Cinnamic acid melting point

Determine the melting point of pure cinnamic acid (133°) and pure urea (133°). Intimately mix approximately equal weights (ca. 01 g.) of the two finely-powdered compounds and determine the melting point a considerable depression of melting point will be observed. Obtain an unknown substance from the demonstrator and, by means of a mixed melting point determination, discover whether it is identical with urea or cinnamic acid. 

It is instructive for the student to construct a rough melting point diagram (compare Section 1,13 and Fig. 1,12, 1) for mixtures of cinnamic acid and urea. Weigh out 1 00 g. each of the two finely powdered components, and divide each into ten approximately equal portions on a sheet of clean, smooth paper. Mix 4 portions of cinnamic acid (A) with 1 portion of urea B) intimately with the aid of a spatula on a glass slide, and determine the melting point (the temperature at which the mixture just becomes completely fluid is noted). Repeat the procedure for 3 parts of A and 2 parts oiB 2 parts of A and 3 parts of B and 1 part of A and 4 parts of B. Tabulate your results as follows —... 

Aromatic carboxyUc acids are produced annually in amounts of several million metric tons. Several aromatic acids occur naturally, eg, benzoic acid (qv), sahcyhc acid (qv), cinnamic acid (qv), and gaUic acids, but those used in commerce are produced synthetically. These acids are generally crystalline sohds with relatively high melting points, attributable to the rigid, planar, aromatic nucleus (see Phthalic acids). 

For (Z)-cinnamic acid [102-94-3], three distinct polymorphic forms have been characteri2ed. The most stable form, referred to as aHocinnamic acid, has a melting point of 68°C, and the two metastable forms, isocinnamic acids, have melting points of 58°C and 42°C, respectively. (E)-Cinnamic acid can be converted to the (Z)-isomer photochemicaHy through kradiation of a solution with ultraviolet light. 

Other methods such as prolonged heating above the melting point or in presence of catalysts have also converted cis into trans forms. Thus in the pair of maleic-fumanic acid, the transform appears to be more stable and the same situation exists in cinnamic acids, stilbenes, 2-butenes and many other compounds. However it cannot be generalized that trans forms are always invariably more stable than the cis forms, because Viche found that in l-bromo-2-fluroethylene or 1,2-dibromoethylene the cis is more stable than the trans. 

In addition to the influence on the dimer morphology, the presence of water molecules strikingly affect apparent photoreaction rate and temperature dependence of the rate (12). Since the topochemical reaction deteriorates pronouncedly at reaction temperatures close to the melting point of the starting crystal, maximal reaction rate is necessarily observed at a specific temperature for individual crystals, for example, at ca. 20"C for a -form crystal of cinnamic acid (mp 132°C) (13). In an aqueous dispersant the apparent maximal rate of photodimerization of 1 is observed about 15°C while the temperature for maximal rate in a non-aqueous dispersant is about 35 °C. The... 

Cinnamic aldehyde (boiling point 253°C with decomposition, melting point -7.5°C, density 1.0497) has a cinnamon odor, hence the name. As it oxidizes in air to cinnamic acid, it should be protected from oxidation. 

Prelab Exercise Predict what the melting points of the three urea-cinnamic acid mixtures will be. 

Melting Points of Pure Urea and Cinnamic Acid... 

Melting Points of Urea-Cinnamic Acid Mixtures... 

For some time four modifications of cinnamic acid, which possess different melting points, have been known. It has been shown recently that three of the forms, which melt at 68°, 58°, and 42°, respectively, are crystallographic modifications of a single acid. When any one of them is melted and the liquid formed is seeded with a crystal, the solid which s parates on cooling has the melting point of the crystal used to induce crystallization. The fact that apparently four forms of the acid existed aroused much interest and led to a number of investigations, because the results appeared to be at variance with the accepted theory as to the structure of organic compounds. 

Obtain a pair of solids whose melting points are within 1 °C of one another and determine whether they indeed depress one another s melting points. Some possible examples are 3-furoic acid and benzoic acid, 1-naphthylacetic acid and 2,5-dimethyl benzoic acid, frans-cinnamic acid and 2-furoic acid, 2-methoxybenzoic acid and malic acid, or dimethyl fumarate, 9-fluorenone, and methyl 4-bromobenzoate. Consult with your instructor regarding these and other possibilities. 

According to Kofler [358], microsublimation is any sublimation procedure in which the sublimate is caught on a smooth plate-usually a small slide-for purposes of microscopic examination (melting point, refraction, crystal structure). Many substances can be directly sublimed in crystalline form from drugs containing them, e. g., caffeine coumarin umbelliferone benzoic, ferulic, salicylic and cinnamic acids emodin etc. 

You are presented with four vials, each containing a white crystalline solid. Two are unlabeled vials containing pure samples of frans-cinnamic acid and urea, respectively. The other two are labeled reference standards for each sample. Devise a method for the proper identification of the unlabeled vials, knowing that the literature melting point for both frans-cinnamic acid and urea is 132.5-133 °C. 

Also obtained from 2,4-dihydroxy-5-propionyl-P-methyl-cinnamic acid by cyclisation with sulfuric acid or on heating above its melting point. It is partly dehydrated yielding the titled ketone [6800]. 

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