It is known that ethyl has a stronger inductive effect than methyl. Its positive inductive effect is greater than methyl.

Then why does ethylamine has a $\mathrm{p}K_\mathrm{b}$ of $3.35$ and methylamine has the $\mathrm{p}K_\mathrm{b}$ of $3.34$ ($\mathrm{p}K_\mathrm{b}$ values in water) Shouldn't the $\mathrm{p}K_\mathrm{b}$ of ethylamine be lower than methylamine?


Ethylamine has a pKb slightly greater than that of methylamine.

As you are saying

It is known that ethyl is better than methyl for inductive effect. It's positive inductive effect is greater than methyl.

is absolutely true. But the basicity in aqueous medium is not only determined by +I effect but also solvation of substituted ammonium cation.

Even though +I effect in ethylamine is more than that in methylamine, ethylamine is more bulkier and sterically hinders the formation of intermolecular H−bonds with water.

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The difference of methyl’s and ethyl’s inductive effects is very small.

Likewise, the difference in $\mathrm{p}K_\mathrm{b}$ values is very small.

Both are in the magnitude of practically identical or within experimenal error. As such, be happy that the theory holds true up to the point that the values are similar and don’t expect it to explain every last digit behind the decimal dot.

  • $\begingroup$ But i believe that in aqueous medium we'll consider the h-bonding with water $\endgroup$ – Vishnu JK Dec 25 '16 at 14:43

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