Amine Basicity

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cloak25

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Why is a tertiary amine less basic than a secondary and primary amine? Methyl groups are electron-donating so wouldn't the more substituted amine be more basic? according to BR, the relative basicity of methyl amines is : R2-NH > R-NH > R3-N > NH3 😕
 
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cloak25 said:
Why is a tertiary amine less basic than a secondary and primary amine? Methyl groups are electron-donating so wouldn't the more substituted amine be more basic? according to BR, the relative basicity of methyl amines is : R2-NH > R-NH > R3-N > NH3 😕
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I think the general take home message is that any alkyl amine is going to be more basic than NH3 due to the inductive effect. I think the relative basicity between alkyl amines is fairly complicated and beyond the scope of a general orgo class. I don't think the MCAT will test you on it.
 
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Ok, you are right. EWG decrease the basicity of an amine, while EDGs decrease the basicity. BUT steric hindrance created by bulky functional groups tends to hinder the ability of an amine to donate its lone pair, and that decreases its basicity. Darn it, I got this wrong on the AAMC 3...so sad..

Here's 'proof' of my source :EK. B/c it's always good to have proof.

KBfia.png
 
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cloak25 said:
Why is a tertiary amine less basic than a secondary and primary amine? Methyl groups are electron-donating so wouldn't the more substituted amine be more basic? according to BR, the relative basicity of methyl amines is : R2-NH > R-NH > R3-N > NH3 😕
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because it has a harder time finding the proton due to steric hinderance.
 
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chiddler said:
i don't. can you please explain what you mean?
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This is from another forum discussing the topic:



The basicity trends of amines are most easily explained by looking at the acidity of ammonium-ions, which (in solution) follows the trend given below:

NH4+ > (CH3)3NH+ > CH3NH3+ > (CH3)2NH2+

That is NH4+-ions are the most acidic (least basic) ones, whereas (CH3)2NH2+-ions are the least acidic (most basic) ones.

This trend can be explained by two conflicting effects - polarizability and solvation.

If you look at gas phase acidities the trend is as follows:

NH4+ > CH3NH3+ > (CH3)2NH2+ > (CH3)3NH+

Which is what we would normally expect since more methyl-groups donates more electrons spreading out the positive charge and therefore stabilizing the ammonium-ion (making it less acidic, and the conjugate base more basic).
On the other hand solvation effects leads to the following trend:

(CH3)3NH+ > (CH3)2NH2+ > CH3NH3+ > NH4+

This trend can be explained by the size of the ammonium-ion - the larger, the least well solvated, therefore least stable and hence more acidic (less basic).
A combination of these two trends gives the acidity (and therefore basicity) trend of ammonium-ions (amines) in solution.
 
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shoehornlettuce said:
This is from another forum discussing the topic:



The basicity trends of amines are most easily explained by looking at the acidity of ammonium-ions, which (in solution) follows the trend given below:

NH4+ > (CH3)3NH+ > CH3NH3+ > (CH3)2NH2+

That is NH4+-ions are the most acidic (least basic) ones, whereas (CH3)2NH2+-ions are the least acidic (most basic) ones.

This trend can be explained by two conflicting effects - polarizability and solvation.

If you look at gas phase acidities the trend is as follows:

NH4+ > CH3NH3+ > (CH3)2NH2+ > (CH3)3NH+

Which is what we would normally expect since more methyl-groups donates more electrons spreading out the positive charge and therefore stabilizing the ammonium-ion (making it less acidic, and the conjugate base more basic).
On the other hand solvation effects leads to the following trend:

(CH3)3NH+ > (CH3)2NH2+ > CH3NH3+ > NH4+

This trend can be explained by the size of the ammonium-ion - the larger, the least well solvated, therefore least stable and hence more acidic (less basic).
A combination of these two trends gives the acidity (and therefore basicity) trend of ammonium-ions (amines) in solution.
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ok you're right, i'll concede that sterics is not the only thing to consider.

but i think it is far simpler to consider sterics than polarizability and solvent interaction.

for example, lets ask the question: why does polarizability change? because the molecule changes size! (steric hinderance) why does this trend reverse in gas phase? because steric hinderance is no longer an issue!
 
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