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Author: Subject: Is potassium amide one of the strongest bases known?
mycotheologist
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[*] posted on 30-7-2012 at 12:10
Is potassium amide one of the strongest bases known?


I read that a good way to estimate the strength of a base is by looking at the pKa of its conjugate acid so I've been thinking about this. I always considered methoxides to be very strong bases but when neutralised, they form methanol which is more slightly acidic. The most basic conjugate acid I can think of is that of the amide ion. Neutralising an amide, you get ammonia. Does this mean amides are far more basic than methoxides and hydroxides or have I got the wrong idea here?
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per.y.ohlin
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[*] posted on 30-7-2012 at 13:26


There are much stronger bases out there. See <a href="http://en.wikipedia.org/wiki/Superbase">http://en.wikipedia.org/wiki/Superbase</a> for a few examples. Since ammonia is a weaker acid than methanol, amide is a stronger base than methoxide.
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ChemGradStudent
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[*] posted on 31-7-2012 at 02:27
bases


Your idea is correct, Na/K amide is more basic than an alkoxide. However, remember that there are even less "acidic" protons out there such as those on hydrocarbons
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weiming1998
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[*] posted on 31-7-2012 at 03:05


Quote: Originally posted by mycotheologist  
I read that a good way to estimate the strength of a base is by looking at the pKa of its conjugate acid so I've been thinking about this. I always considered methoxides to be very strong bases but when neutralised, they form methanol which is more slightly acidic. The most basic conjugate acid I can think of is that of the amide ion. Neutralising an amide, you get ammonia. Does this mean amides are far more basic than methoxides and hydroxides or have I got the wrong idea here?


Nitrides would be much, much stronger bases than even amides, because N(3-) is the conjugate base of NH(2-) which is the conjugate base of NH2-. The ammonia in nitrides are deprotonated 3 times instead of just once like amides. One easily available nitride is lithium nitride, made by burning lithium in a pure nitrogen atmosphere. Magnesium nitride can be made in the same way, but is more acidic due to the Mg2+ ion. Other nitrides are hard to make and almost impossible to buy.

Bases like butyllithium compounds are very strong too, since their conjugate acid (C4H10, the conjugate acid for butyllithium compounds, have a pKa of over 50, if my memory is correct) but they are much nastier to work with than nitrides, and is highly pyrophoric.

Anyway, amides are certainly stronger bases than methoxides. Methoxides hydrolyzes in an equilibrium in water (CH3O- + H2O<===>CH3OH+OH-, with the equilibrium usually leaning towards the left slightly because methanol has a slightly higher pKa than water (about the same). Amides, however, have much high pKa (as you stated) and directly and irreversibly hydrolyses to ammonia.



[Edited on 31-7-2012 by weiming1998]
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mycotheologist
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[*] posted on 16-11-2012 at 04:28


Quote: Originally posted by ChemGradStudent  
Your idea is correct, Na/K amide is more basic than an alkoxide. However, remember that there are even less "acidic" protons out there such as those on hydrocarbons


So its not about basicity of the conjugate acid, but rather lack of acidity of the conjugate acid? For example, t-butyl lithium, t-butane is harder to deprotonate than ammonia (well, I don't know that I'm just going on what you told me), so is t-butyl lithium a stronger base than sodium amide?
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[*] posted on 16-11-2012 at 05:20


Amides are a lot stronger than alkoxides as bases but they are weak compared with alkyl lithiums. Sodium hydride will deprotonate alcohols, amines and more beside.
Other factors do come into play apart from the lack of acidity of the conjugate acid; solubility of the base is important as is the loss of the conjugate acid as a gas eg methyl lithium and sodium hydride, it is also worth noting that Schlosser's base is much stronger than butyl lithium or potassium t butoxide on their own.
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