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Author: Subject: Does the amount of solvent effect reactions?
beastmaster
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[*] posted on 9-10-2010 at 02:21
Does the amount of solvent effect reactions?


What effect on a reaction does the amount of solvent have? Is there "a right amount"? Of course if the reactants are solids you need enough to dissolve them, but can you have to much or to little?
This is a general question. I'm sure there are many different situations. Thank you for any help. Beastmaster
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[*] posted on 9-10-2010 at 05:41


Yes:
One can think about it in terms of collision theory. If there is more solvent, there are fewer reactant particles per dm^3. The fewer particles, the less chance there is of two (or more) colliding successfully to react.
This isn't always the case however, as in reactions which have more than one step there could be reactants whose concentration has no effect upon the rate of reaction.

To find a 'right amount', you need to consider various factors:

Some reactions that are safe at low concentration are very dangerous at higher concentrations.
Some reactions will only work with more concentrated reagents (e.g. dilute H2SO4 will not react with NaCl to produce HCl)
Some reactants act differently at different concentrations (e.g. in the previous example, or with HNO3 which will act as an acid at lower concentrations and as an acid and an oxidising agent at higher concentrations).

There is also an obvious advantage in keeping the concentrations of all the reactants the same, e.g. 1 molar, or whatever because it makes calculations much easier.

[Edited on 9-10-2010 by Mossydie]
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[*] posted on 9-10-2010 at 08:18


Also it's important to anticipate the solubility of byproducts. Will they crash out and is that acceptable?

Some reactions (crown ethers, polymerizations, etc.) need dilute concentrations because they are relying on the molecules coming in almost one at a time and forming the desired molecule. In these cases, even a difference of a hundredth of a molar on a solution can cause the final product to be off.

And there is no reason you need enough solvent to dissolve all of your solvent reactants. That can be a fools game in both organic and inorganic chemistry. Some reactions run in organic solvents might us a hydrochloride salt or other salt that is nearly insoluble but becomes a soluble product during the course of reaction, some inorganic reactions like the reaction between calcium fluoride and sulfuric acid might run as a paste and end up as a cement. You should always know what a realistic expectation is for solubility of the starter and products because adding too much solvent can be a waste whereas increasing loading might even lead to better yields in addition to savings on overall solvent use.




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peach
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[*] posted on 9-10-2010 at 22:40


It depends what reaction you're talking about.

For example, there is one called the Finkelstein that can swap a bromide for an iodide, with nothing other than an iodide salt present and the solvent. Which you would be easily excused for thinking, wouldn't work, as the bromide is a higher halide. It relies on the solubilities of the bromides and iodides being vastly different in that particular solvent.

In others, like a fair bit of organic chemistry, the amount you add can be determined by things like.... can I even stir this? With emergency additions being needed as it thickens up to get it going again.

Then there are finer details, like is the reactant going to come in contact with the substrate too rapidly with this solvent, or this amount of it, or will it be too slow? The kinetics.

In organics particularly, again, there is something called phase transfer, where catalysts are added to drag one component from one phase to the other, in a controllable manner. Your washing up powder is doing something similar. Hair conditions and shampoos can have PTCs in them too. I often thought PTC glass chemistry was kind of funny, "just do a pressure equalized, dropwise addition of some shampoo now and run the PTFE spin cycle...."

Then there are the common bronsted lowry acids & bases, where their disassociation will change, and their dehydrating or oxidizing potential, and the substrate & product solubilities, based on how much water is there. With the water acting as the solvent for the acid or base to split and ionize. Meaning, sometimes, you're reaction will work better with more dilute acid.

Dilute sulphuric on salt will release hydrogen chloride, but it'll then be immediately absorbed into the water and tons of the stuff will absorb into water before it saturates. Hence, to generate hydrogen chloride, you really want the salt and acid as dry as you can get them, because a lot of your product will go missing with what would normally be thought of as small amounts of water present if you're after the gas.

Lead acid batteries use fairly dilute sulphuric, yet energy density is a major concern for the guys producing them and they have access to acid three times more concentrated. But choose the more dilute acid over the 98% or oleum. The same is true for the guys who do plating, who often use dilute acids in the baths over the super concentrated forms.

Then there's reactions that are precisely the opposite.

And then there's biology, and membrane chemistry, or biochemistry, and physics, where osmotic pressures and diffusion rates start coming into play.

Apples and oranges, solvents and solutions. :P

[Edited on 10-10-2010 by peach]




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[*] posted on 12-10-2010 at 14:03


So.......yes!

Often folks ask this question, when they want to reduce the amount of solvent used, in a published reaction sequence.

Like, they want to reduce the amount of solvent in a peroxide/formic acid, alkene oxidation.

Keep in mind, that one of the most important functions of a solvent, may be acting as a heat-sink. Too little solvent, too much heat build up.........Very bad outcome!

Had some friends that suffered excessive solvent loss during a large LiAlH4 reduction. The reaction was complete, and they were hydrolysing the reduction complex, when the reaction got away from them. They had lost a lot of ether during the course of the reaction; the reaction mass started to heat up, and they couldn't calm it down.

Hydrogen gas, ether vapor, and steam.....Were generated in such volume, that the mixture was unable to vent adequately through their condenser. Internal pressure forced the vessel open, the gas vented into the surrounding atmosphere, and detonated.

They had run out of the building in a panic, and thereby survived. The building was not so lucky. It required many thousands of dollars in repairs. Miracle it didn't catch on fire and burn down.
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[*] posted on 12-10-2010 at 14:31


That's the price they pay for trying kilos prior to grams hey. :P



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[*] posted on 12-10-2010 at 15:05


Inorganic and organic chemistry being different, it seems like a broad topic.

There is a book called Solvents and Solvent Effects in Organic Chemistry. It touches on a couple points for that.

I think too little water is at the root of some members' inorganic salt gel precipitates over the years.
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[*] posted on 21-10-2010 at 03:34


Peach,

It wasn't a question of abruptly trying a large scale reaction, without prior experience. These boys were very adept at hydride reductions. Large scale ones, at that.

Ether loss is a common phenomena during some reactions. The boys were going larger than usual. They were short on solvent. Their larger than usual vessel proved difficult to adequately cool, and attempts to conserve solvent via an extended condenser assembly, failed.

They had pushed the limits in the past, without serious incident. This time, they had unwittingly pushed their luck just a little too far. And, they had abruptly run smack-dab into potentially lethal trouble.

Until the last moments, they were unaware that they had finally cut one too many corners, and they were due to pay the piper.

The reaction ran away, all efforts to calm it down, failed..........AND, within minutes.....Ka-blooie!

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peach
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[*] posted on 29-10-2010 at 14:44


Sorry, it's just when I hear the words hydride, solvent loss and large scale, that makes me think of drugs and guys pushing their luck.

Nothing wrong with drugs, provided you keep it as your own problem.

But some of them (I mean, a lot of them) do get way too heavily into the improvising method. I feel for them, trying to get inventive and sometimes coming up with really good ideas, but I also feel for them when they're doing something that just WILL NOT function due to the physics or chemistry involved. Trying to cool things with no where near enough surface area or temperature difference.

I'd rather they successfully made the gear to get high than incinerated the neighbors. :P

{edit}As much as I disagree with arbitary drug laws, I do agree with the authorities that burning down someone else's house or gassing them isn't really fair. :D

"Sorry....."
"Sorry!?! You burnt my ****ing house down!"

I read a police report on meth labs at one point, which said that the majority of them were found because the fire brigade was in the process of putting the place out after they managed to start an ether fire.

[Edited on 29-10-2010 by peach]




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