Purification of the Hünig Base

Question

My group uses a lot of Hünig base which needs to be dry and pure. The common procedure we use is to first distill from ninhydrin then from calcium chloride.

Apparently the problem is that Hünig base decomposes to give primary amines (which are removed by the ninhydrin). What I don't quite understand is why we need the second distillation (why can't we just distill from ninhydrin in the presence of a mild drying agent) but also why for triethyl amine we can just distill off calcium hydride (in told this drying agent causes Hünig base to decompose but I can't see how this would work given that the calcium hudride isn't a strong enough base to take a proton from the groups on the amine).

Answer 1

_{Ref: Purification of Laboratory Chemicals, 7 ed.}

In the first case, your labs 'common procedure' isn't the 'common procedure' that most people tend to follow. Generally, it should be carried out as follows

1. Distill the DIPEA from ninhydrin at atmospheric pressure. The ninhydrin reacts with any primary/secondary amines, removing these impurities from the DIPEA. This ninhydrin step liberates water, as it works by condensing the primary/secondary amines. Since the boiling point of DIPEA isn't that distinct from that of water, people tend not to try fractionally distilling the two away from one another. The DIPEA that comes across in this first distillation is free of primary/secondary amines, but contaminated with up to ~1% water (which is far too much to get away with using in moisture sensitive chemistry).

_{Ref: Chemistry LibreText}

2. Re-distil the 'wet' DIPEA from KOH, again at atmospheric pressure, which removes water and provides clean and dry base.

The reason why calcium hydride is often not used is because it is thought to promote the degradation of DIPEA by cleavage of the isopropyl groups (they can eliminate to give stable products. Certainly, distillation from calcium hydride is usually fine, but the DIPEA shouldn't be stored over it for prolonged periods.