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Topic: chloroform polarity  (Read 5813 times)

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Offline kapital

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chloroform polarity
« on: June 22, 2013, 06:29:28 AM »
I read this article http://themoleculechloroform.blogspot.com/2011/02/polarity-of-chloroform.html
and it says chloroform is polar which is logic. But why is then not more soluble in water and in particulart C-Cl bond "do" molecules more hydrophobic. Why?

Offline Dan

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Re: chloroform polarity
« Reply #1 on: June 22, 2013, 06:46:45 AM »
Polarity is not the only factor that affects water solubility - can you think of other intermolecular forces that might be important here?
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Offline kapital

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Re: chloroform polarity
« Reply #2 on: June 22, 2013, 09:02:40 AM »
Forces between what? Chloroform   or chlorofrom-water os somthing else?

Offline Dan

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Re: chloroform polarity
« Reply #3 on: June 22, 2013, 09:54:53 AM »
Quote from: kapital
Forces between what? Chloroform   or chlorofrom-water os somthing else?

All combinations.

Start generally with the forces between molecules (intermolecular forces). Which types of attractive forces are there?

Now think about water - which forces are most important between two water molecules?

Can chloroform behave in a similar way?
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Offline kapital

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Re: chloroform polarity
« Reply #4 on: June 22, 2013, 10:29:22 AM »
Hydrogen bonds of course, but is this reason for very low solubility? Chlorofrm neverthe less in quite polar?

Offline Dan

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Re: chloroform polarity
« Reply #5 on: June 23, 2013, 05:42:29 AM »
Yes, the inability of chloroform to form strong hydrogen bonds is key to it's low water solubility.

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Chlorofrm neverthe less in quite polar?

It has a dipole moment, so it's polar, but it is classed as a low polarity solvent ("non-polar" solvent). It is not polar enough to dissolve in water.
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Offline kapital

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Re: chloroform polarity
« Reply #6 on: June 23, 2013, 07:49:10 AM »
Ok, but why is low polar?(it has three very polar bonds).  Perhaps in chloroform bond dipoles in some percent cancell each other out, but C-Cl bond always contribute to more hydrophobic carracter of molecules, why?

Offline Dan

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Re: chloroform polarity
« Reply #7 on: June 23, 2013, 08:21:36 AM »
Ok, but why is low polar?(it has three very polar bonds).  Perhaps in chloroform bond dipoles in some percent cancell each other out,

To some extent, yes. The dipole moment and dielectric constant of chloroform is are the lower end of the scale of polarity for common solvents. You may find this useful for comparison:

http://en.wikipedia.org/wiki/Solvent#Properties_table_of_common_solvents

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but C-Cl bond always contribute to more hydrophobic carracter of molecules, why?

Not true. Chloroform is much more water soluble than methane, for example.
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Offline kapital

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Re: chloroform polarity
« Reply #8 on: June 23, 2013, 09:57:48 AM »
Ok, then I have som other question now. Why does not chloroform form hydrogen bonds, it has comparable electronegativity with oxygen?

Offline TheOrganic

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Re: chloroform polarity
« Reply #9 on: June 23, 2013, 05:44:18 PM »
Ok, then I have som other question now. Why does not chloroform form hydrogen bonds, it has comparable electronegativity with oxygen?

Don't you think size is an important factor (or rather the important factor ) in case of such intermolecular bondings ? How do the sizes of Oxygen and Chlorine compare?

Offline kapital

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Re: chloroform polarity
« Reply #10 on: June 23, 2013, 07:37:48 PM »
Chlorine is biger, but how does this influence on that? Possibly biger atom --> smaller charfe per volume ??

Offline orgopete

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Re: chloroform polarity
« Reply #11 on: June 24, 2013, 11:15:57 PM »
Ok, then I have som other question now. Why does not chloroform form hydrogen bonds, it has comparable electronegativity with oxygen?

I don't think electronegativity is a good measure of the relative polarity. HCl is a moch stronger acid than water. Hence I might conclude chloride ion forms weaker hydrogen bonds than water. If so, then I might expect chloroform to form very weak hydrogen bonds, if any. Therefore, hydrogen bonding will not increase solubility.
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Offline TheOrganic

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Re: chloroform polarity
« Reply #12 on: June 25, 2013, 10:39:41 AM »
Ok, then I have som other question now. Why does not chloroform form hydrogen bonds, it has comparable electronegativity with oxygen?

I don't think electronegativity is a good measure of the relative polarity. HCl is a moch stronger acid than water. Hence I might conclude chloride ion forms weaker hydrogen bonds than water. If so, then I might expect chloroform to form very weak hydrogen bonds, if any. Therefore, hydrogen bonding will not increase solubility.

+1.

I would also like to add that the Hydrogen bonding ability of an element depends on it's size. Hydrogen bond itself is not a very strong bond - but the entire network of hydrogen bonding can have much pronounced effects on chemical properties, which we perceive. We would expect a smaller atom (fluorine) to better form a hydrogen-bond-network than a larger atom ( chlorine ). Electronegativity is not the key here, although surprisingly most definitions of Hydrogen bonding involve the term electronegativity. If you go by Electronegativity you will find many contradictions in Chemistry, which shouldn't exist.

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