Phosphate - not a base?

[Jules7890]

I have this multiple choice question asking me which functional group is a base, an amino group (NH2) or a phosphate group (PO4-)

 

It says the answer is the amino group, and I get that NH2 would accept a hydrogen ion to become ammonia, but I don't understand why a phosphate group wouldn't.

 

.... when I don't understand something, I obsess over it, so bear with me here ...

 

A phosphate group seems like a perfect candidate to accept H+, right?? I mean, in its ionized state. It's got TWO perfectly good negatively charged oxygen atoms with only one bond each sitting there, just asking for a proton, so why does my answer key tell me a phosphate group isn't a base??

Does anyone know?

 

(thank you for any help beforehand)

[ecoli]

phosphate ions exist in basic conditions. When there's H+ ions around, it does indeed get protonated to form hydrogenphosphate and phosphoric acid, when things are more acidic.

 

http://en.wikipedia.org/wiki/Phosphate

 

was the question which is a base, or which is a stronger base?

Edited June 29, 2008 by ecoli

[Jules7890]

The wording of the question is: "Which of the functional groups behaves as a base?"

 

(.... btw, why do the italicized words beside your name describe your name? I thought that was just kind of a ranking system)

[ecoli]

The wording of the question is: "Which of the functional groups behaves as a base?"

Phosphate is the conjugate base of hydrogenphosphate, so I don't know why this is considered wrong.

 

(.... btw, why do the italicized words beside your name describe your name? I thought that was just kind of a ranking system)

moderator privilege.

[Gilded]

Multiple choice questions are quite annoying if they have more than one technically correct answer, which seems like what has happened here. When this happens, you're better off with the "most correct" one, which in this case is NH2- since it's an incredibly strong base compared to phosphate ions.

 

Also, if this is a homework assignment from a chemistry book you should also call the people who made it. Inform them of this in a polite manner, such as exclaiming "WTF IS THIS S***???"

Edited June 29, 2008 by Gilded

[Jules7890]

thanks.

 

(and it didn't happen to make any difference that it was an ionized phosphate group, right? Not just phosphate by itself?)

[Gilded]

thanks.

 

(and it didn't happen to make any difference that it was an ionized phosphate group, right? Not just phosphate by itself?)

 

Hmm. A nitrogen atom attached to three other atoms (with single bonds) can obviously capture a H+ to form an ammonium ion, but it won't make it a very strong base; even ammonia is a weak base. I'm not sure about phosphate groups in this respect though. They are slightly polar but I've forgotten how this H+ capturing business goes with double bonded oxygen.

[ecoli]

I think that phosphate groups in molecules will still be basic. Phosphate groups in DNA (for example) still have negative charges, and could theoretically take up a hydrogen under acidic conditions.

[John Cuthber]

Somewhere, something is the strongest acid. Next to it, everything is a base. The alternative would be that the protonated form of some material was a stronger acid than the original "strongest acid".

By similar logic virtually everything is an acid.

The improtant question is under what circumstances you are talking about acidity and basicity.

In Conc sulphuric, nitric acid is a base.

In magic acid, sulphuric acid is a base.

[ecoli]

... which serves to make the original question even more irrelevant.