Sciencemadness Discussion Board

Effect of impure reagents on reactions?

Refinery - 23-6-2020 at 12:33

This is a huge generalization, but what is the effect on using impure reagents for synthesis?

Obvious are side effects, and through that, yield losses, especially if reagents are not compensated for losses, but also side reactions could occur that could even hinder the main reaction. A dangerous side effect could be if some side product react with one of the reagents violently. The impurities could also cause cumulative losses in yields?

Probably one of the major fields where impure reagents are used is in clandestine chemistry, where isolation and purification steps are often skipped due to savings in time and equipment, and looking at the results, the reactions can still be carried through for the most part. Industry does also a lot of optimizing between purity and efficiency, finding some sort of compromise. Compositions of fuel and even bulk steel can have quite a large tolerances.

I have also seen a compromise where multi-step synthesis is carried out so that the leading steps involving cheap feedstock are done with maximum efficiency, usually using the resulting batch directly for second step, and only the end product resulting from these steps is processed with effort to yield the maximum purity.

Some reactions produce a lot of impurities and at least isolating the wanted component is essential by the most efficient method suitable, which can be as simple as filtering and drying, but possibly the result can be pure enough to be used as is. I've seen many legitimate sources cite that a resulting reagent not needing further purification for use.

Some reactions are carried out in sole purpose of obtaining the wanted substance, for example simply for wanting to have it on your shelf, and in this instance the time and yield losses are secondary next to obtaining the purest form possible.

So, how important it is to get highly pure reagent for synthesis use?

mackolol - 23-6-2020 at 13:15

I can tell you a nice example. Of course it's more of a curiosity. I'll copy this from wikipedia:

"In 1976, a 23-year-old graduate student in chemistry named Barry Kidston was searching for a way to make a legal recreational drug. Having read the paper by Ziering and Lee, he deduced that he could make a drug with pethidine's effects without its legal restrictions, since desmethylprodine is a different molecule and had never been addressed by law. Kidston successfully synthesized and used desmethylprodine for several months, after which he suddenly came down with the symptoms of Parkinson's disease and was hospitalized. Physicians were perplexed, since Parkinson's disease would be a great rarity in someone so young, but L-dopa, the standard drug for Parkinson's, relieved his symptoms. L-dopa is a precursor for dopamine, the neurotransmitter whose lack produces Parkinson's symptoms. It was later found that his development of Parkinson's was due to a common impurity in the synthesis of MPPP called MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), a neurotoxin that specifically targets dopamine producing neurons."
https://en.wikipedia.org/wiki/Desmethylprodine

morganbw - 23-6-2020 at 13:24

@mackolol
gave you a very good example.
If you are going to ingest, please research what bad cooties could come from your synth.

Refinery - 23-6-2020 at 13:53

That's a good point. Substances meant for ingestion should be paid extra caution, although this does not seem to be of priority in recreational use. I personally do not ingest anything, except food I make in kitchen.

My priority was more on reaction yields and stability.

Sulaiman - 23-6-2020 at 16:42

Almost every reaction that an amateur chemist is likely to perform has been done before, many times, and documented.
If there are any special requirements for purity, or removal of specific common impurities, it will be documented.

Purification / work up by filtering, distillation, crystalisation, solvent extraction ...
is a common requirement as co/by-products are produced in most chemical reactions,
checking purity of results by b.p. or m.p. is common.

If you create your own experiment(s) then it is often very difficult to analyze your results/products,
as evidenced by the many questions similar to "WTF did I just make ?"

It would be nice to have access to 99.99999% pure reactants,
but for me, half of the fun is in the unexpected :P

karlos³ - 23-6-2020 at 22:09

Hey wait, what makes you think that clandestine chemists use less pure reagents? :o
First, clandestine chemists are well connected among each other, I would even bet most of them are able to access some things you would be envious about.
Second, the things they have to make themselves... reagent purification is something everyone can and should do, and they really put effort into this.
Otherwise you throw willingly product away, with the resulting less product also being less pure initially and requiring more work for purification.
Thats a stupid calculation not to purify the precursor, and then instead using twice as long to purify half as much product? :o

I don't know what people you have in mind, but they do not belong to us.
Maybe you talk about cooks or something like that, or the people who run mafia scale operations, whatever.
But those are not chemists and I refuse to have me and my friends put into the same category as such people.

And also, if someone is able to run a reaction sequence without intermediate purifications, AND all that without resulting in like two third lesser yield, than this has not much to do with the purity of his precursors(and if, then they must be pretty pure), if someone is able to do this, with shortcuts and all and ultimately ending up with a yield as good as the normal length way to do this....
Then this is someone who mastered the art of organic chemistry.
I can guarantee you that 8-9 of 10 people wouldn't be able to make use of well thought out shortcuts without causing more problems than its worth.
Someone who can do this, knows his reactions very well and knows what he is doing in great detail.

[Edited on 24-6-2020 by karlos³]

dawt - 23-6-2020 at 22:37

In some cases impurities also improve the reaction:
Quote:

At first, we attempted the decarboxylation according to the peroxide method using cyclohexanol as the solvent. However, it was difficult to establish a general method because both the reaction time and amount of catalyst required varied to complete the decarboxylation even under the same scale reaction. For instance, when 4-hydroxy-L-proline was treated with peroxide catalyst (tert-butyl peroxide or tetralin hydroperoxide) in cyclohexanol under reflux, the peroxide consumed ranged between 3% and 10% and decarboxylation took between 7 h and 28 h to complete. This variation was found to greatly depend on the quality of the cyclohexanol used. Strangely, the decarboxylation rate gradually decreased in proportion to increase in the purity of the cyclohexanol. For example, in the case of 99.3% purity cyclohexanol, it took 28 h reaction time and the addition of 10% peroxide. However, with 98.1% purity cyclohexanol, the reaction was completed in 7 h by the addition of only 3% peroxide. These results suggest that there was some effective substance contained in the cyclohexanol. The most effective way to determine this substance was by the use of its UV spectrum. cyclohexanol of 99% purity has no peak between 200 to 320 nm, but 98% purity cyclohexanol showed a strong peak at 225 nm. Only 2-cyclohexen-1-one has a peak at 225 nm within some impurities contained in cyclohexanol. Quantitative analysis by gas chromatography showed that it makes up less than 0.1% in 99% purity cyclohexanol but more than 0.3% in 98% purity cyclohexanol.

https://erowid.org/archive/rhodium/chemistry/trp.decarbox.en...

outer_limits - 23-6-2020 at 23:03

Quote: Originally posted by karlos³  
Hey wait, what makes you think that clandestine chemists use less pure reagents? :o


There are chemists and "chemists". The latter are people without the knowledge, skills and experience especially when it comes to extraction and purification.

Tsjerk - 23-6-2020 at 23:46

Quote: Originally posted by morganbw  
@mackolol
gave you a very good example.
If you are going to ingest, please research what bad cooties could come from your synth.


He ran the reaction too hot which caused the dehydration of a tertiary alcohol intermediate.

Refinery - 24-6-2020 at 00:18

Quote: Originally posted by karlos³  
Hey wait, what makes you think that clandestine chemists use less pure reagents? :o


This was said more out of respect for being able to pull off complex processes from impure junk.