267. CHEM Week 8. Alkenes From Alcohols. Labnotes (revised 7/10).
Include the Prelab Exercise on p. 205 of the lab text in your prelab outline. Answer
the same for 2-butanol. Review Chapter 15 on Molecular Mechanics, paying particular
attention to the section on local vs global minima (p. 302, 303) and also p. 304, 305.
Review Wade, Section 7-7 (A-C) and your previous Molecular Modeling handout and
report. A lthough the lab experiment is short and straightforward, you wil l
n eed to put additional effort into the report . I t is expected that such
a dditional effort will be expended. Note also that unlike previous
e xperiments, in which the procedure was spelled out quite exactly in the
h andout, this experiment and others to f ollow will rely more on the
p rocedures given in the lab text. Be sure to carefully read the section below on
calculations before you carry them out.
Experiment Notes: B e extremely careful in using conc sulfuric acid . Instead of
the 10 cm column/collection tube described in the book, use the longer
chromatography column to collect the product. Much of the first slow bubbling that
occurs is simply air being displaced from the heated reaction tube. Once product
formation begins, the bubbling becomes rapid. To ensure that the butenes and not
just air is being collected, allow the reaction to proceed until the tube is at least about
3/4 full.
Calculations: From the GC analysis, determine the relative amounts of the three
isomers produced in the reaction. These will be used to compute energies which will
be compared with calculated and other experimental values. Note that the
assumption that peak area is directly proportional to molar amount is not necessarily
correct. The detector responds differently to different compounds. Usually however,
for structurally similar compounds, as in this case, the assumption is valid. Air will
elute first, followed by the butenes in order of boiling point.
For the equilibrium as drawn above, calculate G, using the relationship, G = RTlnKeq, where R = 1.99 cal/mol deg Kelvin, T = 373 Kelvin, and Keq = N (cis) / N (1-butene),
where N (1-butene) = mol fraction of 1-butene and N (cis) = mol fraction of cis -2-butene.
(Why is T = 373 K used?) Be careful to use the correct sign for G for the
equilibrium and to not mix units (cal vs kcal).
In the same way, calculate G for the following equilibrium:
Other experimental data - heats of hydrogenation: use the heat of hydrogenation
data given in Wade, Table 7-1 (for example, H = -1.0 kcal/mol for the equilibrium cis
to trans 2-butene). (Heats of combustion, if available could also be compared and
would provide the same qualitative results, within experimental error.) As stated in the
other handout for this experiment, An Experimental and Computational Investigation
of the Dehydration of 2-Butene, you will find additional experimental heat of
formation data from the web.
Computational Chemistry: Instructions for using the SPARTAN molecular modeling
program is provided in the handout, An Experimental and Computational Investigation
of the Dehydration of 2-Butanol. Using the SPARTAN computer program, calculate
the strain energy (steric energy) for the three butene isomers using molecular
mechanics (MMFF) calculations, and also calculate the heats of formation using semiempirical MO calculations (AM1).
A word about local minima. Recall the molecular mechanics calculations that you did
previously for butane. If you had minimized the energy when the molecule was in or
near the gauche conformation, you would have concluded that the minimum energy
conformation of butane was the gauche conformation. This is because the gauche
conformation is in an energy well (surrounded by energy barriers). In other words, the
energy goes up as you rotate the C-C bond slightly one way or the other from the
gauche conformation. The program therefore thinks that the gauche is the minimum
energy conformation. The gauche conformation actually represents a local minimum.
If you rotate the C-C bond from the gauche conformation or to near the anti
conformation, then minimize, the program would tell you that the minimum energy
conformation of butane is the anti conformation. Indeed, the anti conformation would
be lower in energy than the gauche. If you rotate around the central C-C bond by
360, as you did in the modeling experiment, checking energy along the way, you
would see that the anti conformation has the lowest energy of all conformations. The
energy of the anti conformation represents the global minimum. This is the
conformation of lowest energy. In doing molecular modeling calculations, you must
always be aware that even though you have minimized the energy of a structure, it
may be at a local, not a global, minimum. The way to get around this problem is to
rotate C-C single bonds in small increments, as you did with butane, and minimize the
energy at each point along the way. This will normally allow you to find the global
minimum. If there are several C-C bonds in the molecule, there may be many local
minima, making the job of finding the global minimum very time-consuming. In the
present case, with the butenes, the SPARTAN program allows this conformer search
to be done automatically. If set to do a conformer search, the program rotates
specified bonds in a systematic way and minimizes the energy for each conformation.
If the correct bonds were specified by the user and the magnitude of rotation chosen
to be sufficiently small, the global minimum would be found. One can imagine the
complexity of doing this for a molecule having a large number of single bonds. Even
for a molecule having only 5 single bonds and stepping through a rotation using 120
(large) increments results in 243 conformations to be calculated. If each calculation
took 10 sec, 40 minutes of computer time would be needed to find the global
minimum. For larger molecules, and for more sophisticated calculations, calculations
for each conformation might take an hour or more. Fortunately for the butenes,
which have only two C-C single bonds, using a rotation step of 120 results in only 9
conformations. Because AM1 calculations are fairly simple and because these
molecules are fairly small, this is an easily-doable task. Note that you could do a
conformer search manually for simple molecules such as these. One would draw a
model, minimize and record the energy, select a C-C bond and rotate it by some
amount, minimize and record the energy, rotate, minimize, and record, until it was
fairly certain that the global minimum had been found. In any case, searching for the
global minimum must be done carefully. If the rotation increment is too large, the
global minimum may be missed.
As part of the report, construct a table (see sample data table at end) which allows
for a clear comparison of all experimental and calculated energy differences.
Comment on the agreement and/or the disagreement of the resulting energies. Do
not expect to see perfect agreement. Concentrate more on the predicted direction of
equilibrium and order of magnitude of the energies. Remember that the sign for G
and H is absolutely crucial and depends upon the way in which the equilibrium is
drawn.
Summary: for the three isomeric butenes produced in the dehydration of 2-butanol
do the following:
1)
determine product distributions using GC, and from those, free energies.
2)
compare your expl values with hydrogenation energies from Wade.
3)
calculate energies by MMFF and AM1.
4)
compare your experimental data with the experimental values from the NIST
website (see other handout) and with your calculated values and comment on the
correlations or lack thereof.
5)
draw the mechanism and energy diagram for the reaction and discuss results
found in terms of transition state energies and structures, how structures of
transition states correspond to structures of products and therefore energies of
products.
6.)
comment on the product distribution predicted from statistical considerations
only vs. what you found.
Sample Data Table
G (your exp)
H (hydrog)
H (NIST)
E (MM)
E (AM1)