Solvent effects on the chemical exchange of a push pull Ethylenes <p> Chemical exchange rates provide a sensitive probe of solvent effects in solution.
The push-pull ethylene methyl 3-dimethylamino-2-cyanocrotonate (MDACC) has three
exchange processes E-Z (the same as E' -Z'), E-E', and Z-Z', and we have measured
these rates in acetone-d6, chloroform-d, tetrahydrofuran-d8, toluene-d8, methanol-d4,
acetonitrile-d3, and methylene chloride-d2 through 1D 1 H.NMR at different
temperatures. From this we obtain ΔG+, ΔHt+, ΔS+ of activation for each solvent by
employing an Eyring plot. As the solvent changes, ΔH+ is approximately constant
whereas ΔS+ follows the solvent polarity. </p> <p> Since the equilibrium constant is less than one, we would expect it to increase
with temperature, but it decreases. This is seen in the Van't Hoff plot where the slope is
positive indicating that the process of going from the major site to the minor site is
exothermic, so not only is the rate governed by entropy effects, but also the equilibrium
constant between E and Z. </p> <p> The rotation between the C-N and the C=C bond occurs through a non-concerted
mechanism. Two reasons why this occurred was apparent through EXSY, since the
intensity of the off diagonal peak varies with different mixing time and second, the
rates from C=C rotation (E-Z) and C-N rotations (E-E' and Z-Z') are different at the
same temperature </p> / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19525 |
| Date | 09 1900 |
| Creators | Khasawneh, Maysoon |
| Contributors | Bain, A.D., Chemistry |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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