PART 1 GENERAL ASPECTS OF CHEMICAL STRUCTURE AND REACTIVITY OF ORGANIC COMPOUNDS

Chapter 1

CHEMICAL STRUCTURE AND BONDING

Probably many students have already acquired some knowledge in a beginner's chemistry course. Nevertheless, it would be expedient to recall in this opening chapter some information on general chemistry and the structural theory of organic chemistry that will be helpful in studying the entire course.

1.1. THE STRUCTURAL THEORY OF ORGANIC COMPOUNDS

In the mid-nineteenth century (1858-1860), F.A. Kekule (in Germany) and A.S. Couper (in Scotland) independently formulated the basis of one of the most fundamental theories in modern organic chemistry, the structural theory. It comprises two main ideas.

• Atoms in organic compounds form a definite number of bonds. A measure of bonding ability of atoms is known today as valence. Thus, a carbon atom forms four bonds when it is linked to other atoms and is tetravalent. Nitrogen usually forms three bonds and is trivalent, oxygen forms two bonds and is divalent, hydrogen and halogens are univalent.

• Carbon atoms can bond to each other to form extended chains of atoms linked together.

At that time, in 1861, the Russian chemist А.М. Butlerov made an outstanding contribution to organic chemistry when he developed new ideas in this field. The main of them stated:

The chemical nature of a complex particle is determined by the nature of its elementary constituent parts, their number and chemical structure.

Butlerov stated that all the atoms in a molecule interact with one another, and a mutual influence is the strongest between directly linked atoms. It was Butlerov who introduced the term chemical structure into chemistry.

Butlerov's theory of chemical structure gave a rational explanation of isomerism.

There are many compounds that have the same molecular formula, but they differ in physical and chemical properties due to their different molecular structure. Such compounds are called isomers (from the Greek isos - equal, and meros -part). One of the simplest examples of isomeric compounds is a pair of ethanol and dimethyl ether with the molecular formula C₂H₆O. The former is a liquid, the latter is a gas at room temperature (their boiling points are 78 and -25 °С, respectively). Ethanol reacts with sodium and dimethyl ether does not. It is quite enough to look at the structural formulas for these compounds to reveal their difference.