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CHM311:PhysicalOrganicChemistryProblemSet1 Instructions: You
have a choice of submitting electronically to
[email protected]; or a neatly written hard copy with
attached graphs. The last date for submission is Friday, Sep 13,
2013 You can discuss and collaborate but the final submitted
version must be yours and written (or typed) in your own words Be
prepared to make a presentation in class to defend your answer Each
problem is worth 15 marks Use values from Clayden
1. The acidity of a series of substituted benzyldimethylammonium
ions has been measured. Determine whether these data are correlated
by the Hammett equation. What is the value of?
2. The following pKas of a series of
phenyl-tetramethyl-guanidinium structures are listed below. Using
the Hammett relationship, decipher whether there is a correlation
between pKa and substituents? Explain. (for 4-CF3 =0.54)
X pKa H 12.2
NO2 9.8 CN 10.9 CF3 11.4 Cl 11.7 Br 11.6
CH3 12.4 OCH3 12.6
3.
a) The equilibrium between a carbonyl compound such as
cyclopentanone (see below) and its hydrate usually favors the
aldehyde or ketone. Propose an arrow pushing mechanism for this
equilibrium. Draw an energy profile to express this and mark the
difference between the free energy. In a similar experiment,
cycopropanone shows an entirely different equilibrium that is in
favor of the product. Comment on this observation.
b) The equilibrium constants for cyanohydrin formation K1 and K2
are measured. Assume that the K1 is 5 and K2 is 200.
Calculate G1 & G2 for the two equilibria at 25 C.
Rationalize the differences in the position of these
equilibria.
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4. For the ionization equilibrium given below, calculate G and S
at 298 K. Draw a Hammett plot for this equilibrium and calculate .
Comment on the sign and magnitude of .
5. For the following equilibrium, the value of equilibrium
constant was determined over a range of temperatures. Graphically
determine H and S.
SS
Ar
Ar S
SAr
Ar
6. The rate constant for this reaction was found to be 1.56 x
10-4 s1. When the hydrogen atoms marked in bold are replaced by
deuterium, the rate constant for this reaction was found to be 1.34
x 10-4 s1. Calculate the kinetic isotope effect. For the following
reaction, propose a reasonable mechanism and clearly label the rate
determining step.
Oconc. H2SO4
O
O
HO O
H
H
7. The following data was obtained when the reaction described
below was conducted. Derive a rate law for this reaction. What is
the order of the reaction? Can you propose a mechanism that
accounts for this rate law?
Time (h) [OH] [ketone]
0 10 10
1 2.31 3.68
2 1.3 1.35
3 0.91 0.498
4 0.7 0.183
5 0.57 0.067
6 0.48 0.025
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8. Sulfonylation of naphthalene proceeds to produce two
products:
Temperature 80C 180C%A 95 10%B 5 90
(a) Propose a reasonable arrow-pushing mechanism for the
formation of A and B. (b) Draw out an energy profile that is
consistent with the above observations with clear labels for
relative energies for each product (c) If A is the preferred
product at low temperatures, how would you account for the
formation of B as the major product at higher temperature? (d) What
strategy should one adopt if we want to get exclusively (>99%)
of A? 9. Assume that the first step is an equilibrium and the
second step is the rate-determining step, write out a rate law for
the reaction. Draw out an energy profile for the reaction and
clearly label the rate determining step.
10.
a) Reaction of the following bromides with tBuOK in tert-butanol
favors elimination reaction. The rate constant for one was found as
4.1 x 10-3 Lmol-1s-1 while the other was 8.0 x 10-6 Lmol-1s-1. Draw
out the two compounds in the respective chair conformations and
explain the rate differences. Identify which bromide would react
slower.
b) Rationalize the observation that A forms an acetal much
slower than B
