The melting point of an organic molecule deteriorate(decompose), if it is above 250 deg.Centigrade.

Dear Wessal, this is certainly a very interesting and fundamental question in organic chemistry. Organic compounds show two fundamentally different behaviors when their melting point is determined. In the case of rutin it is stated in the literature that this compound melts at 214-215 °C under decomposition. This means that at this temperature the compound decomposes / deteriorates. Upon colling to room temperature the original compound cannot be recovered. Often this decomposition at melting temperature is accomanied by darkening and / or gas evolution. In such a case it is stated in the literature: "m.p. 214-215 °C (dec.)". On the other hand, many organic compounds melt without decomposition. For example, cholesterol has a melting pont of 148-150 °C. At this temperature the compound becomes a liquid, and upon cooling it solidifies again and still is unchanged cholesterol.

I hope this answers your question. Good luck with your work and best wishes, Frank Edelmann

Dear all, it is not a general rule, melting may be a just a change in the physical state from solid to liquid without change in the basic chemical structure. However, compounds with heat sensitive 'labile' bonds may not keep the integrity of the basic chemical structure upon melting. This is sometimes favored by the presence of oxygen, humidity, and other influencing parameters. My Regards

Concerning melting points above 250 oC, I have prepared, in the past, a significant number of compounds, mostly nitrogen heterocycles, whose melting points were well above 300 oC without decomposition. A sharp melting point is indicative of purity.

Dear Wessal Ouedrhiri I fully agree with all points mentioned in the answer given by Jack M Hinkley. Melting and decomposition temperatures of organic compounds can vary in a very large range. There may be compounds which already decompose e.g. around 60 °C, while others melt without decomposition above 300 °C. Typical examples for such high-melting organic compounds are polycyclic aromatic hydrocarbons. As a good example please have a look at the data for tetracene (Wikipedia entry: https://en.wikipedia.org/wiki/Tetracene). Tetracene forms orange crystals which melt without decomposition at 357 °C! In the 19th century melting points were important data for characterization of new compounds besides elemental analysis. At that time chemists normally tried hard to purify their new compound bey recrystallization until a constant melting point was reached. Pure compounds always have higher melting points than impure samples.

Please stay safe and healthy!

The melting point of an organic solid can be determined by introducing a tiny amount into a small capillary tube, attaching this to the stem of a thermometer centred in a heating bath, heating the bath slowly, and observing the temperatures at which melting begins and is complete.

http://www.chem.ucalgary.ca › melti...

Melting point determination

Impurities, even when present in small amounts, usually lower the melting point and broaden the melting point range. A wide melting point range (more than 5°C) usually indicates that the substance is impure; a narrow melting point range (O. 5-2°C) usually indicates that the substance is fairly pure.

https://iowacconline.instructure.com › ...

Melting Point Determination: Purity and Identity of Crystalline

The presence of even a small amount of impurity will lower a compound's melting point by a few degrees and broaden the melting point temperature range. Because the impurity causes defects in the crystalline lattice, it is easier to overcome the intermolecular interactions between the molecules.May 18, 2009

https://www.chm.uri.edu › chm228

Using Melting Point to Determine Purity of Crystalline Solids

When molecules are tightly packed together, a substance has a higher melting point than a substance with molecules that do not pack well. ... Molecular size also affects the melting point. When other factors are equal, smaller molecules melt at lower temperatures than larger molecules.Apr 13, 2018

https://sciencing.com › factors-affect...

What Factors Affect Melting Point? - Sciencing

Pure samples usually have sharp melting points, for example 149.5-150°C or 189-190°C; impure samples of the same compounds melt at lower temperatures and over a wider range, for example 145-148°C or 186-189°C. .

http://www.chem.ucalgary.ca › ...PDF

Melting point determination

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Dear Dr Wessal Ouedrhiri . There is no direct relationship between melting point and deterioration . This depends on the nature of the compound .

Dear colleagues, In fact, I was looking to encapsulate some plants secondary metabolites in magnesium oxide, like in this paper :Article Exploring the therapeutic potential of Hibiscus rosa sinensi...

However, this application needs calcination at 300°C, and I think that can deteriorate the plant's molecules. I don't know if you have any advice.

Dear Wessal Ouedrhiri, though it is a high temperature, but it is of prime importance to check the thermal behavior of this plant extracts via TGA. Also think about other possibilities of encapsulation. My Regards

Dear Wessal Ouedrhiri many thanks for your kind response and explanation. This shows that it is very important to formulate technical questions on RG as specific as possible in order to receive qualified answers. Your new explanation sheds completely new light on the issue. In your case your final goal was obviously to synthesize MgO nanoparticles with the use of plant extracts. This can be called a "green synthesis" of MgO NPs. The effect of the calcination step at 300 °C is exactly to destroy the organic material (plant molecules) and leave only the pure MgO nanoparticles behind. Thus it involves a deliberate decomposition / deterioration of the organic constituents.

Good luck with your work and best wishes, Frank Edelmann

Generally melting of a compound is a physical change from solid to liquid without change in the basic chemical structure. Therefore pure organic compounds don't deteriorate in it's melting point but compounds with heat sensitive and impure condition may react at below the meeting point.

melting is not necessarily accompanied by deterioration of the molecular structure or as also known as decomposition. while most of the compositions are fairly stable at normal conditions and are not at risk of oxidisation, some are oxidised or thermally unstable and are susceptible to many chemical reactions with oxygen in the air or breaking down to immediate intermediates such as hydrolysis of the esters and all these are happening in minute amounts and tiny seconds while being tested for melting point measurements but, if, by accident, a composition is exposed to a heat somehow, leading to its melting, it is definitely worth it and re-analyse it for assessing its purity.

Dear Wessal Ouedrhiri P.S. If your final goal is the encapsulation of plant's secondary metabolites in magnesium oxide then I assume that the calcination step at 300 °C will destroy most, if not all, the organic molecules. Thus I suggest that you try to find a literature reference in which an alternative synthetic method without the calcination step is described. In this context, please have a look at the following potentially useful article which come at least close to this issue:

Nanomaterial-based encapsulation for controlled gastrointestinal delivery of viable probiotic bacteria

Article Nanomaterial-based encapsulation for controlled gastrointest...

Unfortunately the paper has not yet been posted as public full text on RG. However, two of the authors have RG profiles, so you can easily contact one of them directly via RG and request the full text from him.

Good luck with your work and please stay safe and healthy!

Dear Wessal Ouedrhiri, yes it's possible but not always. Stable compounds do not decompose even on melting. After cooling such compounds again solidify and show same m.p. But we must consider that during checking of melting point, we are heating the compound in air and if the compound is sensitive to aerial oxidation (specially when m.p is high / needs heating at higher temperature) can lead to decomposition. Thus it becomes impure. In many cases at m.p, the compound undergo decomposition due to certain thermal reaction and on cooling the starting compound may not come back. Dicarboxylic acids like succinic acid, maleic acid etc. on heating during checking of m.p forms respective anhydride impurity. I have many work on synthesis of dihydrobenzacridines via thermal cyclization of suitable anil hydrochlorides and chlorovinyl imine derivatives. For such compounds melting temperature is the reaction temperature. At melting point such compounds undergo vigorous reaction (via thermal cyclization) to form the products and thus on cooling the starting material are not obtained back at all. In such cases normally we report m. as .........oC (d), where (d) signifies decomposition.

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Usually melting is a physical change from solid to liquid state, without any chemical consequences.

However, some compounds are sensitive to high temperatures and may decompose on heating. For example, alpha-beta unsaturated or beta-keto carboxylic acids decarboxylate on heating.

Deterioration of an organic compound depends on the maximum temperature range upto which its physical and/or chemical property(ies)) remain stable. After that, it may lose either its physical or chemical identity and may/may not behave differently.

And the melting point of an organic compound greatly represents the temperature at which its physical state changes from solid to liquid, however the chemical consequences may/may not necessarily change.

Obviously, different atoms, functional groups, type of bonding, orientation, structural aspects, and other parameters may influence the stability of your's selective compound for encapsulation at such a high temperature (300 oC) without undergoing decomposition.

If the compd. is hygroscopic or oxidized by whether then the melting point of an organic molecule deteriorate.