[davidenarb]

Hi all,

RMgX + NH3 give RH + H2NMgX 

in the textbook, it says that RMgX react as a base, so how can we have reaction between two bases ?

Thanks

[orgopete]

The answer has been written with the question. What should that be telling you? Is NH3 acting as an acid (proton donor) or a base?

[spirochete]

You learn about ammonia in general chemistry as being only a base. That's because in water, the predominant reaction that occurs is the protonation of ammonia by water to form hydroxide. Water will not, to any significant extent, depronate ammonia. In other chemical environments, however, ammonia can also act as an acid.

In organic chemistry it's time to stop only memorizing definitions and think about reactions logically. Grignard reagents act essentially like SP3 hybridized carbanions with a one minus charge. The anion in the product of the reaction is an SP3 hybridized nitrogen anion with a one minus charge. Other things equal, which do you think is more stable: a carbon anion or a nitrogen anion?

[PhDoc]

David,

It's important to get to know your pKa values. This is one of the few areas where memorization skills are valuable.

The pKa for an alkane is 50 (depending upon where you get your numbers from), and for ammonia is 35. We can use this information to calculate the equilibrium constant for the reaction of RMgX with NH3:

RMgX  +  NH3  -->  RH  +  MgXNH2

Keq = 10^-50/10^-35 = 10^15

When you get a Keq of this magnitude, that proton is going to practically fly off!

As mentioned by others, acid-base chemistry in Gen Chem focuses mostly upon reactions in water. O-Chem is very different. Gen Chem uses mostly the Arrhenius definition of an acid/base. O-Chem uses the Bronsted-Lowry and Lewis definitions of acids & bases.

So here's a question, what's the stronger base? H2SO4 or HNO3? The question may sound "wacked," however it's really quite logical. What's most important is that this question is eventually going to require an answer from you.

Study smart!

[AromaticAcrobatic]

Also, Electronegativity is your friend. Become familiar with what that means and it's implications and every thing should fall into place.