Racemic mixtures boiling point

The hydrochloride addition salt of the above reaction product was prepared in customary fashion, that is, by reaction with hydrochloric acid, followed by fractional crystallization from a mixture of alcohol and ether. The two possible racemic forms were obtained thereby. The difficultly soluble racemate had a melting point of 169° to 170°C and the more readily soluble racemate had a boiling point of 145° to 148°C. 

Meso compounds contain chirality centers but are achiral overall because they have a plane of symmetry. Racemic mixtures, or racemates, are 50 50 mixtures of (+) and (-) enantiomers. Racemic mixtures and individual diastereomers differ in their physical properties, such as solubility, melting point, and boiling point. 

It should be recalled that whereas the enantiomers in the mixture (or racemate) (1) have identical physical properties (except for their action on the plane of polarised light), the diastereoisomers (2) and (3) have physical properties (e.g. solubility, boiling points, chromatographic behaviour, etc.) which are frequently significantly different. Resolution of the mixture (or racemate) can then be achieved provided that one of the diastereoisomers may be obtained in a pure state, and that regeneration from it of the pure enantiomorphous form is not accompanied by any degree of racemisation. 

B is correct The compounds are diastereomers, so they have different physical properties (like boiling points) and could thus be separated by distillation. A and D are wrong because both compounds have the same mass and the same functional groups. Neither group rotates polarized light, since Reaction 1 produces a meso compound and Reaction 2 produces a racemic mixture. 

Racemic 4-(4-fluorophenyl)-l-methyl-l,2,3,6-tetrahydropyridine (50 g) was dissolved in a mixture of 21.6 ml of concentrated sulfuric acid and 50 ml of water. To the solution were added 25 ml of concentrated hydrochloric acid and 22.4 ml of 37% formaldehyde solution. The mixture was refluxed for 5 h, cooled, and 125 ml of concentrated ammonia were added. The mixture was extracted with 50 ml of toluene. Drying of the toluene solution and distillation gave 38 g of 4-(4-fluorophenyl)-3-hydroxymethyl-l-methyl-l,2,3,6-tetrahydropyridine with boiling point 110°-120°C at 0.1 mm Hg. 

Besides optical rotation, other physical properties of a racemate are not readily predicted. The melting point and boiling point of a racemic mixture are not necessarily the same as either pure enantiomer, and this fact is not easily explained. The physical properties of two enantiomers and their racemic mixture are summarized in Table 5.1. 

Chemical and Physical Properties Trichlorfon is an organophosphate compound, which has an empirical formula of C4HgCl304P and a molecular weight of 257.44. It is a racemic mixture of two isomers. Trichlorfon is a pale clear, white or yellow crystalline powder, melting point 75-84°C, boiling point 100°C, vapor pressure 7.8mmHg at 20°C, and is stable at normal temperatures and pressure. At higher temperatures and pH less than 5.5, trichlorfon decomposes to form dichlorvos (0,0-dimethyl-0-(2,2-dichlorovinyl) phosphate, DDVP). It is readily soluble in chloroform and methylene chloride, and less soluble in water, benzene, and diethyl ether. 

Diastereoisomers (unlike enantiomers) have different physical properties such as boiling point, density, etc. These differences between diastereoisomers can be exploited to resolve (or separate) mixtures of enantiomers. The principle behind this technique is to resolve the mixture of enantiomers by chemically converting them into a pair of diastereoisomers. This is achieved by reacting the racemic mixture with an optically pure reagent. These reagents are usually natural products for example, if the racemic mixture contains acidic compounds, reaction is with an optically pure alkaloid such as strychnine or brucine. 

When a racemic mixture reacts with a single enantiomer of another compound, a mixture of diastereomers results, and diastereomers, because they have different melting points, boiling points, and solubilities, can be separated by conventional means. 

Separation of a racemic mixture into the pure enantiomers is known as chiral resolution and traditionally is considered the most challenging in separation sciences. Because two optical isomers (R- and 5-enantiomers) have identical physical and chemical properties (boiling points, melting points. 

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