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Author Topic: pKb of strong bases  (Read 4591 times)

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CrimpJiggler

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pKb of strong bases
« on: December 02, 2011, 02:04:13 PM »

I can find pKa tables but have yet to see a comprehensive pKb chart. I'm trying to get an idea of the strength of various bases so I can use them as references. For acids, I know that acetic acid pKa = 4.6, H2SO4 pKa = -3, HI pKa = -10, trifluoromethanesulfonic acid pKa = 16 etc. so I can use them as references in my head but I have no numbers for the bases I know of. For example, I don't know if sodium amide is a stronger base than sodium methoxide or if lithium hydride is stronger than tert-butyllithium. Also if I'm not mistaken, the strongest acid known to man is fluoroantimonic acid. Whats the strongest known base?
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Vidya

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Re: pKb of strong bases
« Reply #1 on: December 02, 2011, 02:29:02 PM »

to compare amide with metoxide you can use the pKa values also
CH3OH <------->CH3O-  + H+
NH3<------->  NH2-   + H+
Compound with low pKa will give you weak base and compound with high pKa will give you strong base.

Dan

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Re: pKb of strong bases
« Reply #2 on: December 03, 2011, 02:33:17 AM »

Yeah, the pKb of a base is directly related to the pKa of its conjugate acid, you can calculate it using Kw for aqueous solutions.

For B- + H3O+ ::equil:: BH + H2O

pKa(BH) + pKb(B-) = 14

By the way, take care with amines as searching for the pKa of an amine will almost always give you the pKa of the conjugate acid (9-11). If you search for the pKa of ammonia, for example, you'll probably be led to believe it's around 9.25 - but this is the pKa of the ammonium ion, not ammonia. The pKa of ammonia itself is around 35-40 I think.
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CrimpJiggler

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Re: pKb of strong bases
« Reply #3 on: December 03, 2011, 05:21:01 PM »

Yeah I've been thinking about bases conjugate acids. I notice that the amide ions conjugate acid is ammonia. According to the Evans pKa tables, ammonia has a pKa of 38 so I'm guessing metal amides are insanely strong bases. I had read that metal methoxides are very strong bases but their conjugate acid is methanol so they can't be all that strong. Hydroxides conjugate base (water) has a pKa of 15.7. Methanol is 15.54. Does that mean sodium hydroxide is a stronger base than sodium methoxide? t-butane has a pKa of 53 so I see why t-butyl lithium is a good base.
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