Basicity of amines

[brood910]

So I know that secondary amine is more basic than tertiary because excessive substitutes on tertiary amine hinder its ability to donate electrons.

But, is tertiary amine less basic than primary and ammonia as well?

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[Czarcasm]

A simple google search would of saved you from posting:

The basicity of amines is often discussed indirectly in terms of the acidity of their respective conjugate acids. Recall that the conjugate acid of a weak base (e.g. like water) is a strong acid (like hydronium ion), while the conjugate acid of a strong base (like hydroxide ion) is a weak acid (like water). The concept of pKa has already been developed as a measure of the acidity of Bronsted acids, and we will also see that a corresponding concept, pKb can be used as a measure of the basicity of bases and that these two quantities are very closely related. Consider the acid dissociation, in dilute aqueous solution, of ammonia and a representative primary, secondary, and tertiary amine:

qNote that the strongest acid (least positive pKa) is ammonia. This means that ammonia is the weakest base of the four bases.


qWe can easily understand this because alkyl groups are electron donating (EDG), so they stabilize the positively charge ammonium ions, i.e., the methyl ammonium ion is more stable than the parent ammonium ion because the alkyl group stabilizes the positive charge on the attached nitrogen atom.


qNote also that the alkyl stabilizing effect is purely inductive! [By looking at possible resonance structures, see if you can see why there is no hyperconjugative resonance stabilization by the alkyl group.


qNotice that the second alkyl group, in the dimethylammonium ion, has only a very slight effect, while the third group (in the trimethylammonium ion) causes an increase in acidity (decrease in basicity) relative to the dimethylammonium ion. Of course, the trimethylammonium ion is still less acidic than ammonia.


qAll of the amines are more basic than ammonia, but primary and secondary amines are the most basic.


qThe effect of the third alkyl group is another instance of steric inhibition of solvation. The presence of three alkyl groups sharply diminishes the ability of the solvent to stabilize the corresponding ammonium ion, thus causing a reversal in the tendency of the alkyl groups to decrease acidity and increase basicity.

Source: http://research.cm.utexas.edu/nbauld/CHAPTER 21.htm

 

[E pur si muove]

I'm going to try and summarize the topic and let me know if you need more.

There are 3 main factors that affect the basicity of amines;

1. Resonance - The less resonance stabilization, the more basic
   
   
2. Electron donating/withdrawing groups - Electron donating groups make nitrogen more electron rich, therefore more unstable (more basic). Electron withdrawing do the opposite.
   Examples of donors: NH2, OH, OR, CH3, NR2
   Examples of withdrawing groups: NO2, NR3, NH3, CN
   
   If you have a weird group, you can determine its affect my looking at its resonance.
   Remember the (-) charge wants to be on the most electronegative atoms (and more polarizable atoms). So if there are predominately resonance structures creating a dipole with the (-) charge farther from the amine and the (+) charges closer to the amine, then the group is electron withdrawing (less basic).
   
   
3. Orbitals - More s-character the orbital, the more stable the nitrogen lone pair will be (and therefore, less basic)
   Ex. sp3 > sp2 > sp (sp3 is more basic than sp2 and sp)

It all depends on the actually molecule. You can't just say secondary is this much more basic without a structure. If you have tertiary amine solely connected to a carbon chain then you can assume NH3 will be a weaker base. When you were talking about the substitutes, and strong withdrawing group will drastically decrease the basicity. On the other hand steric hindrance is not really taken into account when determining the basicity. The relative amount of hindrance would be taken into account for nucleophilicity.

I wrote this a 12:00 AM..... so please say if you think anything I wrote is incorrect or misleading.