guy
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Choosing the solvent
In most cases I can figure out why some solvents are prefered for some reactions, but there are a few that I dont get (cuz I havent taken O-chem yet).
One problem I have is the difference between using CH2Cl2 and CHCl3?
Another I guess would be DMSO and DMF?
[Edited on 1/15/2007 by guy]
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roamingnome
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well since you mentioned DMSO
its a very dynamic electrostatic world down in relms of 500 mile per hour Boltzmann gases and liquids, point group symmetrys and im just throwing out
fancy words here...
the face of a protein active site called the epitope is just a topo map with magnetic attractions, changing the pH one extra unit and the whole
protein unravels.
its like saying do you want a massage or a hot rock treatment, thats the difference say between using methanol over IPA
im putting my money on DMSO for a reaction im trying to wrap up here namely microwave akabori...
i suspect after you take o chem you will only have more questions....hehe
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12AX7
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Methinks roamingnome == gil == semiautomatically generated posting. Out of curiosity, IP check, mods?
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woelen
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As stated by 12AX7, sciencemadness would be better off without roamingnome than with roamingnome.
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CH2Cl2 is more often used as solvent, because it is less reactive. CHCl3 is quite reactive, especially in contact with bases. The H-atom is split off
fairly easily and then a very reactive species remains (CCl3 radical or CCl3(-) ion?). It easily reacts with many other organics. A nice example is
with aldehydes and ketones. The best known reaction of that type is formation of chlorbutol.
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roamingnome
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| Quote: |
CH2Cl2 is more often used as solvent, because it is less reactive
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define reactive its terms of electrostatics sir
and pick up a physical chemsitry book to see what point group symmetry is,,, and maybye just maybye imbrace a walsh diagram....
its less reactive... thats really deep
12AX7: you are mistaken, and im am pacing in circles 12 times here Mods
what Novel chemistry is anyone doing here..really..
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guy
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| Quote: | Originally posted by roamingnome
| Quote: |
CH2Cl2 is more often used as solvent, because it is less reactive
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define reactive its terms of electrostatics sir
and pick up a physical chemsitry book to see what point group symmetry is,,, and maybye just maybye imbrace a walsh diagram....
its less reactive... thats really deep
12AX7: you are mistaken, and im am pacing in circles 12 times here Mods
what Novel chemistry is anyone doing here..really.. |
It is less reactive. Chloroform can easily form a dichlorocarbene species in a base solution, whereas it is more difficult for DCM to.
I fail to see how point group symmetry relates to this though.
And could you explain your first post? I dont get what you are trying to say
[Edited on 1/16/2007 by guy]
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unionised
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Interesting thought, if this
"Methinks roamingnome == gil == semiautomatically generated posting. Out of curiosity, IP check, mods?"
is not correct then roamingnome is a person who fails Turing's test for inteligence.
For what it's worth, symmetry does have a vague bearing on the matter because it affects polarity and, therefore, efficiency as a solvent.
CHCl3 used to be the chlorinated solvent that everybdy used because it was readily available from bleach and alcohol. In the last 15 or 20 years it
has gone out of fashion because it is notorious as a drug, it's rather toxic and DCM works just as well.
For DMSO vs DMF there are a number of reasons why one might chose one or the other. solvency, freezing point, reactivity, the fact that one smells of
rotten fish but the other leaves you with a funny taste in your mouth.
Also they are sometimes use not just as the solvent but as a reagent.
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roamingnome
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I truly respect Guy’s question of solvent choice as being one of the deepest of chemistry.
Molecules are vibrating bundles of charge, even though the charge is moving super fast like an electron orbit or whatever, it takes on a probability
density of charge, approximated by a static charge, or Van der Waals force.
These balls, but there not just spherical balls, because of the point group theory, are colliding and reacting and making chemistry interesting.
Im not trying to put woelen down here, but reactivity is a relative scale as is most chemical scales actually.
If my analogy’s are out of place then I can except that, but I was trying give you this painting of imagination of how a solvent plays into the
reaction. How it handles and distributes charge. How it makes or brakes a reaction, in the dynamic flutter of the quantum world. In the case of DMSO,
im finding out that it can sequester water from a reaction nicely. I have no experience with DMF.
Im am here to learn, gather information and exchange information with interested parties.
I am not here to make enemies, neither am I here to make friends.
I hope Gil becomes the L-PAC master with his taste tests…. And would love to sip some turpentine with anyone who’s nuts enough to do it.
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Ozone
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OK...
In order to describe how complicated this question is, I am trying to put at least this one question into a nutshell (10,000mt of crap into a 20g
nutshell). I apologize in advance if there is a mistake.
It is *not* simply an issue of polar or nonpolar. Polarity is split into protic and aprotic, that is, one which can donate a proton and one which
sequesters them. Symmetry effects "polarity" little, that is, it has little effect on the dielectric constant of the solvent. It does, however, have a
profound effect on another important colligative property, dipolar interaction (and induced dipolar effects where the inductive effect draws electron
density toward the more electronegative atom in a pair to produce an opposed partial positive and negative charge pair; this is a dipole, dipoles can
then "bond" with other dipoles, etc.). Here, we see a difference in the dipole moment of the molecule; this is reliant upon assymetry of groups
effecting the dipole.
For example, our mutual friends, di and trichloromethane do not at first appear to dissimilar, but when we consider induced dipolar forces as force
vectors, all becomes clear. In Dichloromethane, there are two Cl and two H's exerting inductive effects on the carbon center. In this case, the
functional group symmetry leads to cancellation of some of the net dipolar force (dielectric constant_=_8.93 at 25°C and the dipole moment is 1.6
Debye). As symmetry increases, and more net forces cancel, the dipole moment decreases, viz. trichloromethane_=_1.1d and tetrachloromethane_=_0.
The dielectric constant also decreases, viz. 8.9, 4.8, and 2.2. This is not, however, a function of symmetry, it is a function of in this case, the
ability to part with a proton (say, in a strongly alkaline environment). Dichloromethane has two H and chloroform has 1 (carbon tet has none).
OK, so now we still have not explained why dichloromethane is not as reactive as chloroform when placed under alkaline conditions (when the
colligative properties indicates that the contrary should occur, viz. 2 instead of 1 proton to abstract...). Now, we have to look at the reaction
intermediates themselves and see what is actually going on:
With chloroform, -B: abstracts the proton to give BH and -:CCl3 which can easily eliminate -Cl to yield the dichlorocarbene (which is an interesting
species with an open shell like Boron, and, like B, it can act as either a nucleophile *or* an electrophile). In this case, it is seen reacting as the
nucleophile with a double bond to create the dichloropropane analog. Now then, what happens to dichloromethane under similar conditions? All goes well
with the proton abstraction and we get the very unstable HCCl2:- anion which will immediately eliminate Cl- to yield HCCl:, a "hot" carbene. This
species does *not* have a completely unfilled orbital and will act as a strong electrophile with any species with greater electron density. In this
case, that species is usally the recently eliminated Cl- *or* chloroform to yield either the original intermediate or chloroform and dichlorocarbene.
This is why many "dichlorocarbenations" (or dichlorocyclopropanylation) reactions are frequently conducted on chloroform in... dichloromethane.
It also appears that we can make dichlorocarbene from carbon tet and Mg, but that is another story.
Whew.
For some further information (but by no rational means all of it) please see:
http://pubs.rsc.org/ej/P2/1999/A903936H.PDF
http://en.wikipedia.org/wiki/Carbene
http://www.mdpi.org/molecules/papers/80800608.pdf
You can take *years* of courses and not know half of it!
I Learn a *little* everyday, and forget twice as much,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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unionised
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"Symmetry effects "polarity" little"
In the very real sense that, while o dichlorobenzene has a fairly large diople moment, the p isomer has zero dipole moment. That's precisely because
of the vector summation you talk about. If the individual vectors are symmetrically arranged in opposition to one another the dipole is zero. If they
are all pointing in one direction they add up. One of the isomers of trichlorobenzene is called the "s" isomer because it's the most symmetrical. It
is the only isomer with zero dipole.
Having said that, there is as you say, a whole lot of stuff that affects solvency.
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Ozone
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Well put,
My context only involved a single carbon center, with say a Bz ring, symmetry is sustained across the structure and the directionality of the
vector(s) becomes far more pronounced. That is to say, the vector direction is fixed rather than rotating about a single carbon.
Should we discuss Van der Waals or London forces to explain why non-polar molecules aggregate? (Yark!)
Cheers,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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roamingnome
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Another thing worth mentioning is that as most things in life are “relative” geometry is more absolute. And even though I hate and suck at even
simple proofs I know that it is applied to chemistry to predict chemical properties. It even defines space and time….
The meat and potatoes today is molecular modeling software though
You can make a water molecule minimize its energy then copy and paste a bunch of them to create a “water box” then stick in a ion or something to
calculate meaningful numbers of some kind. Even simple calculations are performing millions of integrals. And jumping up to more complex systems like
proteins it gets well… complex
I’ve heard that even Deep Blue gets bored after awhile and tells its programmers that it would rather play some chess and beat out a human…… how
about that for an autonomic response spam mailer 
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franklyn
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Solubility Parameters
Everything you didn't know about solvents
Solubility Parameters: Theory and Application
http://aic.stanford.edu/sg/bpg/annual/v03/bp03-04.html
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