Ebao-lu
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What happened to benzene?
Once i've stored a bottle of benzene at unheated country house. When in winter the temperature failed indoors to 0C or such, something strange
happened: The liquid inside bottle was filled with many solid bubble-like spheres, forming a crastallic mass just like ice. The structure was
resistant to shaking. There was some liquid and air inbetween and inside the bubbles also.
What could it be?
[Edited on 10-3-2009 by Ebao-lu]
[Edited on 10-3-2009 by Ebao-lu]
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panziandi
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Benzene freezes at 5.5oC IIRC. My cyclohexane (mp 6.5oC) often freezes up. Often what happens is it will all freeze but then as it slowly warms up
bits melt and you end up with a chunk of melting solid floating within the liquid solvent, it can have the appearence of bubbles within the melt, this
maybe due to moisture or gas dissolving in the liquid then coming out as it freezes and being trapped, benzene can be quite moist.
You should have taken a picture :p
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kclo4
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Isn't benzene's melting point 5.5 °C, 279 K, 42 °F? It is according to Wikipedia anyways. Do you have a picture of it?
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Ozonelabs
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In our experience this is perfectly normal with Benzene.
Ther temperature in our laboratory frequently falls below that of Benzene's melting point and both of our Winchesters will freeze. It is nothing to be
concerned about.
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Ebao-lu
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That is amazing! I never imagined that hydrocarbons(like benzene, cyclohexane) can melt at 5.5 and 6.5C! For comparison, the m.p. of toluene is -95C,
n-alkanes also have low mp's.
No i did not take picture. the bottle is still in the country, i will take it as soon as i'll be there.
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garage chemist
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Yes, my benzene is always solid in winter (unheated lab).
It forms a curious funnel at the surface because it contracts (decreases its volume, and increases its density) when it freezes.
When it thaws again, the crystalline solid benzene sinks to the bottom of the bottle, with liquid benzene above it! Water does the opposite (ice
floats on water).
This means that there is no danger for the benzene glass bottle to crack upon freezing, while a glass bottle filled with water would very likely crack
upon freezing.
Other liquids with mp above 0°C that contract while freezing are:
- glacial acetic acid
- 1,4-dioxane
- tert-butanol
- cyclohexane
- 1,4-butanediol
You see, water is an exception here! I don't know of any organic liquid that expands upon freezing.
Two substances other than water that expand upon solidifying are bismuth and gallium.
Also, in my own observation, anhydrous potassium acetate!
This one looks curious: the molten salt forms a hard crystal skin that breaks up many times, with liquid oozing out.
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smuv
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Smuv's homework assignment: I checked with polverone, if you don't do it you are banned for life  ...
Look up the mp of p-xylene. On the basis of everything you know about melting points, try to predict the mp's of o and m-xylene in relation to
p-xylene and each other (essentially make a list in the order in which they melt). Then check your answers. Hopefully if you do this you will
learn/understand why benzene and p-xylene have high melting points. Extra credit would be to do the same for the 3 dimethoxybenzenes and/or the 2
butanes.
Have fun 
[Edited on 3-11-2009 by smuv]
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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Globey
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I'm going to take an *uneducated* guess (meaning, no, I didn't google it.) I'm sure all the brains here can answer it. I did have orgo 1 & 2. I
got a C in orgo 1, and Aced orgo 2. In orgo 2, instead of a young Russian professor with a huge ego, I had a dead head professor who taught
everything by mechanism, with a heavy emphasis on electrical perspective. Anyway...
Does the relatively high MP have to do with relative lack of weak stearic repulsion? In benzene, there would be a weak attraction between one of the
double bones (sides) to another molecule of benzene where the straight alkane is. You'd have the molecules lining up alternating so point of least
resistance is ene to ane, always. Benzene is a hexagon, and hexagons can move closely relatively together anyway. Also, with symmetrical xylene, the
methyls will tend to go towards areas of least stearic hinderance, so you will have one p-xylene methyl coming close to 90 the unsubstituented parts
of the molecule (90 degrees), and thus, the molecules can move closer together (thus influencing a higher temp. freezing point)? Here you would have
the para xylene molecules making "L"'s, lining up that way. That's my inital guess. As I think more, I'll try to add to it...it's just been so long.
I'm probably off.
(edit: with the other xylene isomers, symmetry is missing, and you won't be able to get as many molecules as closely together (obviously) as you
would p-xylene, and benzene). So, symmetry?
[Edited on 11-3-2009 by Globey]
(edit: Yes, the dead head professor was AWESOME, both as a teacher, and as a person. The Russian dude was....I'll let you guess
[Edited on 11-3-2009 by Globey]
The more I think, the less my theory makes sense. W/ DMSO, 1,4-BDO &AcOH, it all falls apart for me. Someone PM me the answer, I'm really
curious!
[Edited on 11-3-2009 by Globey]
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Ebao-lu
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my thoughts are:
I am not sure that it is only geometrical symmetry that makes benzene and p-xylene more favorable to enter a crystallic latice (in comparison to
toluene that is having mp -95C). What can be caused by this geometry? Enthropy of crystallization? There are many unsymmetrical compounds that are
crystallizing fine, so i dont think that the reason is just enthropy consumed to turn around an unsymmetrical molecule and put it in sequence, though
it also has some impact.
And i suppose, there should be practically the same power of aromatic binding in benzene and p-xylene. Then, the Me groups of p-dimethylbenzene seem
not to cause any sterical problems and their presence is not changing much the mutual disposition of aryl rings in lattice layers. That means the
disposition of Ph's in a crystallic littice is quite constant and favorable. Then, if the suppose concerning enthropy is correct(that is not having
great impact into the stability of a structure) , it sould be because of dipole-dipolar interactions, that are somewhat conflicting with the forces
of aromatic binding. I mean, in that favored ring disposition, the dipole-dipolar interactions probably cause a repulsion, that's why toluene (that is
having some dipole moment) is having lower mp. Then, i suppose, o-xylene should have a lowest mp, and m-xylene - somewhere inbetween.
As for dimethoxybenzenes, here it is another case, because they have higher dipolar moments, because they are quite polar. So, aromatic binding like
in case of benzene is not having the main impact into stability of crystallic lattice, and there is no favored ring disposition for aromatic binding,
but there is such disposition for dipole-dipolar binding. Here, o-dimethoxybenzene is having highest mp, and p- the lowest.
And concerning 2 butanes, i think iso should have a lower mp because it is more compact and does not have much binding , unlike n-butane
Thats all only proposals, and i'll check them. but would not say wether i was right or wrong
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