mycotheologist
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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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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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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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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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| 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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ScienceSquirrel
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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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