Q.Which of the following has the highest basic strength ?
a. Trimethylamine b. Methylamine c. Ammonia d. Dimethylamine

Step 1. Recall each option's pKb from Table 13.4. Methylamine (a primary amine, close to ethanamine's listed 3.29) is around pKb 3.4; ammonia is pKb 4.75; dimethylamine (N-methylmethanamine) is pKb 3.27; trimethylamine (N,N-dimethylmethanamine) is pKb 4.22.

Step 2. Recall the rule linking pKb to basic strength. A SMALLER pKb corresponds to a STRONGER base (section 13.5).

Step 3. Compare all four. Of these four values (roughly 3.4, 4.75, 3.27, 4.22), dimethylamine's pKb (3.27) is actually the smallest, making IT the strongest base among the four -- but dimethylamine is not one of the four given options here. Restricting strictly to the four LISTED options -- trimethylamine (4.22), methylamine (~3.4), ammonia (4.75), dimethylamine (3.27, not listed as an option in this particular question, whose choices are only trimethylamine/methylamine/ammonia/dimethylamine as printed) -- re-reading the actual f …

Reading each option's pKb value directly from Table 13.4 and picking the smallest (strongest- …

• Assuming trimethylamine, having the MOST alkyl groups, must be the strongest base purely by the +I inductive-effect argument, without checking Table 13.4's actual OBSERVED values -- section 13.5.1 explicitly warns that the tertiary amine is in fact the WEAKEST of the three aliphatic amine classes (secondary is strongest), because of poor solvation of its conjugate acid …