Isomer
Just as early chemists were beginning to understand the nature of chemical compounds, a new twist complicated the picture. They found that the molecular formula for two different compounds can be the same. Because the atoms are connected in different ways, a single formula can represent more than one compound, each with its own distinct physical and chemical properties. For example, a molecule containing 6 atoms of carbon and 12 of hydrogen (C6 H 12) can form a number of hydrocarbons, such as cyclohexane and hexene-1, each of which behaves quite differently.
This concept, called isomerism, was first noted by French chemist Joseph Gay-Lussac in 1814. He raised the issue again 10 years later, when he was editing a scientific journal and noticed that two papers used the same formula to refer to different chemicals, yet both were correct. Gay-Lussac concluded that it was the arrangement of a compound's particles that determined the character of a substance.
At first, this revolutionary idea was rejected by the scientific community. A recognized expert in the field, Swedish chemist Jöns Berzelius, flatly refused to believe it. But when he investigated the concept for himself, it proved to be true in other cases. In 1830, Berzelius gave the name isomers (from the Greek for equal parts) to compounds that have identical formulas but different properties, due to different atomic structures. The more atoms a molecule contains, it was found, the greater the number of isomers it can form. Eventually, Gay-Lussac' s discovery helped explain the theory of atomic structure developed by German chemist Friedrich Kekulé around 1860.
In addition to such "structural" isomers, other types of isomers have been found. The existence of nuclear isomers, which differ in their level of energy, was discovered by Soviet physicist Igor Kurchatov (1903-1960) in the 1930s. Another type was found to exist, called "optical" isomers or stereoisomers. French bacteriologist Louis Pasteur discovered in 1848 that two forms of tartaric acid crystals would rotate polarized light in opposite directions. Although the structure of the crystals was identical, their chemical groups had different spatial arrangements. Since then, scientists have learned that many of the body's important substances, such as amino acids, are optical isomers.
These biological isomers have important implication in the development of drugs. In many cases, certain isomeric materials behave differently in the body even though they have the same atomic composition. Unfortunately, the chemical reactions that we use to produce synthetic versions invariably result in a mixture of these isomers. For this reason, learning to separate these stereisomers has been the focus of ongoing research.
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