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How can molecules with the same chemical composition have different structures?
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Louis Pasteur
In the 19th century Louis Pasteur's work established the germ theory of disease, proving that microorganisms cause fermentation and sickness, which led to the development of pasteurization, aseptic techniques in medicine, and effective vaccines for diseases such as rabies. (Painting by Albert Edelfelt, 1885.)
Molecules that share the same chemical composition but possess different structures are known as isomers. This phenomenon occurs because atoms can be arranged in various ways while maintaining the same molecular formula.12
There are two primary ways these structural differences manifest:
- Constitutional Isomers: These molecules differ in their connectivity, meaning the order in which the atoms are bonded to one another is different.1
- Stereoisomers: In these molecules, the connectivity is identical, but the atoms are oriented differently in space.1
Louis Pasteur made a foundational contribution to this field by recognizing that optical activity arises from the dissymmetric arrangement of atoms in molecules. He observed that certain compounds, such as those found in tartaric acid, can exist in mirror-image arrangements that cannot be superimposed on one another—a property known as chirality or molecular asymmetry. This discovery laid the groundwork for the branch of science now called stereochemistry.34