'A' Level Qn.
Methylamine is more basic than trimethylamine, less basic than
dimethylamine. Electron-donating group on the N makes the electrons
more available for protonation, so with an increase in the number
of electron-donating group, it makes the amine more basic. Instead,
trimethylamine is less basic than methylamine though it contains
more methyl (electron-donating) group.
I remembered it has something to do with trimethylamine not
being able to form hydrogen bonds with water thus the lower
basicity. Or is it due to steric hindrance?
Thanks. 
The stability of a conjugate acid (ie. when a base is
protonated), depends on stabilization by induction, resonance,
and solvation (hydration).
Recall that bond forming is exothermic (ie. products less energy
hence more stable than reactants; energy lost to surroundings), and
therefore favourable (enthalpic) intermolecular
interactions such as ion-dipole interactions, hydrogen bonds,
dipole-dipole interactions, all stabilize the species.
In the case of ammonium and alkyl ammonium cations (eg.
protonated conjugate acid forms of methylamine, dimethylamine
and trimethylamine), there is no resonance stabilization to speak
of.
Purely in terms of induction effects (ie. the electron donating
alkyl groups (in turn because C is more electronegative than H)
stabilizing the positive formal charge on the N atom (a Grp V
element with 0 lone pairs and 4 bond pairs) after protonation
of the ammonia or alkylamine base), the greatest stabilization
conferred by induction would be experienced in the tertiary alkyl
ammonium cation, followed by secondary alkyl ammonium
cation, followed by primary alkyl ammonium cation, and lastly
the ammonium cation.
Purely in terms of solvation (hydration) effects, specifically
hydrogen bonding with water molecules, the greatest stabilization
conferred by solvation would be experienced in the ammonium cation
(4 H-bonds), followed by primary alkyl ammonium cation (3
H-bonds), followed by secondary alkyl ammonium cation (2
H-bonds), and lastly the tertiary ammonium cation (1 H-bond).
(Note that hydrogen bonds rather than ion-dipole
interactions, exist for the protonated cation in aqueous
solution; the positive formal charged N atom is surrounded by
hydrogens and/or alkyl groups tetrahedrally, preventing effective
ion-dipole interactions with the polar water solvent
molecules.)
Acknowledgements & Recommendation : Graham Patrick's
"Organic Chemistry (Bios Instant Notes)" available from NUS co-op,
Clementi bookstore, Amazon.com, etc. - Google's Book Search snapshot of page 92 ("Solvation
effects").
Just as in real-life, it is often the (fascinating) case in
Chemistry, that we have to consider opposing patterns/trends (for
instance, solubility of Grp II compounds, boiling points of
hydrogen halides, etc), and apply our theoretical conceptual
understanding to explain observed experimental data.
In other words, based on experimental evidence, we know
that such-and-such principle outweighs such-and-such principle at
such-and-such a point. (A good case in point would be enthalpy vs
entropy in solubility of Grp II carbonates - theoretically,
enthalpy dictates carbonates become less soluble down Grp II;
theoretically, entropy dictates that carbonates become more soluble
down Grp II; based on experimental evidence, we know that enthalpy
effect outweighs entropy effect from Be to Sr, and entropy
effect outweighs enthalpy effect from Sr to Ba. Visit
Jim Clark's ChemGuide.co.uk's "Solubility of Grp II
compounds").
Chemistry (and all Sciences) teaches us that is
an overall complex balance of (under varying
circumstances, often) opposing principles/patterns/trends, and
for the wise with the eyes to see, the mind to comprehend and
the heart to feel; this may well serve as a ethical,
moral, philosophical and spiritual lesson for
humanity.
That in truth (or as Einstein would say, "relative leading edge
truth"), there is no "right" and "wrong" (ie. dogma), but
there is Ethics ("do unto others as you would like others to do
unto you") and there is CosmoEthics ("that which results in
the greatest and highest good for all beings involved and
ultimately for the entire universe").
