[AlbertoA]

let's consider the (Me)2NH3+ pKa=10.7
piperidineH+ pKa =11.2
pyrrolidineH+ pKa= 11.3

although the 3 amines are secondary, the pyrrolidine is more basic, it is a five member ring, so, I think that the angle strain is a factor but I'm not shure how that applies to the acyclic amine

[fledarmus]

Diethylamine is actually a better comparator than dimethylamine - most of the difference between dimethyl amine and either of the other two can be accounted for by the difference in electronics between a methyl group and a higher chain alkane. My table shows 11.02 for diethylamine, 11.13 for piperidine, and 11.27 for pyrrolidine. (10.68 for dimethylamine). I believe the remaining differences can be accounted for by the difference in axial interactions in the two rings for lone pairs and protons.