EXAM 1
CHEMISTRY 220
Friday, October 7, 2011
NAME (print): ________________________________________________________
TA:_________________ Sect. Day:_________________ Sect. Time:_____________
Fill in the information above. Place your name on the top right of subsequent pages.
Take a few moments to look over the exam. Answer each question on the exam paper.
No calculators, electronic devices or earbuds. You may use molecular models. Important clues and structures are in bold. There is a Periodic Table on pg. 8 and a pKa Table on pg. 9.
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 ______
2. (20 pts) Potpourri (4 of 5) ______
3. (20 pts) Thermochemistry, et al. ______
4. (20 pts) Bond Dissociation Energies ______
5. (20 pts) Concepts (3 of 4) ______
______________________________________________
Total (100 pts)
1. Conformation: (20 pts) For the eclipsed and staggered conformations of 2-methylbutane 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, 3.6 kcal/mol; CH3/CH3, gauche, 0.7 kcal/mol] Show your work!
2. Potpourri: (20 pts.; equal weight) Circle the best
answer(s) where applicable in 4 of 5 of the following questions. If you do all five questions, cross out the
one that you do not want graded.
a) Sertraline (Zoloft), an antidepressant, has the molecular formula
C17H17Cl2N. How many degrees of unsaturation
does it have? Show work.
b) Circle the compounds that are readily deprotonated by KNH2.
ethanol CH3CO2H ethylene NH4+ acetylene
c) Circle the species with sp hybridized atoms.
HCN ethylene CS2 CH2=C=CH2 [BeCl3]-1
d) Briefly explain and illustrate why ClHC=C=CHCl has a net dipole moment while
(E)-ClCH=CHCl does not.
e) Rank the following acids in order of decreasing pKa (high pKa to low pKa) with the numbers 1 – 5, respectively. [Least acidic carboxylic acid gets number 1.]
3. Thermodynamics, et al.: (20 pts.; equal weight)
a) a) Draw all six alkenes with the molecular formula C5H10.
b) Provide the IUPAC name for the most stable alkene in a). Explain briefly and illustrate how hyperconjugation stabilizes this compound.
c) Provide the IUPAC name of the alkene in part a) that has the greatest heat of combustion.
d) Provide the IUPAC names of two alkenes in a) that have a difference of ΔHfo = ~1 kcal/mol. Which one is less stable? Why?
e) Given the following data (kcal/mol): n-pentane, ΔHfo = -35.1; 1-butene, ΔHfo = 0. Determine the heat of hydrogenation of the alkene in part c).
4. Bond Dissociation Energies: (20 pts.) The heat of hydrogenation of (E)-3-hexene is given in the illustration shown below. Using bond dissociation energies (bonds made, bonds broken), determine the energy of the π-bond in the alkene. Show work. Typical BDEŐs (kcal/mol): CH3CH2-H, +101; (CH3) 2CH-H, +99; (CH3) 3C-H, +97; H2, +104.
5. Concepts:
(20 pts.) Provide a brief explanation and illustration for 3 of 4 of the following concepts. If
you do all four questions, cross out the one that you do not want graded.
a) For the series of compounds 1) ethane, 2) ethylene and 3) acetylene, pKa decreases from 1 3 while bond dissociation energy increases in the same order.
b) The Hammond Postulate
Écontinued
c) The rate limiting step in an exothermic reaction having an intermediate carbocation.
d) For the alkane R-H, the relationship between bond dissociation energy and the heats of formation of R-H, the R radical and the hydrogen atom.
Periodic Table
Work Sheet
Work Sheet
Work Sheet