Aldehydes melting point

Being crystalline compounds which usually have sharp melting-points, they are used to characterise the parent aldehydes and ketones. 

Reagent A is particularly useful for the treatment of the lower aliphatic aldehydes and ketones which are soluble in water cf. acetaldehyde, p. 342 acetone, p. 346). The Recent is a very dilute solution of the dinitrophenylhydrazine, and therefore is used more to detect the presence of a carbonyl group in a compound than to isolate sufficient of the hydrazone for effective recrystallisation and melting-point determination. 

The melting points of some crystalline derivatives of a number of selected aliphatic aldehydes are collected in Table 111,70. 

Physical constants such as melting point boiling point and solubility in water are collected for a variety of aldehydes and ketones in Appendix 1... 

Hydrogen bonding m carboxylic acids raises their melting points and boiling points above those of comparably constituted alkanes alcohols aldehydes and ketones... 

Acetaldehyde can be isolated and identified by the characteristic melting points of the crystalline compounds formed with hydrazines, semicarbazides, etc these derivatives of aldehydes can be separated by paper and column chromatography (104,113). Acetaldehyde has been separated quantitatively from other carbonyl compounds on an ion-exchange resin in the bisulfite form the aldehyde is then eluted from the column with a solution of sodium chloride (114). In larger quantities, acetaldehyde may be isolated by passing the vapor into ether, then saturating with dry ammonia acetaldehyde—ammonia crystallizes from the solution. Reactions with bisulfite, hydrazines, oximes, semicarb azides, and 5,5-dimethyl-1,3-cyclohexanedione [126-81 -8] (dimedone) have also been used to isolate acetaldehyde from various solutions. 

Aldehyde CAS Registry Number Molecular formula Molecular weight Melting point, °C Boiling point, °C Solubiity, g/lOOg water... 

The location of the hydroxyl and aldehyde groups ortho to one another in saUcylaldehyde permits intramolecular hydrogen bonding, and this results in the lower melting point and boiling point and the higher acid dissociation constant observed relative to -hydroxybenzaldehyde. 

Table 4. Melting Points of Some Peroxy Compounds from Aldehydes and Hydrogen Peroxide ...

An unusual method for the preparation of syndiotactic polybutadiene was reported by The Goodyear Tire Rubber Co. (43) a preformed cobalt-type catalyst prepared under anhydrous conditions was found to polymerize 1,3-butadiene in an emulsion-type recipe to give syndiotactic polybutadienes of various melting points (120—190°C). These polymers were characterized by infrared spectroscopy and nuclear magnetic resonance (44—46). Both the Ube Industries catalyst mentioned previously and the Goodyear catalyst were further modified to control the molecular weight and melting point of syndio-polybutadiene by the addition of various modifiers such as alcohols, nitriles, aldehydes, ketones, ethers, and cyano compounds. 

In the example below, Bhardwaj and coworkers synthesized tetramethoxyflavone 36 this flavonol was believed to be the structure of a compound isolated from Artemisia annua Methyl ketone 37 and aldehyde 38 were smoothly condensed to afford chalcone 39 in 73% yield. 39 was then converted to 40 under slightly modified AFO conditions in low yield. Selective demethylation of 40 gave 36. However, spectral data and melting point data of 36 did not match up with the compound isolated from the plant. Hence, the original structure was misassigned and was not flavonol 36. 

Sehlogl has shown that para-aminophenylglycine, para-amino-phenyloxamic acid, and para-aminoacetanilide form compounds with aldehydes, which have sharp melting-points and are suitable for the characterisation of aldehydes. With p-aminophenylglycine condensation takes place when the glycine, mixed with alcohol and the aldehyde in question, is warmed. For the purpose of condensing with p-aminophenyloxamic acid and with p-aminoacetaldehyde the alcoholic suspension of the amino-hody is acidulated slightly with hydrochloric acid and the solution is warmed after the aldehyde has heen added. This method yields the hydrochloride of the condensation products. 

Schauwecker, that when citronellal acetal is oxidised by permanganate, there is formed the acetal of the semi-aldehyde of j8-methyladipic acid and a glycol. The indistinct melting-points observed in citronellal derivatives (for example in the semi-carbazone) can also thereby be explained. It would appear that commercial citronellal contains the two isomeric forms in varying proportions. 

The crude product is dissolved in benzene-hexane (1 1) and applied to a column containing 125 g. of silicic acid (Note 10). Elution with the same solvent gives traces (less than 5 mg. each) of the two fast-moving components in fractions 2 and 4 (125-ml. fractions) and chromatographically pure cholane-24-aI in fractions 5-8 (Note 11). Evaporation of the pooled fractions yields 870 mg. (84%) of the pure crystalline aldehyde, m.p. 102-104°. Recrystallization from 5 ml. of acetone raises the melting point to 103-104° (Note 12). 

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