Outline for CHEM 267 at UMass
Outline for CHEM 267 at UMass
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Date Created: 02/06/15
CHEM 267 Week 8 Alkenes From Alcohols Labnotes revised 708 Include the Prelab Exercise on p 215 of the lab text in your prelab outline Answer the same for 2butanol Review Chapter 15 on Molecular Mechanics paying particular attention to the section on local vs global minima p 313 and also p 321 and the first two paragraphs on p 322 Review Wade Section 77 AC and your previous Molecular Modeling handout and report Although the lab experiment is short and straightforward you will need to put additional effort into the report It is expected that such additional effort will be expended Note also that unlike previous experiments in which the procedure was spelled out quite exactly in the handout this experiment and others to follow will rely more on the procedures given in the lab text Be sure to carefully read the section below on calculations before you carry them out Experiment Notes Be extremely careful in using conc sulfuric acid Instead of the 10 cm columncollection 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 34 full Emulation 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 WT w For the equilibrium as drawn above calculate AG using the relationship AG RTaneq where R 199 calmol deg Kelvin T 373 Kelvin and Keq N my N buts Where N Me mol fraction of lbutene and N 05 mol fraction of cis 2butene Why is T 373 K used Be careful to use the correct sign for AG for the equilibrium and to not mix units cal vs kcal In the same way calculate AG for the following equilibrium Other experimental data heats of hydrogenation use the heat of hydrogenation data given in Wade Table 71 for example AH I 0 kcalmol for the equilibrium cis to trans 2butene 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 2Butene you will find additional experimental heat of formation data from the web WW Instructions for using the SPARTAN molecular modeling program is provided in the handout An Experimental and Computational Investigation of the Dehydration of ZButanol 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 semi empirical MO calculations AMI 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 CC bond from the gauche conformation to or 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 CC 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 W The way to get around this problem is to rotate CC 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 CC bonds in the molecule there may be many local minima making the job of finding the global minimum very timeconsuming 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 CC single bonds using a rotation step of 120 results in only 9 conformations Because AMl calculations are fairly simple and because these molecules are fairly small this is an easilydoable 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 CC 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 andor 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 AG and AH 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 2butanol do the following 1 determine product distributions using GC and from those free energies 2 compare your exp l values with hydrogenation energies from Wade 3 calculate energies by MMFF and AMl 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 M w AG your exp AH hydrog AH NIST AE MM AE AM1
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