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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: rleung on February 18, 2006, 05:41:34 PM

Title: rate of aminolysis
Post by: rleung on February 18, 2006, 05:41:34 PM

Hi,

My book asks me to explain why the rate of aminolysis of an ester cannot be increased by HO-, or RO-.  The solutions manual says that HO- and RO- are both stronger nucleophiles than the amine and therefore would be more apt to attack the electrophilic carbon.  However, isn't the pKa of an amine usually above 30, while the pKa's of HO- and RO- are around 15-16???  So I am confused as to why HO- and RO- are better nucleophiles....the only explanation I can come up with is that the solvent is polar, so the amine (being the stronger base) would interact more strongly with the solvent molecules and be less powerful as a nucleophile.  

Any thoughts?  Thanks so much :)  I appreciate it.

Ryan

Title: Re:rate of aminolysis
Post by: Albert on February 19, 2006, 06:30:24 AM

Quote from: rleung on February 18, 2006, 05:41:34 PM

the pKa's of HO- and RO- are around 15-16

Are you sure?

Title: Re:rate of aminolysis
Post by: movies on February 19, 2006, 03:10:25 PM

Yes, don't get confused by the way we commonly refer to pKas.  Remember what they represent: the equillibrium constant for the dissociation of a hydrogen from a particular molecule.

Title: Re:rate of aminolysis
Post by: rleung on February 19, 2006, 05:59:18 PM

Hmmm, I am still not quite understanding.  I thought that the more basic (higher pKa) of something, the better it acts as a nucleophile (unless there are solvent factors, in which case the weaker base would serve as the stronger nucleophile).  Is my understanding of this wrong?

Ryan

Title: Re:rate of aminolysis
Post by: rleung on February 19, 2006, 06:10:00 PM

Ohh!  I think I see now.  Since RO- and HO- do not have hydrogens to really donate, it must have a very high pKa, higher than the very basic 36 for NH3, right?

Ryan

Title: Re:rate of aminolysis
Post by: movies on February 21, 2006, 03:30:13 PM

Yep.  We commonly say that the pKa of OH- is 15, but that's really the pKa of water.