Why is phenylamine a weaker organic base than ethylamine?

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Phenylamine contains a phenyl group attached to the amine. Phenyl groups have a delocalised system of electrons as a result of p orbitals from the 6 adjacent carbon atoms overlapping and "merging" together to form a 9very stable) delocalised pi bond with 6 electrons. The lone pair of the nitrogen atom in the amine group is "drawn in" towards this delocalisation and this interaction makes the lone pair less available to bond to an incoming H+ ion. In comparison, ethylamine has no delocalisation in the molecule. In fact, there is actually a positive inductive effect contributed from the alkyl groups of the ethylamine which pushes the lone pair away from the N atom and makes the lone pair more available to bond to an H+ ion. The availability of a lone pair of electrons on a base determines its strength as it is these electrons that will "mop up" H+ ions in solution and hence increase pH towards more alkaline conditions. Therefore, phenylamine is a weaker base than ethylamine because its lone pair is less available.

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