Pure compounds physical constants

Determination of the physical constants and the establishment of the purity of the compound. For a solid, the melting point is of great importance if recrystalhsation does not alter it, the compound may be regarded as pure. For a hquid, the boiling point is first determined if most of it distils over a narrow range (say, 1-2°), it is reasonably pure. (Constant boUing point mixtures, compare Section 1,4, are, however known.) The refractive index and the density, from which the molecular refractivity may be calculated, are also valuable constants for liquids. 

Physical Properties. An overview of the metallurgy (qv) and soUd-state physics of the rare earths is available (6). The rare earths form aUoys with most metals. They can be present interstitiaUy, in soUd solutions, or as intermetaUic compounds in a second phase. Alloying with other elements can make the rare earths either pyrophoric or corrosion resistant. It is extremely important, when determining physical constants, that the materials are very pure and weU characteri2ed. AU impurity levels in the sample should be known. Some properties of the lanthanides are Usted in Table 3. 

The references which we obtain in a successful search are of various kinds. The main work of Richter , as already mentioned, first refers us to Beilstein s Handbuch der organischen Chemie, which may now be briefly described. The third edition of this work, in four volumes and as many supplementary volumes, gives a brief description of all pure organic compounds prepared up to July 1, 1899, with their physical constants, methods important reactions, and all references to the literature. 

The development of reliable methods for structural analysis of mixtures is very laborious. Physical data of pure compounds may serve as a base for the investigations. It has, however, been proved that not in all cases can these data be simply correlated with those of the mixtures. Thus correlations of physical data of pure, individual hydrocarbons often prove not to be valid in the analysis of mineral oils. In this case physical constants of mineral oil fractions of widely different origin form a more reliable basis for the structural analysis, provided that their structure has been determined by absolute methods. 

For the structural analysis of cyclic fatty acid derivatives (polymerized drying oils, copolymerization products of fatty oils with various hydrocarbons), in principle the same graphical methods can be developed as have been described for the investigation of hydrocarbon mixtures. However, the construction of useful graphical representations is hampered by the fact that reliable data on physical constants are restricted to the normal saturated fatty acids and their methyl and ethyl esters the synthesis of pure unsaturated fatty acids is already extremely difficult, to say nothing of more complicated cyclic or branched compounds. 

Lavoisier discovered the law of conservation of matter, which states that matter cannot be created or destroyed during chemical reactions or physical changes. This generalization increased chemists efforts to measure the masses of elements in compounds and resulted in two more laws. The law of definite proportions states that the percentage of each element in any sample of a pure compound is always the same. According to the law of multiple proportions, if the mass of one of the elements in two or more compounds of the same elements is held constant, the masses of each other element form a small, whole-number ratio. (Section 3.1)... 

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. 

Biochemistry deals with an enormous number of chemical compounds with widely differing properties. Some are gases, some are liquids and some are solids either crystalline or amorphous. Wide variation in stability is encountered. Obviously no single fractionation technique will be the most effective for the separation of each t q)e. Since the determination of purity requires maximum separating power under conditions of complete stability, a choice of the most effective method must be made. Fortunately, the different methods often supplement each other and wherever possible, more than one method must be applied. Measurement of physical constants with adequate precision always must be done. Agreement of physical constants implies a degree of purity but is not vigorous proof that the substance is pure. 

A guide to the analysis, as well as to the ordinary physical properties of pure oxygenated compounds is provided for in a work by Huntress and Mulliken (52). The physical constants (njy, b.p., and m.p.) with references to the original literature for a selected number of organic peroxides are given by Mesrobian (72). 

A molecule is the smallest entity of a pure compound that retains its characteristic chemical properties, and consequently has constant mass and atomic composition. It is an assembly of nonmetallic atoms held together into specific shapes by covalent bonds. As much as a car is a single unit made up of many parts, a molecule is a unit made up of atoms bonded around each other in certain fixed geometries. Shapes influence the physical and chemical properties and consequently much of the chemistry of a molecule. 

Octamethylpyrophosphoramide, when very pure, is a colorless somewhat viscous oil with the following physical constants b.p. 120 to 122° at 0.5 mm. 154° at 2 mm. sp. gr., 1.09. Normally the color is yellow to amber. The compound is miscible with water, soluble in most organic solvents, and insoluble in higher aliphatic hydrocarbons. It is a very toxic substance with a MLD50 to white mice of 17 mg./kg. by intraperitoneal injection. 

DIPPR Pure Compound Database Dortmund Data Bank Enzyme Nomenclature Database EDM Reference Spectra Databases EIZ Chemie Berlin EIZ Karlsruhe-ICSD Eundamental Physical Constants GmeUn... 

CLASSIFICATIONS OF MATTER (SECTION 1.2) Matter exists in three physical states, gas, liquid, and solid, which are known as the states of matter. There are two kinds of pure substances elements and compounds. Each element has a single kind of atom and is represented by a chemical symbol consisting of one or two letters, with the first letter capitalized. Compounds are composed of two or more elements joined chemically. The law of constant composition, also called the law of definite proportions, states that the elemental composition of a pure compound is always the same. Most matter consists of a mixture of substances. Mixtures have variable compositions and can be either homogeneous or heterogeneous homogeneous mixtures are called solutions. 

For experiments in which a compound is isolated from a particular source and is not prepared from other reagents, some information described in this section will not be applicable. Such experiments are called isolation experiments. A typical isolation experiment involves isolating a pure compound from a natural source. Examples include isolating caffeine from tea or isolating cinnamaldehyde from cinnamon. Although isolation experiments require somewhat different advance preparation, this advance study may include looking up physical constants for the compound isolated and outlining the isolation procedure. A detailed examination of the separation scheme is important here because it is the heart of such an experiment. 

Presently such errors are no longer admissible because, in many cases, it is easy to obtain the necessary pure samples from different sources, as for example from the Chemical Division of the National Bureau of Standards, in Washii ton, or from the National Chemical Laboratory in Teddington (England) it Is also easy to find out what is known about tbe methods of purification and the numerical values of physical constants in my own book. Physicochemical Constants of Pure Organic Compounds. published by Elsevier (Amsterdam - New York) in 1950, to which a Supplement of Addenda and Corrigenda will soon be published. 

The goal of synthetic organic chemistry is twofold how to prepare the desired compound with the highest yield, and how to obtain that compound in the purest form possible. Often these two goals coincide the best way to get a pure product is to make it with a high yield. The reaction product will then already have a high content of the desired product, which facillitates further purification. The desire for very pure products is based in the first place on the need to determine the exact chemical composition and some characteristic physical constants. But of course, for whatever application the product is destined, a pure form is essential to get a reliable test of its performance. 

No completely accurate and specific analytical method is available for the determination of glucuronic acid and development of such a method would be of tremendous value in studies of the chemistry and physiology of this compound. At the present time isolation of the crystalline product or one of its derivatives appears to be the only reliable method of demonstrating its presence. Table I lists the physical constants of pure compounds that have been isolated. Analytical methods now available are helpful, but interfering substances often lead to erroneous conclusions when these methods are used for analysis of biological fluids or synthetic reaction mixtures. 

It is important to point out that pure compounds are necessary for the measurement of physical constants and spectra, and that the isolation of such substances is usually very difficult. Gas chromatography is very suitable for the determination of purity, because it can also be used for the isolation of a pure individual compound (see p. 133). 

Timmermans, J. "Physical-Chemical Constants of Pure Organic Compounds," Vol. 1-2, Elsevier, Amsterdam, 1950-65. 

Hydrogen peroxide, when pure, is an almost colourless (very pale blue) liquid, less volatile than water and somewhat more dense and viscous. Its more important physical properties are in Table 14.11 (cf. H2O, p. 623). The compound is miscible with water in all proportions and forms a hydrate H2O2.H2O, mp —52°. Addition of water increases the already high dielectric constant of H2O2 (70.7) to a maximum value of 121 at 35% H2O2, i.e. substantially higher than the value of water itself (78.4 at 25°). 

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