/r/Chempros

Photograph via snooOG

A subreddit for chemistry professionals.

The chemistry subreddit is increasingly overrun with material that is not interesting or novel to working chemists. The content here will strive to be at the level of a working professional chemist or graduate student.

Posts seeking help on technical matters (synthesis issues, spectra interpretation, instrument advice, etc) are heartily encouraged.

Rules

  1. No memes.

  2. No garden variety, or undergrad level chemistry. Especially homework.

  3. Be nice, there's a real person on the other keyboard.

  4. When possible, cite your sources. Use DOI URL that way it can be put into scihub most easily.

  5. If it's a crystal post, it better be especially unique.

  6. Don't ask about getting into grad school or how to get a job. That's annoying and low content, and furthermore, there is plenty of info elsewhere on the net.

  7. No asking how to isolate psychoactive or medicinal products for personal consumption.

/r/Chempros

17,280 Subscribers

1 Comment
2024/06/22
14:08 UTC

1

Bruker Laser Offset?

Hi everyone,

We have a Bruker Invenio IR that I had to replace the laser on. For the first week following that l, the system was great. We are now receiving a laser offset error. Can anyone provide any insight for a remedy?

Thanks!

1 Comment
2024/06/22
14:07 UTC

13

DCM columns

Hi friends, my PI is looking to remove DCM from our lab with the upcoming ban in the US. The one sticky part we're running into is we have to run the occasional MeOH/DCM columns on amino acid derivatives (think similar to Fmoc-lysine, where the side chain is unprotected). Ideally we'd prefer to purify this by normal phase, but reverse phase is something we can look into. Does anyone have suggestions for alternative solvent systems?

43 Comments
2024/06/21
21:37 UTC

3

Purification of mCPBA

Does anyone know a method to purify mCPBA? Currently following the way in Purification of Organic Chemicals but it is not clear.

4 Comments
2024/06/21
19:23 UTC

5

Protons Coupled to two Carbons in HSQC

Hi would anyone happen to understand or know why in my HSQC I'm seeing correlation for both of these protons (part of a double bond) to two separate carbons? Perhaps my understanding of HSQC is flawed, but I was only aware that single bond correlations (J1) would be visible, which is why I've been stuck trying to interpret this...

Any input is appreciated.

https://preview.redd.it/q08jrj85ay7d1.png?width=1628&format=png&auto=webp&s=e486213e34b4944edc95f37c9847e4ef2e2c6f99

8 Comments
2024/06/21
16:20 UTC

3

KF issues

We're having problems with one of our coulometric KFs. The reagent had become very dark and it was failing to reach start drift. On replacing the reagents it remained at a high drift and inspection revealed iodine forming at the indicator electrode (!).

I've triple checked the electrodes are connected properly and tried a new indicator electrode. Generator looks ok, I've not tested electrical continuity to the grills but could do. The anolyte could be a bit old now I think about it but this seems like odd behaviour if that were the only issue.

Ideas? Was thinking perhaps generator is damaged or blocked and hence current is flowing out through the indicator?

3 Comments
2024/06/21
16:00 UTC

4

"Reverse streaking" on silica flash chromatography + TLC

Have this one compound containing esters and a heterocyclic amine that when ran on TLC and flash chromatography streaks but in kinda backwards, where it has a slow draggy front that gets more concentrated and an abrupt drop off at the end of the peak.

Has anyone experienced this before, and if so did you find a way to get this to not happen?

20 Comments
2024/06/21
13:23 UTC

2

Filtration Techniques

I've been working for a while with polymers in a small scale lab. We have a filtration process to remove silica/clays from polymers that we currently just use filter papers and buchners. These need to be kept hot to maintain any level of flow. Anyone got any tips on doing this a different way?

12 Comments
2024/06/21
12:56 UTC

3

HPLC-ICP-MS not showing any counts during analysis

Hi!

I’m praying to the gods of analytical chemistry that someone here know the solution to this issue. This is a problem that is happening quite often with this setup.

The counts and sensitivity are fine during manual tuning but when the batch starts and the first sample is injected there are none. Simply zero counts during the analysis.

Almost everything has been tried and tested, CAN cables swapped, remote connection cable wiggled and reinstalled, batch has been changed but nothing helps.

But then some other day everything is working fine and analysis runs smoothly.

Uploading the same batch from a “successful” day does not help on the days that counts do not show up.

Has anyone encountered this issue or has a maybe some tips what and how to fix this issue?

Many thanks!

5 Comments
2024/06/21
11:48 UTC

21

Chemical Synthesis Companion

Hello all. I've been making molecules for a while now and sometimes run into issues when trying to reproduce procedures from the literature. This predicament, I quickly learned, was quite common in the academic setting at least. Not only can this lead to lots of wasted time, but also creeping doubts on whether the problem lies with the chemistry or the chemist. Although there are reputable journals whose role is to ameliorate this issue (i.e. Organic and Inorganic Syntheses), I always wished that more background information was added to the procedures presented therein. In attempting to better address the reproducibility problems, I've started a forum with synthetic procedures for compounds from all across the periodic table. Within it, users will find background information, synthesis protocols, other useful data, and relevant references. I think a forum-style resource of this kind is great because it allows users to comment on potential pitfalls and additional anecdotal information. Feel free to check the link below and let me know your thoughts. I've been gradually adding entries for organic, main-group, and transition-metal compounds that I'm sure synthetic chemists will find useful. Hope it helps!

https://chemdotes.discourse.group/

3 Comments
2024/06/21
03:06 UTC

7

How does NaBH4 reduce this ester?

https://imgur.com/JnGibCr

This reaction worked very well. But why? Shouldn't esters be very poorly reduced by NaBH4, if at all?

And how come it doesn't reduce both ester groups?

14 Comments
2024/06/20
23:24 UTC

2

CHN analysis lab needed

I have some samples that need CHN analysis from a reliable lab. The problem is that our default option is on holidays for the next month....

Please share your analysis labs and experiences in reliability of the results. I can send to the US, but we live in Europe.

11 Comments
2024/06/20
16:53 UTC

3

Correct Interpretation of a Cyclic Voltagramm

https://preview.redd.it/y9r9mhpuvq7d1.png?width=474&format=png&auto=webp&s=b1fbd6f770c443d57be84fcaf4a928ce767a6cf7

Good day everyone,
I came across the following paper (https://www.science.org/doi/10.1126/science.abg2371), and was trying to understand some of their reported results.
In particular, the cyclic votagramm curve has a shape that is very different from what I have come across in the electrochemistry books I have. My guess is that this is due to a very low exchange current density, but I was hoping I could get your insights, and perhaps advice to determine kinetic parameters form this curve.
If there is any way I can improve the question please let me know and I will edit it, I much prefer that over being insulted. Thank you all.
Best regards.

3 Comments
2024/06/20
15:33 UTC

4

AOBChem USA

Does anyone have experience ordering from a supplier called AOBChem USA? How are lead times and fine chemical quality?

2 Comments
2024/06/19
14:57 UTC

7

Boc and tBu ester pH stability during enamine hydrolysis

I have a molecule which has both N-Boc and tBu ester protecting groups. I am forming an aldehyde in the reaction, and have discovered that most of the aldehyde is trapped as a pyrrolidine enamine, which I need to hydrolyse. The main reaction is pretty much a new procedure, so very little literature on this specifically, but from the literature on enamine hydrolysis it appears to hydrolyse them you need to acidify to pH <4, stir for a period and then neutralise. Obviously the issue here is that both protecting groups are potentially unstable at this pH. I was wondering if anyone had any experience of working with these protecting groups during acid-base workup and could give me any pointers that would help me make them survive.

My plan was to trial something like 0.5-1M HCl to acidify and stir for ~30 minutes, before neutralising with 0.5-1M NaOH, all done at ~0degC.

9 Comments
2024/06/19
05:52 UTC

0

Bromination of 2, 5-dihydroxyterepthalic acid

I would to to brominate 2,5-dihydroxyterepthalic acid. I found a procedure that brominated terepthalic acid with bromine in conc acid. Is that likely to work for the aforementioned product? If not, why not? Other alternatives?

5 Comments
2024/06/18
22:14 UTC

3

Control in a Diels-Alder reaction

If you had a DA reaction woth an excess of diene would the cyclohexene product of a DA reaction act as a dieneophile in a subsequent DA reaction? Or would the retro DA be more favored?

My thought in this case is that the furfural is pretty electron poor, and the product dienophile is also electron poor, that they'd simply not react.

8 Comments
2024/06/18
16:42 UTC

2

Microwave irradiation assisted organic synthesis

Hello. When heating a reaction mass using microwave irradiation, does the watt power need to be low (like 60 watts) and constant throughout the reaction to achieve wanted temperature, or can it be heated with high watt power on and off (like how normal microwaves work)? Would it affect the reaction rate and/or yield?

15 Comments
2024/06/18
15:40 UTC

0

Software for successful ADMET prediction

Hi guys. I'm just wondering if anyone has had any success with either commercial or open-source ADMET prediction software? I've trialled a few options but nothing giving any kind of decent correlation with real life data yet unfortunately.

7 Comments
2024/06/18
14:13 UTC

4

Cholic acid oxidation selectivity

I came across this recent publication https://doi.org/10.1021/acs.oprd.4c00121 and found the reaction of cholic acid oxidation quite fascinating. It is possible to selectively oxidize one of three different secondary alcohols with really good yield, but the authors didn't explain the origins of this selectivity. Do you have any ideas? From an electronic and steric point of view, all three positions seem quite similar to me.

https://preview.redd.it/svyt960av97d1.png?width=794&format=png&auto=webp&s=5d875d90e755a4908d30ef7c47a40065c8185554

3 Comments
2024/06/18
06:12 UTC

0

Chitosan TPP Nanoparticles

Hello,

I'm currently working on synthesizing nanoparticles, but I'm encountering some issues with particle size and isolation. First, when I increase the concentration of chitosan (dissolved in 1% acetic acid) relative to TPP, the size of the nanoparticles, as measured by DLS, does not consistently increase; it seems quite erratic. The second issue arises during drying: instead of obtaining a powder, I end up with a sheet-like material that resembles cotton. I'm using a lyophilizer to dry the nanoparticles. These reactions are performed via ionic gelation at a pH of 4.6, with chitosan concentrations ranging from 1 mg/mL to 5 mg/mL and TPP concentrations ranging from 0.5 mg/mL to 2 mg/mL. Is this outcome typical?

I am certain that the higher concentration will not give me nanoparticles and the lower concentrations are where I should be focusing i.e 1mg/ml of chitosan in 1% acetic acid to 0.5mg of TPP.

Any advice would be much appreciated.

9 Comments
2024/06/17
18:14 UTC

1

USP

I’m not sure if this is against the rules, but does anyone know where one could download general chapters of the USP for free? I’ve tried general googling and it’s come up with some files that have been clearly kicking around the internet but I’m having issues unzipping them, so they may be borked. Any suggestions welcome.

9 Comments
2024/06/17
15:58 UTC

7

Removal of trace water from a reaction

Hey Chempros, I have a reaction that's I've been working on that's causing me some trouble.

On concentrating a solution of my product, the product decomposes. I've worked out that it's due to water being present when removing solvent.

The solvent in question is acetonitrile (and cannot be changed), and is bought anhydrous (<10 ppm H2O). I've also dried all of my reactants as best as possible to get total water concentration around 200 ppm.

Do you guys have any way to remove such a small amount of water at the end or beginning of my reaction? Any help would be appreciated!!

32 Comments
2024/06/17
14:52 UTC

9

Maintaining DMF and POCI3 Dry

Hello,

What are some ways to keep DMF and POCl3 dry? We currently have the DMF with a rubber stopper and under argon, but it keeps deteriorating the rubber stopper over time. The same thing happens with the POCl3. Are there any stoppers that are compatible with storing DMF and POCl3?

Also, are there any molecular sieves compatible with keeping POCl3 dry? I tried searching online, but there isn't much information about them.

13 Comments
2024/06/15
19:37 UTC

7

Buying needles for lab in the US

TL;DR Where do I buy needles in the US? Is it not common here?

I grew up in Europe where it was very standard for every organic chemistry lab to use syringes with needles for transporting liquids (across many labs in different countries). Now I'm setting a lab in the US and although I found the same syringes, buying the needles seems to be impossible.

The most common needles that nearly every lab had were:

  • 21G 50mm (green)
  • 21G 120mm (green)
  • sometimes 18G 50mm (pink)
  • (I might be misremembering the exact lengths)

In Europe, I could get a pack of 100 needles for < $5 in every other pharmacy. The most common brand was Sterican.

In the US, I just cannot find anyone selling these - I've looked through companies like Sigma, Fisher, medical/surgical supply companies, Amazon, .... I found a couple websites selling 1" or 1-1/2" 21G needles but those are just too short to reach through septum into a vial or flask.

Is it not common to use surgical needles for organic chemistry in the US?

Do you know where to buy the needles?

30 Comments
2024/06/15
17:56 UTC

9

Can someone please tell me how to get rid of these lime green circles/blobs around my assignments in MNova? :(((

4 Comments
2024/06/15
10:13 UTC

29

Troubleshooting a difficult Heck reaction

30 Comments
2024/06/14
21:33 UTC

7

Alternatives to ReactIR

Hello Pros

Working with flow chemistry, the ReactIR equipped with a flow cell is a very nice and very needed in-line analytical tool. However, it also very expensive and not something you just buy on a whim. Are there similar FTIR devices with flow cells? I haven‘t really found anything from Agilent or Thermo. If you have any ideas or tips I would be very happy to learn about them!

10 Comments
2024/06/14
08:21 UTC

4

Low(est) Temperature DSC measurements

Hello folks,

I'm counting on the hive mind: I'm looking for a calorimetry device that can go to temperatures below 100K, as low as 10K. All the commercially available devices I found (Netzsch, TA instruments, Malvern etc.) can go to -150 or -160 °C, which is not low enough for me. Background: I have some compounds and I want to see if there are phase transitions below liquid nitrogen temps and if there are, what the heats of the transitions are.

Thanks!

3 Comments
2024/06/14
08:05 UTC

1

Intermediate isolation

I'm running a reaction at ~230 C, it is in equilibrium between a metal oxide(powder/NP)/metal oleate/oleic acid/oleic anhydride/water. The problem is: any water catalytically destroys the anhydride, which in turn produces more water. Doing things dry would for sure help and that is in the efforts, but preferably there may be a better way.

I would like to use something that is an irreversible desiccant, or at least one that won't release water back into solution at 230 C (so 3A and 4A mol sieves are out). Or, something that can react with the anhydride as it is produced so that it can be more easily isolated/detected/not destroyed by water/etc. It cannot be an amine (likely also not an amide but that's unconfirmed) as they will inhibit the production of anhydrides all together.

I cannot decrease the temperature as the reaction proceeds at minimum at 230 C, It can be increased though. Any, suggestion would be greatly appreciated.

12 Comments
2024/06/14
00:10 UTC

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