Which base is stronger methylamine or propylamine? I know that it depends on the strength of inductive effect of alkyl group. Am I right? But how can I determine the difference between methyl and propyl substituent?
Answer
Different sources quote different values.
Wikipedia: Methylamine $\mathrm{pK_a~= 10.64}$ and Propylamine $\mathrm{pK_a~= 10.71}$
Pubchem: Methylamine $\mathrm{pK_a~= 10.6}$ and Propylamine $\mathrm{pK_a~= 10.71}$
$\mathrm{pK_a}$ is also temperature dependent and this effect may be larger than the difference in the inductive effect between the two amines in question.
http://www.zirchrom.com/organic.htm: Methylamine $\mathrm{pK_a~= 10.63~, 25^\circ C}$ and Propylamine $\mathrm{pK_a~= 10.60~, 20^\circ C}$
Overall there seems to be a consensus that propylamine is slightly more basic than methylamine at the same temperature but there is very little difference. Theoretically this can be explained by the slightly greater inductive effect of the propyl group versus the methyl group. In an experimental context I suspect that there is virtually no difference between the behaviours of the two amines.
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