Melting point analysis

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Beaker
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Melting point analysis

Post by Beaker » Mon Mar 09, 2020 10:48 pm

Just watching a news story casually showing a USB device for sequencing a virus. What was once housed in an entire room can now get lost if you put a journal down on it.

When I was at the uni bench, melting point and elemental analysis had to be done on every compound. I remember the sadness of taking the last couple of mg of hard won compound, putting in a Victorian melting point machine, and watching it through a lens as it slowly turned into tar. Like anyone was ever going to use it to determine if they had repeated your synthesis.

Question to those doing organic chemistry today - do you still have to do melting point? Or roll the dice of elemental analysis?

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FredM
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Re: Melting point analysis

Post by FredM » Tue Mar 10, 2020 11:41 am

AFAIK still required but modern methods allow you to use capillary tubes so sample size can be reduced significantly. The RSC has guidelines here.

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science_fox
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Re: Melting point analysis

Post by science_fox » Tue Mar 10, 2020 12:01 pm

I think as much as possible every technique is required to confirm precise synthesis of something - Mass Spec (<5ppm mass error), NMR with full assignment, IR/UV etc. determining stereo-chemistry can very hard.

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Gentleman Jim
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Re: Melting point analysis

Post by Gentleman Jim » Tue Mar 10, 2020 12:08 pm

Have a look in any pharmacopoeia and see the range of techniques and tests still required
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Boustrophedon
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Re: Melting point analysis

Post by Boustrophedon » Tue Mar 10, 2020 2:20 pm

I last did a melting point test at A Level with Doc "Give me summat to titrate" Sykes. We had just made methyl salicylate. I haven't been a chemist since then though.
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Gentleman Jim
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Re: Melting point analysis

Post by Gentleman Jim » Tue Mar 10, 2020 3:01 pm

Boustrophedon wrote:
Tue Mar 10, 2020 2:20 pm
I last did a melting point test at A Level with Doc "Give me summat to titrate" Sykes. We had just made methyl salicylate. I haven't been a chemist since then though.
<pedant>
Methyl salicylate is a liquid (Oil of wintergreen)
Schools tend to make Acetyl salicylate (Aspirin)
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Beaker
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Re: Melting point analysis

Post by Beaker » Tue Mar 10, 2020 6:16 pm

Thanks FredM for the link. So yes, RSC still asking for melting point as an ‘important physical property.’ Humbug.

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Boustrophedon
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Re: Melting point analysis

Post by Boustrophedon » Tue Mar 10, 2020 10:53 pm

Gentleman Jim wrote:
Tue Mar 10, 2020 3:01 pm
Boustrophedon wrote:
Tue Mar 10, 2020 2:20 pm
I last did a melting point test at A Level with Doc "Give me summat to titrate" Sykes. We had just made methyl salicylate. I haven't been a chemist since then though.
<pedant>
Methyl salicylate is a liquid (Oil of wintergreen)
Schools tend to make Acetyl salicylate (Aspirin)
We definitely made methyl salicylate, but as you point out that is already a liquid. I wonder what it was that we made for the melting point test? Nuffield A level chemistry circa 1976.
...zoologically improbable and/or terrifying to small children.

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Gentleman Jim
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Re: Melting point analysis

Post by Gentleman Jim » Wed Mar 11, 2020 8:53 am

Boustrophedon wrote:
Tue Mar 10, 2020 10:53 pm
Gentleman Jim wrote:
Tue Mar 10, 2020 3:01 pm
Boustrophedon wrote:
Tue Mar 10, 2020 2:20 pm
I last did a melting point test at A Level with Doc "Give me summat to titrate" Sykes. We had just made methyl salicylate. I haven't been a chemist since then though.
<pedant>
Methyl salicylate is a liquid (Oil of wintergreen)
Schools tend to make Acetyl salicylate (Aspirin)
We definitely made methyl salicylate, but as you point out that is already a liquid. I wonder what it was that we made for the melting point test? Nuffield A level chemistry circa 1976.
Ah. Same as me then :lol:

Probably measured the melting points of the ester derivatives:
ESTERS
These must be hydrolyzed into their constituent carboxylic acid and alcohol. The products must be separated and
isolated, then individually characterized. If you were given an ester, you will not have a second component in your
unknown.
Hydrolysis: In a 50 mL boiling flask, add 0.310 mol of NaOH pellets, 10 mL of the ester, and 15 mL water. Reflux
this mixture with good mixing for 1 hr buried in a sand bath. The resulting reaction mixture will contain the neutral
alcohol and the anion of the carboxylic acid. It is up to you to figure out how to physically separate, recover, and identify
each of these compounds. Remember that either or both of the components of the ester may be water-soluble.
Group Recommended Solid Derivatives
Alcohol - phenylurethane, p-nitrobenzoate or 3,5-dinitrobenzoate
Carboxylic Acid - amide, anilide, or p-toluidide
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FlammableFlower
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Re: Melting point analysis

Post by FlammableFlower » Thu Mar 12, 2020 9:03 pm

Melting point is an interesting one. We still get our undergrads to take the marketing point of every solid they make and compare to literature values, but actually as an analytical technique/test of purity its rarely used due to being superseded by NMR and MS. But as an important indicator of properties of that compound, if a novel compound or different polymorph, then it can tell you interesting things, for example: in drug development is a good indicator of its solubility.

Equally, elemental analysis has almost entirely been replaced by HRMS. One of the teeth-grindingly annoying things about elemental analysis is that you need a very pure sample or the results were meaningless. Which meant you could spend days on a rapidly disappearing amount of your compound trying to get it sufficiently clean. Whereas with high res mass spec you can throw in all kinds of sh.t and as long as your compound is there you'll get a m/z peak with an isotope pattern match to 5 dp and can still claim to have your compound.

However, there are certain compounds for which HRMS is no good, and they then need to go off for elemental analysis. Sadly, our dept's machine reached to the point the uni wouldn't pay for repairs as they cost too much against the amount of use it had, so now they get shipped out.

Spoiler:

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Turdly
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Re: Melting point analysis

Post by Turdly » Wed Mar 18, 2020 3:01 am

NMR and MS are terrible measures of purity despite becoming the norm for testing it (NaCl contamination for e.g. is unlikely to be seen in either).

Elemental analysis is still used routinely for purity.

I have an automated melting point apparatus in my lab, but we mainly use it to determine cloud points.
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