Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: chunkmartinez on June 22, 2007, 02:50:55 AM
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i need help in figuring out polarities. I used to think that dielectric constants had to do with polarity, now i think im wrong!
can anyone help? :-\
Anyways, i see that formic acids dielectric constant is 58, does it mean that it's more polar than [methyl,ethyl,propyl] alcohol(~33,~24,~20)??
Or like i said, does DC not matter?
also, i noticed acetic acid had a dC of 6.15!!, while methylenechloride's(non polar) is ~9.
I suppose im not sure of how to tell rather more polar or non-polar, although, i do know of 3 different groups(polar protic,dipolar aprotic, and non-polar).
So if dielectric constants are not a good way of determining polarability, than what is?
Thanks for any help BTW: may sound a little newbieish, sorry :-\
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What an interesting question. I'm afraid that I cannot give you a better answer than the following:
The dielectric constant of acetic acid is quite low, but the solvent is miscible in water due to dissociation of the acidic proton to form a charged species and also because of its ability to participate in hydrogen bonding. For the purposes of most discussions, methylene chloride does not dissociate in water and does not participate in hydrogen bonding interactions, therefore its behavior is in accord with one's expectations given its dielectric constant.
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besides acetic acids being miscible in water, would it count as a polar solvent than?
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I would consider both acetic acid and methylene chloride to be polar organic solvents. They both have a significant dipole moment. But you cannot ignore the chemical structure when comparing their behavior.
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Acetic acid (and a lot of other carboxylic acids) have unusually low dielectric constants because the molecules dimerize when at high concentrations or when they're in fairly nonpolar solvents (see: http://en.wikipedia.org/wiki/Acetic_acid#Cyclic_dimer). Since the dimer has very little or no net dipole, pure acetic acid has a low dielectric constant. However, acetic acid itself is polar and can interact with many polar molecules through dipole-dipole interactions or hydrogen bonding. So, despite a low dielectric constant, acetic acid is still a polar solvent.
(Derek over at In the Pipeline has an interesting anecdote about carboxylic acid dimers. http://pipeline.corante.com/archives/2005/04/28/speak_for_themselves_they_do.php)