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?
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$\begingroup$ Propyl has a little stronger inductive effect, but makes also some steric hinderence for protontion. $\endgroup$– MithoronFeb 1, 2015 at 16:56
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2 Answers
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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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$\begingroup$ This differences are ridiculous! Accuracy of such measurements (esp over history and whodunit) is less than 0.1 digits! $\endgroup$– GeorgFeb 2, 2015 at 11:32
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It does not make much of a difference. Both are short chain primary alkyl amines, so chemically much alike ($\ce{pKa} \approx 10.7$ due to the $\ce{-NH2}$ group). In what context do you want to know the answer to this question? That could help me direct my answer better.
