Solutions, formaldehyde Refractivity

Specifications and Analytical Methods. The commercial aqueous solution is specified as 34% minimum butynediol, as determined by bromination or refractive index. Propargyl alcohol is limited to 0.2% and formaldehyde to 0.7%. 

Density, refractive index, viscosity, and isothermal vapour-liquid equilibrium data for hexafluoroacetone-water mixtures, i.e. solutions of the gem-diol (CFa)gC(OH)t, are now available, and so are thermochemical parameters for the addition of water and methanol across the C=0 bond of the ketone to give the hydrate, (CF3)2C(OH)a, and the hemiacetal, (CFa)2C(OH)OMe, respectively. The value for the heat of hydration of hexafluoroacetone [Affr°(soln.) = —93.6 kJ mol" cf. MeCO-CHjCI, -8.4 MeCO CHCl -23.0 MeCHO, -21.3 and CCI.-CHO, -58.5 kJ mol ], taken with that estimated for acetone [-4 4 kJ mol" (too low to have been measured reliably)] and the difference in the heats of hydrogenation of formaldehyde (-92 kJ mol- ) and acetone (—54 kJ mol- ), leads to the conclusion that two CFj groups destabili2e a carbonyl bond by ca. 50 kJ mol-, a factor which correlates with the remarkable reactivity of hexafluoroacetone in carbonyl additions. ... 

Density and Refractivity The density and refractive index of commercial formaldehyde solutions vary with methanol content and temperature. Table 4, prepared from the data of Natta and Biiccninddn, gives... 

Occasionally formaldehyde containing little or no methanol has been marketed at a concentration of 30 per cent, since a solution of this concentration does not have to be kept at an elevated temperature to prevent polymer precipitation. At 1S°C, this solution has a density of U0910 and a refractive index of 1.3676 . 

Density and Refractivity. The density and refractivity of pure formaldehyde solutions bear a simple linear relation to formaldehyde concentration. Differences due to variations in the relative proportions of simple and polymeric formaldehyde hydrates in solutions of the same concentration have practically no effect on these figures as ordinarily measured. Such variations can be measured only b the extremely sensitive methods of dilatometry and interferometiy. 

A recent study of the density and refractivity of formaldehyde solutions... 

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. 

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