difference in basicity for amines in aqueous vs. gaseous phase

[SaintJude]

Kaplan is telling me that the order of basicity in ammonia is different in gaseous phases vs. aqueous phases due to hydrogen bonding.

So in gaseous phases it's the well known: 3' > 2' > 1'> NH3. Just so we're on the same page, this is due to the inductive effect of electron donating alkyl groups--they destabilize the lone pair on nitrogen atom, making nitrogen more basic.

But in aqueous phase, the order is slightly different: 2'> 1'> 3' > NH3.

Kaplan says the positive charge on the quaternary ammonium ion formed after amine is protonated in the aqueous solution is stabilized via hydrogen bonding. The more stable the conjugate acid, the compound is more basic. Following this logic, Kaplan concedes that we would expect ammonia to be most basic. But in real world, it's not exactly like that, but they don't say why.

Did you guys just memorize the difference?

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[a_zed24]

Tertiary amines are weak bases due to the steric bulk of its three alkyl groups , which prevent the attack to the H+, and even if protonation occurs, remeber that there is no H left for a hydrogen-bond in the 3° amine molecule: no hydrogen-bond, no stabilization of the produced ammonium ion.

Ammonia is less basic than the other amines because the atoms surrounding the N (which after protonation will bear a postive-one formal charge) are all hydrogens and they do very little to stabilize its +ve charge...therefore it'd be less stable than the ammonium ions produced by the other amines

 

[aldol16]

I wouldn't worry about memorizing small details like this. Just understand how the logic generally works and you'll be fine. You won't be expected to know deviations from trends when scientists can't adequately explain them in any sort of predictive manner.