The Bouncer Analogy for Solvation

Imagine the nitrogen atom in an amine is like a bouncer at a nightclub. The bouncer's job is to "grab" a proton ($H^{+}$) and hold it. The better the bouncer can grab and hold the proton, the stronger the base.

Now, when the bouncer grabs the proton, he immediately needs backup from friends (water molecules) who surround him and stabilise the situation. These "water friends" are the solvation shell.

• Primary amine (1 H on N, 1 alkyl group): Bouncer has a bit of muscle (+I effect from one alkyl group) and his friends can easily reach him to help. Good base.
• Secondary amine (2 alkyl groups): More muscular bouncer. Friends can still reach him well. Best base in water.
• Tertiary amine (3 alkyl groups): Very muscular, but now THREE big coats (alkyl groups) are in the way. His water friends can't get close to help stabilise the grab. So even though he's strong, his "grab" doesn't last long — poor base in water.

The Aniline Exception: Now imagine the bouncer isn't even in the nightclub — he's been sent to guard the back door (the benzene ring) by resonance. His lone pair is tied up doing another job (stabilising the ring). So when a proton comes along, he's barely there to grab it. Very weak base.

The one-sentence takeaway: In water, secondary amine wins because it has both muscle (+I effect) AND backup (solvation); tertiary loses its backup due to steric bulk blocking water.