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Author Topic: hydroxy acid  (Read 1975 times)

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thedy

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hydroxy acid
« on: February 14, 2012, 01:05:47 AM »

Is hydroxy acids ampholytes?
I think,that they are,because they have carboxy group,which is acid and OH.OH have lone electron pair,so it can attract hydrogen cation.But in my book chemistry is writen,that hydroxy acid are not ampholytes.SO where is true?
Thanks
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Rutherford

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Re: hydroxy acid
« Reply #1 on: February 14, 2012, 09:47:42 AM »

Isn't the carboxy group substituted with a hydroxyl group?
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thedy

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Re: hydroxy acid
« Reply #2 on: February 15, 2012, 12:31:35 AM »

Isn't the carboxy group substituted with a hydroxyl group?
I m not sure,but for example lactic acid has OH and COOH group....So lone pair on O in OH can take H+,and COOH can slip off the H+...But,probably I m wrong....and I don t know,where I make mistake...
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Rutherford

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Re: hydroxy acid
« Reply #3 on: February 15, 2012, 04:28:34 AM »

When dissolving something in water the most polar bonds are going to be broken, so the bond COO"-"H is more polar the C"-"OH so I think that the hydroxil-carbon bond is not broken when dissoluting.
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Dan

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Re: hydroxy acid
« Reply #4 on: February 15, 2012, 10:38:30 AM »

OH have lone electron pair,so it can attract hydrogen cation.But in my book chemistry is writen,that hydroxy acid are not ampholytes.SO where is true?
Thanks

Lone pairs on uncharged O are not very basic. They can accept H+ and become protonated, but only in the presence of strong acids. Note that the pKa of H3O+ is approximately -2, which means it is strongly acidic and it follows that H2O is weakly basic. Alcohols behave similarly.

Your theory is absolutely correct though, the O lone pair can accept H+, and indeed it is thought to do so in many chemical processes. The issue here is with the definition - as far as I know the definition is simply that it can "act as an acid or base" and I'm not sure this is within a defined pH range (I hope someone can comment on this). Generally speaking, a molecule is only considered ampholytic if it can accept H+ at moderately acidic pH and donate at moderately alkaline pH - an example would be an amino acid.

Under the broadest definition, almost every compound you can draw can both accept and donate H+ under extreme conditions.
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fledarmus

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Re: hydroxy acid
« Reply #5 on: February 16, 2012, 02:52:29 AM »


The issue here is with the definition - as far as I know the definition is simply that it can "act as an acid or base" and I'm not sure this is within a defined pH range (I hope someone can comment on this).

I think you are giving the definition of "amphoteric" rather than "ampholytic". An ampholytic molecule is one that has both acidic and basic groups, and that there is a pH range in which the compound is zwitterionic - that is, the acid group is deprotonated and the basic group protonated at the same time. That does not happen with hydroxy acids - a hydroxy group is not a strong enough base to deprotonate a carboxylic acid, so that you would end up with a protonated hydroxy group and a deprotonated carboxylic acid group.

An amino acid can be ampholytic - the amine is usually basic enough to deprotonate a carboxylic acid, giving the protonated amine and deprotonated carboxylic acid on the same molecule.
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Dan

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Re: hydroxy acid
« Reply #6 on: February 16, 2012, 05:32:59 AM »


The issue here is with the definition - as far as I know the definition is simply that it can "act as an acid or base" and I'm not sure this is within a defined pH range (I hope someone can comment on this).

I think you are giving the definition of "amphoteric" rather than "ampholytic". An ampholytic molecule is one that has both acidic and basic groups, and that there is a pH range in which the compound is zwitterionic

God catch, thanks for clearing that up.
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moondams

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Re: hydroxy acid
« Reply #7 on: March 05, 2012, 06:25:33 AM »


Hydroxy acids are not all ampholytes. Lactic acid CH3-CHOH-COOH, for example, is not amphoteric !
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