Physical Properties of Organic Compounds

TABLE 2-2 Physical Properties of Organic Compounds iContinued)... 

Predict the influence of hydrogen bonding on the physical properties of organic compounds (Self-Test 19.2). 

The process that is outlined on the next page will help you to predict the physical properties of organic compounds by examining the intermolecular forces between molecules. As you progress through the chapter, referring back to this process will enable you to understand the reasons behind trends in physical properties. 

It was at Columbia that Hammett carried out his research on the physical properties of organic compounds that made him famous. His first accomplishment was his development of a concept now known as the acidity function, a new interpretation of the behavior of acids in concentrated and dilute solutions. He also derived the Hammett equation and Hammett function for organic substances, accomplishments for which he is perhaps best known today. In 1940, he published Uistextbook Physical-Organic Chemistry, which some chemists have called "one of the great textbooks in the history of chemistry."... 

The important physical properties of organic compounds include melting and boiling points and solubility and density. Whenever you compare physical properties of different compounds, stick to compounds with similar molecular weights. 

This time, there were no polemics and no fight over priority (neither of the principals had Keknle s driving ambition to be the first and only inventor of the theory) - in fact, in his snbseqnent papers on stereochemistry, van t Hoff was careful to point out Le Bel s contributions, and may have saved them from obscurity. In both 1874 papers, the authors asserted that the physical properties of organic compounds, especially their optical activity, could be accounted for by specifying that the molecules contained an asymmetric atom corresponding to a tetrahedron surrounded by four different groups. While van t Hoff s paper concentrated on the tetrahedron as the basis for optical activity, Le Bel s paper was more wide-ranging, and allowed other chiral shapes to be considered. 

In a parallel development, structural effects on the chemical reactivity and physical properties of organic compounds were modelled quantitatively by the Hammett equation 8). The topic is well reviewed by Shorter 9>. Hansen 10) attempted to apply the Hammett equation to biological activities, while Zahradnik U) suggested an analogous equation applicable to biological activities. The major step forward is due to the work of Hansch and Fujita12), who showed that a correlation equation which accounted for both electrical and hydrophobic effects could successfully model bioactivities. In later work, steric parameters were included 13). 

From the foregoing you may anticipate that the chemistry of carbon compounds will be largely the chemistry of covalent compounds and will not at all resemble the chemistry of inorganic salts such as sodium chloride. You also may anticipate that the major differences in chemical and physical properties of organic compounds will arise from the nature of the other elements bonded to carbon. Thus methane is not expected to, nor does it have, the same chemistry as other one-carbon compounds such as methyllithium, CH3Li, or methyl fluoride, CH3F. 

A great number of investigations was dedicated to studying the physical properties of organic compounds of germanium, tin and lead. We discuss here only those that include a comparative study of these compounds as well as studies which verify or develop theoretical concepts of their electronic structure. 

This is a continuation of Mr. Dreisbach s compilation of physical properties of organic compounds. Data on 511 cyclic compounds were published in 1955 as Number 15 of the Advances in Chemistry Series under the title of Physical Properties of Chemical Compounds. The present volume includes 476 acyclic compounds. 

When two molecules approach, they attract or repel each other. This interaction can be described fairly simply in the case of atoms (like the noble gases) or simple molecules such as H2 or Cl2. In general, the forces are attractive until the molecules come so close that they infringe on each other s van der Waals radius. When this happens, the small attractive force quickly becomes a large repulsive force, and the molecules bounce off each other. With complicated organic molecules, these attractive and repulsive forces are more difficult to predict. We can still describe the nature of the forces, however, and we can show how they affect the physical properties of organic compounds. 

Hydrogen bonding has a large effect on the physical properties of organic compounds, as shown by the boiling points of ethanol (ethyl alcohol) and dimethyl ether, two isomers of molecular formula C2H60 ... 

A rectangular range of data in an Excel worksheet can be used as a list or database. A database consists of a number of records, each of which can contain a number of fields. For example, a compilation of physical properties of organic compounds, such as the one in the CRC Handbook of Chemistry and Physics, is a database the row of data for a particular compound is a record and the values for the melting point, boiling point, solubility, etc. are the data fields within the record. In Excel, a list or database must be arranged in tabular form, with row or column labels that s the only requirement. 

Gakh, A. A., Gakh, E.G., Sumpter, B.G. and Noid, D.W. (1994). Neural Network-Graph Theory Approach to the Prediction of the Physical Properties of Organic Compounds. J.Chem.Inf. Comput.Sci.,34,832-839. 

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