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Among pyridine and 2,6-di-tert-butylpyridine, which one would be more basic?

My guess was that since two bulky groups are substituted at ortho position in the latter, it will experience steric repulsions and hence it's lone pairs will easily be donated.

However, that doesn't seem to be the case.

Can someone please explain why steric factor does not lead to easy donation of lone pairs in this case?

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1 Answer 1

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Among pyridine and 2,6-di-tert-butylpyridine, which one would be more basic?

As Mithoron pointed out in the comment section, it's more fundamental than pretty obvious thing that large steric groups simply block access to atoms (and lone pairs). Thus it is fact that 2,6-di-tert-butylpyridine is a weaker base than pyridine because access to its lone pair is blocked by steric hindrance (Ref.1). According to Ref.1, the $\mathrm{p}K_\mathrm{a}$ of conjugate acid of 2,6-di-tert-butylpyridine in 50% aqueous ethanol at $\pu{25 ^\circ C}$ is 3.58 while that of conjugate acid of pyridine is 4.38. Brown and Kanner attributed this unusually weaker basicity of 2,6-di-tert-butylpyridine to the steric strain involving the bound proton (Ref.1).

Keep in mind that the alkyl (electron donating) groups at appropriate positions would increase the basicity of pyridine by inductive effect. For example, the $\mathrm{p}K_\mathrm{a}$ of conjugate acid of 2,6-di-metylpyridine is 5.77 while that of conjugate acid of 2,6-di-isopropylpyridine is 5.34. However, the severe steric hindrance coursed by replacing the hydrogen atom in each of isopropyl group by a methyl group reversed this positive effect to other direction (Ref.1).

It is recommended that OP should read Ref.2 for additional information.

References:

  1. Herbert C. Brown and Bernard Kanner, "2,6-Di-butylpyridine-An unusual pyridine base," J. Am. Chem. Soc. 1953, 75(15), 3865-3865 (DOI: https://doi.org/10.1021/ja01111a530).
  2. Robert L. Benoit, Monique Fréchette, and Diane Lefebvre, "2,6-Di-tert-butylpyridine: an unusually weak base in dimethylsulfoxide," Canadian Journal of Chemistry 1988, 66(5), 1159-1162 (DOI: https://doi.org/10.1139/v88-190).
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