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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: kodos on September 03, 2010, 04:32:54 PM

Title: Solubility of organic compounds in concentrated sulfuric acid
Post by: kodos on September 03, 2010, 04:32:54 PM

Explain why practically every organic compound that contains oxygen dissolves in concentrated H₂SO₄ (cold), obtaining a solution from which the compound can be retrieved by dissolving with water.

WHAT I THINK: This excercise is about bond polarity. Most organic compounds containing oxygen probably are polar molecules, so they're expected to be soluble in polar solvents like H₂SO₄.

Is this right?

DOUBTS:

• Are most molecules with O polar?
• Why cold?
• how can I possibly retrieve it by adding water?
• I would expect a redox, being H₂SO₄ an oxidative agent.

Any help?

Title: Re: Solubility of organic compounds in concentrated sulfuric acid
Post by: Borek on September 03, 2010, 06:34:24 PM

Sulfuric acid is very strong, oxygen has lone pairs.

Title: Re: Solubility of organic compounds in concentrated sulfuric acid
Post by: kodos on September 03, 2010, 06:45:14 PM

Yes, I don't doubt that. I meant organic compunds with oxygen.

I'm a bit lost...

Title: Re: Solubility of organic compounds in concentrated sulfuric acid
Post by: Borek on September 04, 2010, 03:10:28 AM

Do you think oxygen in a compound has no lone pairs?

I have presented you with two most important facts that are necessary to answer the question. THINK.

Third important fact is that ionic compounds are much better soluble in polar solvents.

Title: Re: Solubility of organic compounds in concentrated sulfuric acid
Post by: kodos on September 04, 2010, 08:16:58 AM

All right, so the reason why these compunds are soluble is that they have lone pairs, so they're expected to be attacked by H⁺ from the acid, thus breaking intermolecular interactions in the solid.

And the addition of water draws the acid from the solution, leaving the other phase with more organic molecules which are less polar and less soluble in water. Then the two phases can be separated.

Am I right so far?

Title: Re: Solubility of organic compounds in concentrated sulfuric acid
Post by: Borek on September 04, 2010, 08:23:06 AM

That's more or less part of the explanation that I would look for. Add to that ion/polar solvent thing.

What I don't like about the original question is that concentrated sulfuric acid will char many organic compounds containing hydrogen and oxygen, so the statement

Quote from: kodos on September 03, 2010, 04:32:54 PM

practically every organic compound that contains oxygen dissolves in concentrated H₂SO₄

seems to be too far fetching.

Title: Re: Solubility of organic compounds in concentrated sulfuric acid
Post by: kodos on September 04, 2010, 08:38:18 AM

Great, thanks a lot for your help and patience.

Yes, I see that the statement is far fetched.

One more thing, could the cold thing be because adding water to an acid releases too much heat?