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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: SemonSays on May 08, 2007, 06:48:17 PM

Title: Why is urea basic and not acidic?
Post by: SemonSays on May 08, 2007, 06:48:17 PM
I can't figure out why urea is basic and not acidic. I can't find any information on the internet and i have to use the molecular structure or Lewis structure of urea to prove it. I also need to find out how urea is chemically changed to nitrate ions.

Can anyone help me out?
Title: Re: Why is urea basic and not acidic?
Post by: Sam (NG) on May 08, 2007, 06:58:44 PM
Please read the Forum Rules (http://www.chemicalforums.com/index.php?topic=9037.0), specifically:
Quote
4.) Please show that you've at least attempted the problem.  We don't mind helping you solve problems but we are ethically opposed to doing homework for you. Violators will have their topic deleted or locked.

If you draw resonance structures for urea, that would be a good start.
Think about conjugate acids and bases.
Title: Re: Why is urea basic and not acidic?
Post by: SemonSays on May 08, 2007, 07:06:54 PM
there are 4 i think.                                                                             http://academic.evergreen.edu/curricular/MANDM2000/Spring/homework/hw6/Image950.gif

That is the Lewis structure of urea (hope the link works). But how does that help? Im so confused. :(
Title: Re: Why is urea basic and not acidic?
Post by: SemonSays on May 08, 2007, 07:07:59 PM
I thought it had something to do with the amino groups or amidogens (NH2), but im not so sure.
Title: Re: Why is urea basic and not acidic?
Post by: Sam (NG) on May 08, 2007, 07:40:45 PM
The lone pairs of electrons in the molecule are available despite resonance because in all of the resonance structures, at least one of the nitrogen atoms has a lone pair on it.
Title: Re: Why is urea basic and not acidic?
Post by: constant thinker on May 08, 2007, 09:43:56 PM
Don't forget what Brønsted-Lowry said about a base, the whole part about a base being simply a proton acceptor.

The nitrogen has that nice lone pair. Looks like a good place to accept a proton to me.
Title: Re: Why is urea basic and not acidic?
Post by: P on May 09, 2007, 04:50:55 AM
Hi guys - sorry for being dumb, but, why is it then that urea dissolved in water is quite acidic - pH being about 1 when I put it in water at about 20%.
Title: Re: Why is urea basic and not acidic?
Post by: Sam (NG) on May 09, 2007, 06:13:22 AM
Quote from: P on May 09, 2007, 04:50:55 AM
Hi guys - sorry for being dumb, but, why is it then that urea dissolved in water is quite acidic - pH being about 1 when I put it in water at about 20%.

This is a good point, the pKa for urea is low, why have you assumed that it is basic Semonsays?
Title: Re: Why is urea basic and not acidic?
Post by: P on May 09, 2007, 09:08:08 AM
Yea  -  my first assumption (which was wrong) was that the solution would be basic   - I guess it was the 2 NH2's that threw me  -  it just 'looks' as though it should be basic to me.  It is, however, quite acidic in solution   -  I might be missing something simple here, but, at the moment I'm not sure why it is acidic.
Title: Re: Why is urea basic and not acidic?
Post by: Sam (NG) on May 09, 2007, 12:00:13 PM
Quote from: P on May 09, 2007, 09:08:08 AM
Yea  -  my first assumption (which was wrong) was that the solution would be basic   - I guess it was the 2 NH2's that threw me  -  it just 'looks' as though it should be basic to me.  It is, however, quite acidic in solution   -  I might be missing something simple here, but, at the moment I'm not sure why it is acidic.

If you push the electrons from the nitrogen into the C-N bond and make it C=N, then you have a positive charge on the NH2 group, loss of hydrogen would fix this?
Title: Re: Why is urea basic and not acidic?
Post by: constant thinker on May 09, 2007, 08:49:48 PM
Hmmm, so it is acidic. The NH2's also threw me.
Title: Re: Why is urea basic and not acidic?
Post by: UnintentionalChaos on May 10, 2007, 10:38:45 PM
Google Search: *urea basic*...second result: http://answers.yahoo.com/question/index?qid=20070508144240AAqawz0
Title: Re: Why is urea basic and not acidic?
Post by: Sam (NG) on May 11, 2007, 04:45:05 AM
Quote from: UnintentionalChaos on May 10, 2007, 10:38:45 PM
Google Search: *urea basic*...second result: http://answers.yahoo.com/question/index?qid=20070508144240AAqawz0

Strange, because the pKa of urea is ~0.2.
Title: Re: Why is urea basic and not acidic?
Post by: P on May 11, 2007, 06:02:20 AM
I'm really sorry gents - feel free to eat my snacks    :-[

It was the urea phosphate I was using that gave the very low pH's (0.2 in water).  The urea in water was around 7.5.  Apologies.
Title: Re: Why is urea basic and not acidic?
Post by: Sam (NG) on May 11, 2007, 10:32:22 AM
Quote from: P on May 11, 2007, 06:02:20 AM
I'm really sorry gents - feel free to eat my snacks    :-[

It was the urea phosphate I was using that gave the very low pH's (0.2 in water).  The urea in water was around 7.5.  Apologies.

That clears up some confusion i guess.