Theoretical principles of organic chemistry

Subject matter of organic chemistry and classification criteria

Historically, views on the subject matter of organic chemistry have changed. In 1851, A. Kekulé defined it simply as the chemistry of carbon compounds. Later, in 1889, C. Schorlemmer refined this concept, defining organic chemistry as the science of hydrocarbons and their derivatives.

Today, the main classification criterion for organic molecules is the presence of carbon–carbon (C–C) bonds in them. However, there are a number of substances in which such bonds are absent. In these disputable cases, classification relies on established chemical traditions and the properties of a specific substance:

• Classified as inorganic: carbon dioxide ($CO_2$), carbonic acid salts (carbonates), tetrachloromethane ($CCl_4$), and iron pentacarbonyl ($Fe(CO)_5$).
• Classified as organic: methane ($CH_4$), chloroform ($CHCl_3$), and formic acid ($CH_2O_2$).

Organic compounds form the basis of living matter. At the same time, maintaining vital activity requires not just any molecules, but strictly specific types. The study of chemical processes that sustain life is the domain of bioorganic chemistry, which emerged as an independent branch in the mid-20th century.

Structural features and states of aggregation

Compared to inorganic substances, organic matter is characterized by a much greater complexity of molecular composition and chemical formulas. Under normal conditions, the vast majority of these compounds are in a liquid or gaseous state.

Liquid organic substances are widely used by humans as fuels, solvents, food products, and flavorings. Solid organic compounds usually have relatively low melting points. Notably, many of them exhibit pronounced volatility, which gives them a specific odor (e.g., phenol, naphthalene, and iodoform).

A special group consists of polymers (high-molecular-weight compounds). These include the well-known cellulose, starch, rubber, as well as various synthetic and natural fibers. Mixtures based on polymers form plastics.

The most important high-molecular-weight substances are proteins. Their giant molecules feature an ideally reproducible structure. In a protein molecule (such as hemoglobin), thousands of atoms occupy strictly defined, fixed positions. The level of structural organization in proteins even surpasses the structure of crystals, since crystal lattices permit the presence of defects.

Chemical activity and conditions for the existence of life

Organic substances exhibit high kinetic stability. Compared to certain inorganic substances (such as ozone, which decomposes extremely rapidly to $O_2$), organic compounds react much more slowly.

This natural sluggishness of chemical reactions is a fundamental condition for the existence of life on the planet. If organic substances possessed the same high chemical reactivity as inorganic ones, all living organisms would instantly be oxidized by ambient oxygen. Rapid combustion to final products such as water ($H_2O$) and carbon dioxide ($CO_2$) would occur.

To ensure that essential biochemical processes still proceed at the required rate, nature employs biological catalysts—enzymes. It is they that ensure the occurrence of complex in vivo reactions (inside a living organism). In industrial production, the use of various artificial catalysts is also one of the key features of organic chemistry.

Structural theory and the uniqueness of the carbon atom

By the mid-19th century, chemists had discovered and described a huge number of carbon compounds. Their chemical and physical properties radically differed from the properties of compounds of other elements in the periodic table. This colossal diversity required a logical and theoretical explanation.

The foundation for understanding these processes was laid by the Russian chemist A.M. Butlerov. In 1869, he published the theory of the structure of organic compounds, the tenets of which were subsequently fully confirmed and gave a powerful impetus to the development of science.

The key reason for the diversity of organic matter lies in the ability of the carbon atom to form chemical bonds simultaneously with several completely different atoms. While in inorganic chemistry elements often bond with a single type of ligand (e.g., aluminum forms the hydride $AlH_3$ or halides $AlCl_3$, $AlF_3$), carbon easily forms molecules of mixed composition (e.g., $CHBrClF$, $C_3H_3F_5$, $C_2H_3ClFI$).

Classification of carbon atoms by bond type

Depending on their position in the molecule and the number of neighboring carbon atoms with which a direct bond is established, carbon atoms are classified into four types:

1. Primary atom: located at the very end of the carbon chain and bonded to only one neighboring carbon atom.
2. Secondary atom: located within a linear section of the chain, forming bonds with two other carbon atoms.
3. Tertiary atom: arises at a branching point of the molecule, directly bonded to three carbon atoms.
4. Quaternary atom: forms at the point of conditional intersection of two carbon chains and is firmly bonded to four other carbon atoms simultaneously.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Structure and spatial organization of the hemoglobin molecule
• Mechanisms of enzyme action in vivo as biological catalysts
• Historical background and main principles of A.M. Butlerov's theory
• Differences in structure and properties between natural and synthetic polymers
• Oxidation processes of organic compounds and the formation of final combustion products

Other topics in "Chapter 14. Theoretical Foundations of Organic Chemistry"

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