaction in the Newman projections below.  [Use the circles as templates for the Newman projections.]  Calculate the total energy (kcal/mol) of both conformations.  Place your answer in the appropriate box.   [H/H, eclipsed, 1.0 kcal/mol; CH3/H eclipsed, 1.3 kcal/mol; CH3/CH3, eclipsed, 3.0 kcal/mol; CH3/CH3, gauche, 0.9 kcal/mol.]  Show your work!

 

             

 

 

 


2.  Conformation II: (20 pts)  Consider the stereoisomer of 1-chloro-1,4-dimethylcyclohexane (1) shown here.

 

 

a) (10 pts)  Draw the chair conformations below so that the equilibrium arrows are correct.  Be sure all groups are clearly labeled axial or equatorial.

 

 

 

 

 

 

 

            chair A                                                                                                   chair B

 

b) (10 pts)  What is the value of D(DGo) for the above equilibrium given the following A- values for the monosubstituted (X) cyclohexane: X = Cl = 0.5 kcal/mol; X = CH3 = 1.8 kcal/mol; X = C2H5 = 1.9 kcal/mol?  Show calculations.

 

 

 

 

 

 

 

 

 

     

 

 

      Chair A energy = ___________                     Chair B energy= ___________

 

 

 


3.   Potpourri: (25 pts.; equal weight)  Circle the best answer(s) where applicable in  each of the following:

 

a)    Estimate the difference in the heat of combustion and heat of formation in kcal/mol for cis- and trans-1,4-dimethylcyclohexane.  Explain briefly.

 

 

 

 

 

 

 

 

 

b)    Circle the ÒacidsÓ that are readily deprotonated by n-butyllithium (n-CH3CH2CH2CH2Li).

 

 

              ethanol                CH3CO2H                 NH3                   NH4+                    acetylene

 

 

c)     Circle the species with sp2 hybrid atoms.

 

 

              HCN                    ethylene              CO2                    CH2=C=CH2               [BeCl3]-1        

 

 

d)    Briefly explain and illustrate why ClCH2CH2Cl has a net dipole while

trans-ClCH=CHCl does not. [Newman projections of the most stable conformations       of 1,2-dichloroethane might be helpful.]

 

 

 

 

 

 

 

 

e)   The following ÒacidsÓ all have resonance stabilized conjugate bases. Rank these acids (low pKa to high pKa) with the numbers 1 – 5, respectively. [Most acidic gets number 1.]

 

          

 

 

 

 

 

 

4.   Thermochemistry: (20 pts.) n-Heptane (DHfo = -44.8 kcal/mol) and 2-methylhexane (DHfo = --46.6 kcal/mol) both have the formula C7H16.

 

a)  (5 pts.) What type of isomers are they?

 

 

 

b) (5 pts.) What is the difference in their heats of combustion?  

 

                         

 

 

c) (10 pts.)  Prove part b) with  a ÒStandard StateÓ illustration of the combustion of the two isomers. [Your diagram should show the products of combustion and the number of moles of each.] Write an equation for the heat of combustion of n-heptane using its heat of formation.


5.  Bonding/Orbitals: (15 pts) The ÒplanarÓ compound 1 has a dipole moment. Explain and illustrate with an orbital diagram how this is possible. 

  

          

 

           

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

          

 

 

 

 


Periodic Table

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Work Sheet
Work Sheet


Work Sheet