# How does base strength vary for piperidine, pyridine, piperazine, and morpholine?

Apr 23, 2017

You want to order them in terms of basic strength?

#### Explanation:

Well, at a (hopefully informed!) guess, base strength should increase in the order:

$\text{pyridine"<"piperidine"~="morpholine"<"piperazine}$

Now we should be able to find $p {K}_{a}$ for the conjugate acids of these nitrogen bases:

$p {K}_{a} , \left({C}_{5} {H}_{5} N {H}^{+}\right) = 5.25$

$p {K}_{a} , \left({C}_{4} {H}_{8} O N {H}^{+}\right) = 8.34$

$p {K}_{a} , \left({C}_{4} {H}_{11} {N}_{2}^{+}\right) = 9.8$

$p {K}_{a} , \left({C}_{5} {H}_{11} N {H}^{+}\right) = 11.22$

Now the higher the $p {K}_{a}$ of the acid (the ammonium salt), the STRONGER the conjugate base, and here the conjugate base is the parent amine.

And the order of reactivity in the carbonyls? Well you have got an aldehyde, versus 2 ketones. It is well-known that aldehydes are much more reactive than the corresponding ketones, by reason of steric hindrance and access to the carbonyl centre.