[Rutherford]

Why is the left compound more basic than the right one?

[clarkstill]

Must be sterics: in quinuclidine the alkyl groups are tied back so the lone pair is more available, so the equilibrium lies slightly more towards the conjugate acid, whereas in triethylamine the alkyl chains have more significant and deleterious steric interactions with the proton...

[Rutherford]

Okay, so the difference is very small. Thanks.

[Rutherford]

Why is the left amine more reactive towards MeI than the right one?

[Rutherford]

Does anyone have any reasonable explanation for this seemingly unexpected result?

[kriggy]

I think the alkyl groups increase the electron density on the nitrogen in the left compound. In the 2nd compound, the inductive efects are not that strong because of electronegative nitrogen.
Just guess.
What do you mean by more reactive? How much?

[Rutherford]

Still, there are two nitrogens in the second compound, that's what confuses me. The inductive effect can't be a match to that.

Don't know, I was only asked to put these in order of decreasing reactivity towards MeI (some other amine/s was/were present, too).

[kriggy]

Maybe the 1st methyl alkylating the nitrogen reduce the reactivity of the 2nd one because of positively charged nitrogen?
Is there a possibility of the results being wrong?

[Rutherford]

That's a very satisfying answer. Combined with the inductive effect you proposed earlier, it is good.
I think that it shouldn't be wrong, as it is from India National Olympiad and they usually correct the incorrect stuff.

[clarkstill]

It can only be inductive effects, and I think maybe you're underestimating their strength Rad.  Given that Et3N is ~1.5 pKa units more basic than NH3, each alkyl group contributes ~0.5 pKa units, which corresponds to a 5-fold shift in the equilibrium.  So inductive effects are clearly able to overcome the influence of simply having a second nitrogen (which should only double the rate, to a first approximation).

I realise I am using a thermodynamic metric to try to explain a kinetic behavior, but I think the inductive HOMO-raising effect of hyperconjugation must be noticeably greater in quinuclidine vs DABCO, since the sigma bonds are much more electron-deficient in DABCO.

Either that, or it's some weird solvent effect, in which case god help us.

[clarkstill]

But this wouldn't decrease the rate of the first methylation...

Maybe the 1st methyl alkylating the nitrogen reduce the reactivity of the 2nd one because of positively charged nitrogen?

[Rutherford]

I think that it is like this: The inductive effect explains the smaller reactiveness of the first nitrogen, then the positive charge (together with the inductive effect) explains the incredibly small reactiveness of the second nitrogen.