有机化学中碱性的比较，六氢吡啶和三甲胺谁的碱性大？为什么？

这个问题其实很纠结。
我查到的数据是 哌啶 pka=11.22。三甲胺 pka=9.87. 很明显哌啶的碱性大。
一般认为烷基是给电子集团，所以烷基取代越多，应该碱性越大。从以上的数据来看，这种观点不对。
我从论坛上查看了一下类似的问题。比较氨，甲胺，二甲胺，三甲胺的碱性大小。其实这类问题要考虑两个方面：极化性和溶剂化效应。我把原帖转过来，希望对你有所帮助：
The basicity trends of amines are most easily explained by looking at the acidity of ammonium-ions, which (in solution) follows the trend given below:

NH4+ > (CH3)3NH+ > CH3NH3+ > (CH3)2NH2+

That is NH4+-ions are the most acidic (least basic) ones, whereas (CH3)2NH2+-ions are the least acidic (most basic) ones.

This trend can be explained by two conflicting effects - polarizability and solvation.

If you look at gas phase acidities the trend is as follows:

NH4+ > CH3NH3+ > (CH3)2NH2+ > (CH3)3NH+

Which is what we would normally expect since more methyl-groups donates more electrons spreading out the positive charge and therefore stabilizing the ammonium-ion (making it less acidic, and the conjugate base more basic).
On the other hand solvation effects leads to the following trend:

(CH3)3NH+ > (CH3)2NH2+ > CH3NH3+ > NH4+

This trend can be explained by the size of the ammonium-ion - the larger, the least well solvated, therefore least stable and hence more acidic (less basic).
A combination of these two trends gives the acidity (and therefore basicity) trend of ammonium-ions (amines) in solution.

六氢吡啶=哌啶

碱性比较：哌啶>三甲胺

原因：胺的碱性比较很复杂，需要考虑的方面很多。一般精确的都是通过实验测量出来的值。比较的话有些规律。

1. 脂肪胺>氨>苯胺。

2. 二甲胺>甲胺>三甲胺。

3. ...

总之，不仅要考虑电子效益，还要考虑位阻效应以及溶剂化效应综合判断。