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When comparing the mesomeric effects of primary vs secondary and tertiary amines, we see that primary amines have a more available lone pair because of less electronegativity of the Nitrogen atom and are therefore better donors.

If this is the case, why are secondary and tertiary amines more basic? Shouldn't the same logic of the lone pair not being as available hold true for basicity too?

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Your understanding about lone pairs is wrong. All amines(primary, secondary and tertiary) have a single lone pair.

The difference in basicity is a mere effect of the inductive effect and steric effect.

The Inductive effect from the alkyl group increases its basicity(the larger/more branched the alkyl group is, the better). But bigger alkyl groups cause a lot of steric crowding of the lone pair, making it unavailable for attack. Striking a balance between the 2 results in the trend of basicity of the amine.

Examples:

tri-methylamine > di-methylamine > methylamine (Since Me group is rather small)

di-ethylamine > tri-ethylamine > ethyl amine (Since Et group is larger than Me, 3 Ethyl groups result in a higher steric effect than 3 Methyl groups)

Now considering a rather large alkyl group.

di-tert-butylamine > tert-butylamine > tri-tert-butylamine

here, tert-butyl group is rather large causing a lot of steric hinderance. hence in the case of tri-tert-butylamine, it causes the lone pair to not be available for attack, making it the least basic.

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  • $\begingroup$ Sorry, I see how my question made it seem like I was implying certain amines have more lone pairs. I know that primary amines show a greater mesomeric effect, but secondary and tertiary amines are more basic. Since both properties involve the donation of a lone pair, I was wondering why there was no correlation between the two. $\endgroup$
    – Aarchaeus
    Commented Nov 8 at 14:08
  • $\begingroup$ @Aarchaeus I fail to understand the relation between the mesomeric effect and BASICITY OF THE GROUP. while a higher positive mesomeric effect increases the basicity of the compound when bonded to a conjugated system, it doesnt affect the basicity of the group itself. $\endgroup$
    – Dev Not Taken
    Commented Nov 8 at 16:10
  • $\begingroup$ @Aarchaeus While on the other hand, If we go by the inductive effect, the alkyl groups(EDG's) attached to the nitrogen exert a negative inductive effect, pushing electron density towards the nitrogen, thus increasing the availability of its lone pair which increases the basicity of the compound $\endgroup$
    – Dev Not Taken
    Commented Nov 8 at 16:13
  • $\begingroup$ The presence of alkyl groups on nitrogen also increases the bond angle between nitrogen and the alkyl groups, increasing s character, and therefore also electronegativity. The increased electronegativity is said to decrease the mesomeric effect of the nitrogen group. At least that's the reason I've come across. I'm wondering why the increased electronegativity doesn't affect the basicity as well. $\endgroup$
    – Aarchaeus
    Commented Nov 10 at 3:32
  • $\begingroup$ The difference in basicity is a mere effect of the inductive effect and steric effect. I think this "mere effect" is at the core of the question and you are not making this any easier to understand. There also recently was a study, that says that the inductive effect of an alkyl group is withdrawing compared to a hydrogen ligand. || Also, you nomenclature is slightly off, for example it should probably be tris(t-butyl)amine (as a common name). $\endgroup$
    – Martin - マーチン ♦
    Commented 2 days ago

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