Storing alkynes, aldehydes and acetals: what actually degrades and how fast

Most of the degradation that ruins a building block happens in the container, not in the reaction. The three groups that cause the most trouble in this catalogue each fail in a different way, which means each needs a different handling habit.

Alkynes: the coupled diyne

Terminal alkynes oxidatively homocouple on standing, especially with trace copper or palladium in the material and air in the headspace. The product is a diyne that is chemically similar to the starting material, so it is easy to miss on a quick check and hard to remove once it is there.

The habit that prevents most of it: keep the container full, keep it closed, keep it dark, and check for the diyne before a step where it would compete for catalyst.

Aldehydes: the acid, and then the ester

Aldehydes oxidise to the corresponding acid in air, and the acid then catalyses further condensation. A short-chain aldehyde can go from clear to cloudy in weeks. There is no way to reverse it; the only reliable approach is to buy the quantity that will be used, keep the headspace small, and store cold.

This is why aldehydes are carried one step and consumed rather than held as stock.

Acetals: hydrolysis is reversible, which is both the point and the problem

An acetal is a masked carbonyl: it survives base and hydride conditions, and it opens under acid. That is exactly why it is useful in the middle of a sequence. The same property means a wet acetal slowly equilibrates with the free aldehyde, and the free aldehyde then oxidises. A lot that smells different from the last one has probably already started.

The one habit

Write the opening date on the container and re-check the certificate before the second use. A lot that has been open for six months is a different material from the one that was released, and the certificate describes the released material. If the entry is hygroscopic, add a water check to the second use rather than assuming.

Storage lines for each functional group are stated on the entry itself, and the safety data sheet takes precedence over any general rule on this page.