Refractive Indices of Pure Compounds

Separations based upon differences in the physical properties of the components. When procedures (1) or (2) are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Sections 11,15 and 11,17) the degree of separation may be determined by the range of boiling points and/or the refractive indices and densities of the different fractions that are collected. A mixture of non-volatile sohds may frequently be separated by making use of the differences in solubilities in inert solvents the separation is usually controlled by m.p. determinations. Sometimes one of the components of the mixture is volatile and can be separated by sublimation (see Section 11,45). 

The refractive index is one of the important physical constants of organic compounds and can be determined accurately. As a criterion of purity it is more reliable than the boiling point. The determination of refractive indices is useful for the identification of an unknown pure liquid. It is also used in analytical work for the determination of the relative amount of a substance in solution. 

The primary reaction products of formaldehyde and hydroxy compounds are hemiacetals. Since these compounds are unstable and have not been isolated in a pure state, evidence for tbeir formation is based principally on physical measurements. The increased density and refractivity of alcoholic solutions of aldelij des are recognized indications of hemiacetal formation". Hemiacetals are apparently formed in alcoholic solutions of many carbonyl compounds, as evidenced by abnormalities in the ultraviolet absorption spectra of these solutions. Hov ever, the results of such measurements indicate that the extent of compound formation decreases as the substituents surrounding the carbonyl group increase". Formaldehyde has the property of forming such deiivatives to an ad "anced degree. 

Figure 9.72 Chromatograms of the action patterns of maltoheptaose after the indicated periods of incubation with a-amylase and a-glucosidase. Peaks 1, glucose 2, maltose 3, maltotriose 4, maltotetraose 5, maltopentaose (x) compound A 6, maltohexaose 7, maltoheptaose. (A) Pure maltoheptaose used for the assay. (B) Blank sample before the addition of substrate. (C-H) Chromatograms after 1, 5, 10,15, 20, and 30 minutes, respectively, of incubation. Chromatographic conditions column, 10 jum Nucleosil SA (250 mm X 4 mm) solvent, acetonitrile-water (72.527.5) flow rate, 0.7 mL/min temperature, 27°C detection, differential refractometer, full scale = 2 X 10-6 refractive index units. (From Haegel et aL, 1981.)...

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