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Which is the stronger Lewis base, and why?

I'm thinking that the answer is "it depends." With regard to the hydrogen proton, which hydroxide ion is obviously the stronger Lewis base. However, with regard to other, more complex Lewis bases, the distinction may not be so clear, as hydroxide ion has several lone pairs (three) and these can weaken the stability of Lewis adducts through lone pair/"bonding pair" (the pair that forms the coordinate covalent bond) repulsions. Ammonia only has one lone pair so there isn't an issue of lone pair/"bonding pair" repulsion.

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  • $\begingroup$ I agree in principle with your premise that "it depends." There's a slew of complicated effects at work here, some related to HSAB and FMO theory (the importance of frontier orbital energies vs. Coulombic interactions), some to solvent effects (reactivity in the gas phase, for example, sometimes differs drastically), and other factors. The nature of the electrophile in question is crucial here: some molecules are highly reactive as nucleophiles with certain substrates, while being comparatively unreactive with others. $\endgroup$ – Greg E. Aug 20 '14 at 1:41
  • $\begingroup$ I think you should also probably distinguish between kinetic and thermodynamic basicity/nucleophilicity. Reaction rates doesn't necessarily correlate well with equilbrium constants in all cases. $\endgroup$ – Greg E. Aug 20 '14 at 1:44
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With respect to Brønsted basicity, definitely hydroxide is the stronger base. With respect to Lewis basicity, I think the same scenario holds. As you said, hydroxide has three lone pairs, whereas ammonia has only one lone pair. As far as the energetics of these situations are concerned, the magnitude of lone pair to lone pair repulsion is far greater than that of lone pair to bond repulsion. Thus, it would make sense for hydroxide to grab the proton instead of ammonia.

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$pKa$ of conjugate acid($NH_4^+$) of ammonia($NH_3$) is $9.9$ and of conjugate acid($H_2O$) of hydroxide($OH^-$) is $15.7$.

So, hydroxide is a strong base.

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  • $\begingroup$ I'm talking about Lewis basicity not just Bronsted basicity. $\endgroup$ – Dissenter Aug 13 '14 at 18:55
  • $\begingroup$ @Dissenter I just treat all acids and bases equally, what's the difference between them? $\endgroup$ – RE60K Aug 13 '14 at 19:00
  • $\begingroup$ @Dissenter the molecule doesn't diffrentiate the two, so why would we. I actually know the difference $\endgroup$ – RE60K Aug 15 '14 at 3:45

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