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| | From:  MikeKL5 (Original Message) | Sent: 9/17/2004 12:19 PM |
Hey Steve,
I finally have a non-biochemistry related question for you. :-) We've been studying Diels-Alder Cycloaddition Rxns in Adv. Synth class, and when our professor lectured on the material he explained the Diels-Alder reaction in terms of Frontier Orbital Theory. The problem that I'm having is trying to figure out how electron-withdrawing/electron-releasing substituents on the Diene/Dienophile distort the HOMO/LUMO's of the Diene/Dienophile. I understand that when the Diene and Dienophile react the orbitals involved have the same phase signs, but our professor mentioned something about the orbitals changing size due to the presence of electron-withdrawing/electron-releasing substituents, and that these sizes needed to be somehow complimentary. Do you have any idea what he was talking about? I'm lost on this one. lol.
Thanks in advance for any help at all,
MikeKL5 |
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Hi Mike, good to hear from you! I may have better luck with a biochem question, I'm pretty rusty with frontier orbitals. Although I could not immediately find illustrations of how electron-donating and electron-withdrawing groups change the shapes of particular molecular orbitals, I would expect that electron-donating groups would result in affected MOs to "spread out" more, and just the opposite when withdrawing groups are present. Electron density "contour maps" of benzene show this effect - there is greater electron density at the ortho and para positions in phenol than in benzene itself, and when a withdrawing group is present, there is less electron density at the ortho and para positions than in benzene, both cases corresponding to the positions of the formal charges at these positions in the resonance structures of phenol and, say, acetophenone.
I quoted a couple of Internet sites on the subject of Diels-Alder reactions below. None mention the effect on the shape of the orbitals; rather, they emphasize the effect on the energy of the orbitals involved, the HOMO of the diene and the LUMO of the dienophile. Apparently the combination of having electron-donating groups on the diene and electron-withdrawing groups on the dienophile brings the energy levels of the HOMO and LUMO MOs closer to each other, facilitating interaction between the orbitals and the resulting concerted bond-breaking and bond forming process. But I agree with you that the shapes or sizes of the orbitals would also be a factor related to the effectiveness of orbital overlap in approaching the transistion state, thus affecting the energy of the transition state and therefore the rate of the reaction.
Hope this helps a little!
Steve
BTW, hope your ferrocene prep went OK! http://lonestar.utsa.edu/mpenick/diels_alder.htm"the electron-withdrawing carbonyl groups also make it easier for the reaction to happen, by lowering the energy of the lowest-unoccupied molecular orbital (LUMO) to approach (but still be higher than) the energy of the highest-occupied molecular orbital (HOMO) of the butadiene. When the energies of the two molecular orbitals approach each other it is easier for electrons to flow from one orbital to another. When it's easier for electrons to flow from one orbital to another, it is easier for them to reorganize and form new bonds." http://www.shodor.org/succeed/compchem/projects/fall00/dielsalder/outlinee.htm"the reaction depends on the interaction between the diene’s highest occupied molecular orbital (HOMO) and the dienophile’s lowest unoccupied molecular orbital (LUMO). The reaction goes on more readily when the energy difference between the two orbitals is small, and electrons are readily traded. In addition, minimal electrostatic repulsion between the products should accelerate the reaction." (cont.) "we can deduce there is indeed a relationship between LUMO energy and reaction rate for Diels-Alder reactions. This relationship states that the reaction rate will increase as LUMO energy decreases, meaning the HOMO/LUMO energy difference will be smaller and electron transfer will be easily facilitated. This is in accordance with FMO theory." |
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Reply
| | | From: MikeKL5 | Sent: 9/17/2004 9:56 PM |
Hey Steve,
So do you think that electron withdrawing groups would cause the MO of
the carbon their bonded to to become relatively larger, or relatively
smaller. I would think relatively smaller since the electron withdrawing
group would pull electrons out of the pi bond system. On the other hand, if
the electron withdrawing group is pulling electrons towards itself out of
the pi system, then the carbon to which the electron withdrawing group is
bonded would be expected to have a relatively negative charge (more than the
normal concentration of negative charge in pi bonds), and the MO would end
up being relatively larger.
I've attached a scanned image of my notes from the lecture. It will
help illustrate the question a little better. Maybe I'm overlooking
something simple. It wouldn't be the first time. :-)
The Ferrocene synthesis went well. I got beautiful, shiny, needle-like
crystals wth a yellow-orange color that reminded me and a few others in my
class of the color of flowers that grow by the highway down here in Ga. All
that being said, my sublimation got screwed up, and although I got pretty
crystals, I'm going to have to report a 30% yeild unless I go through all
the long process of drying more product, subliming it, yadda yadda yadda. So
it was a mixed experience. On Monday I'm supposed to get an NMR spectra of
my product, and I've had about twenty minutes of training on the NMR
spectrometer. I figure that it'll take me about five minutes to break it.
lol.
Take care man,
MikeKL5
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>> do you think that electron withdrawing groups would cause the MO of the carbon their bonded to to become relatively larger, or relatively smaller <<
I am assuming that in general, nearby withdrawing groups --> a smaller orbital, while donating groups --> a larger orbital (in the latter case, that's what I meant about the orbital becoming more "spread out").
 Imagine an electron-donating ring activating group on one of the carbons. The ortho and para carbons will have larger areas of electron density around them compared to the meta carbons. If a withdrawing group is present instead, the contours around the ortho and para carbons are smaller than those around the meta carbons. I need some better illustrations, but this fits well with the resonance stuctures we can draw for monosubstituted benzenes like phenol and acetophenone. There are five resonance structures in each case, with three of the structures having formal charges on the ortho and para carbons, accounting for the greater reactivity of those carbons towards electrophiles when a ring activator (electron-donating group) is present, and the lesser reactivity of those carbons towards electrophiles when a ring deactivator (electron-withdrawing group) is present. Unfortunately, many texts don't show these resonance structures, but they do make it easier to understand the directional effect these groups have in EAS reactions from this perspective. >> I figure that it'll take me about five minutes to break it. lol << That sounds familiar! The morning after the first time I used a departmental 1H-NMR instrument, the fellow in charge came by and informed me that I forgot to set a so-and-so knob back to where it belonged, etc., etc. Rather embarrassing.
It is more fun to work with compound that have nice colors! It's often hard to get all the product extracted or sublimed out of the crude mixture though, so lower than expected yields are not uncommon.
Steve
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Reply
| | | From: MikeKL5 | Sent: 9/18/2004 6:05 PM |
Hey Steve,
I think that I've figured out where I'm going wrong. I've been working
under the assumption that MO's combine more easily when they are of
relatively like size. But the only explanation that fits the observation is
that MO's combine more easily when they are of relatively unlike size,
probably so that they can reach some given level of stability. Which is
right?
I cannot thank you enough for all of your help. We need professors like
you. Come and get a job at Georgia State! :-)
MikeKL5
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Relative size of orbitals vs. energy of orbitals vs. rate of reaction (related to the energy of the transition state)... there is probably not always a clear correlation. I believe we can safely say in a gereral way that, the more easily the required orbitals of the reactant molecules can contact and overlap, the greater will be the rate of the reaction and the stability of the newly-formed bonds formed from these orbitals. But I do not know, in the case of the Diels-Alder reaction, if the actual sizes of the molecular orbitals involved are significantly affected by the inductive effects of other groups. I could not find anything definitive on the subject. Intuition suggests that orbitals of similar sizes can interact the most favorably. For example, compare the bond energies of the hydrogen halides -
Bond Bond Energy, kJ/mol Valence Energy Level of Halogen (n)
H-F 570 2
H-Cl 432 3
H-Br 366 4
H-I 298 5
We can describe these bonds as resulting from overlap of the hydrogen 1s orbital with a halogen sp3 hybrid orbital. In this series, note that the strongest bond results from the interaction of valence orbitals of similar energies (n=1 and n=2 orbitals -> HF, while in HI the valence orbitals have n=1 and n=5, very different from each other). But also, the relative sizes of the orbitals are more similar if their energy levels are similar, although with the shapes of the orbitals being so different here (s vs. sp3), this is not a very exact statement.
I was also going to try to make a correlation of orbital sizes with relative rates of SN2 reactions as a counter-example, in which the better nucleophiles, presumably with their reactive electron pair in larger orbitals, result in the higher reaction rate. But the reason for the rate trend is probably not because of the difference in orbital sizes of nucleophile and substrate, but rather because, in larger orbitals the valence pair(s) of electrons in the nucleophile are simply not held as tightly and therefore they react more easily in these reactions.
In describing the interactions of orbitals to form new bonds, I rarely see any mention of the relative sizes of the orbitals and how this affects the reaction. Two effects I'm thinking of are: 1) the rate of the reaction, and 2) the overall thermodynamics of the reaction. These two aspects of reactions are not related to each other, however. In the above example of hydrogen halides, we're looking at a thermodynamic parameter, the bond energy, but in the Diels-Alder reaction, we're looking at kinetics. The overall stability of Diels-Alder product is probably not very different whether electron-donating or withdrawing groups are present or not, so in this case our attention is focused more on the energy of the transition state relative to the reactants.
I haven't see any examples of any attempt to facilitate Diels-Alder reactions using an opposite logic, with electron-withdrawing groups on the diene and electron-donating groups on the dienophile. From an orbital size point of view, we might expect that the reaction should be just as favorable. But in terms of the relative energies of the LUMO of the dienophile and HOMO of the diene, this would increase the energy difference rather than reducing it.
Well, in summary, this is a complicated subject! The relative sizes of orbitals is rarely mentioned in the context of reaction rate or favorability, so I think you have raised a good point. I don't think we can generalize that orbital of unlike size combine more favorably in reactions. The answer probably lies more in the "mathematical" outcome of MO calculations, in terms of energies, which may not "translate" well into more familiar concepts.
At this rate I wouldn't get very far at Georgia State, but thanks for the thought!
Steve
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Reply
| | | From: MikeKL5 | Sent: 9/22/2004 10:08 PM |
Hey Steve,
Sorry I wasn't able to get back to you right away. I asked my synthesis
professor to clear up this frontier orbital question, and this is how he
explained it. We know that the diene needs an electron donating group. and
the obital that reacts in the diene is the HOMO. The dienophile needs an
electron withdrawing group, and the orbital that reacts in the dienophile is
the LUMO.
In the diene, the electron donating group changes the size of the
orbitals. One orbital is relatively smaller (the one next to the edg), and
the other orbital (if the edg is bonded to C1, then the other is C4) becomes
relatively larger. In the dienophile, the HOMO is distorted such that at the
carbon bonded to the ewg, the MO is relatively larger, and at C2 the orbital
is relatively smaller. According to my professor, since the HOMO of the
dienophile is distorted such that @C1=larger and @C2=smaller, in the LUMO of
the dienophile the order is reversed, and @C1 the LUMO orbital is relatively
smaller, and @C2 the LUMO orbital is relatively larger.
So the answer to the question appears to be, that MO's of like size
combine. (of course, the phases of the MO's must be the same. This is
indicated in the attached graphic and non-colored/colored lobes)
To answer your question, yes, it appears that you can switch the ewg and
edg such that the ewg is on the diene, and the edg is on the dienophile.
(From Graham Solomons and Craig Fryhle's Organic Chemistry, Seventh Edition
Upgrade, pg 605) "Research (by C.K. Bradsher of Duke University) has shown
that the locatios of the electron-withdrawing and electron-releasing groups
in the dineophile and diene can be reversed without reducing the yields of
the adducts. Dienes with electron-withdrawing groups have been found to
react readily with dienophiles containing electron-releasing groups."
I have no idea who C.K. Bradsher is, but I intend to research it when I
get my massive load of Calc/Phys/Enzymology homework done. lol.
Talk to you later,
MikeKL5
P.S. I tried to attach a graphic that would explain a bit more clearly what
I'm trying to say, but I wasn't able to get the message through with the
graphic attached.
----Original Message Follows----
From: "·Steve·" <[email protected]>
Reply-To: "Chemistry Corner" <[email protected]>
To: "Chemistry Corner" <[email protected]>
Subject: Re: Diels-Alder Reactions
Date: Sun, 19 Sep 2004 23:54:14 -0700
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Hey Mike, yeah, that's about the "size" of it (couldn't resist!). I finally had a chance to dig up one of my old texts, Mechanism and Theory in Organic Reactions, by Lowry & Richardson, and see how much I've forgotten since I last saw this back in grad school. I scanned a couple of pages to go with what you said in your last message. Here is a partial quote:
"...the circles represent the near lobes of the p orbitals , shaded for positive sign and unshaded for negative. The relative sizes of the circles represent the relative contributions of the respective p orbitals to the HOMO [in the diene]. (Orbitals at positions 2 and 3 are omitted.) It has been shown by Anh and others that the condensation will occur so as to bring together the ends with the largest coefficients. In the normal electron demand example," (continues on the following scanned page)
They also comment about switching donating and withdrawing groups:
"Although the reaction occurs in the unsubstituted case, it is most successful when the diene and the alkene (referred to in this context as the dienophile) bear substituents of complimentary electronic influence. Although these are most commonly an electron-donating group on the diene and an electron-withdrawing group on the dienophile, there are also a number of instances that illustrate inverse electron demand, that is, electron-withdrawing groups on the diene and donating groups on the dienophile."
The following diagram illustrates the three possibilities (unsubstituted, "normal electron demand", and "inverse electron demand". Notice that the HOMO-LUMO roles are reversed depending on what type of groups are present.
These scans look rather fuzzy on my screen, hope the show more clearly on yours.
The text also mentions that "Frontier orbital theory also explains the endo principle. This time secondary interactions are important; these are interactions between portions of the interacting HOMO and LUMO other than the loci of new bond formation."
Good topic and discussion! But boy, am I rusty! I see what you were getting at, how the groups affect the sizes of the reacting orbitals, which in turn determine the stereochemistry of the reaction.
Steve
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