The sulfur atom belongs to the Group VIa of elements, the same as the oxygen atom. However, it possesses unique properties which are not found in the oxygen atom. For example, the sulfur atom has a lone pair of electrons making the bivalent sulfur atom soft with a strong nucleophilicity. However, it has poor affinity for hard acids such as protons, and many different types of reactions can easily cleave the carbon-sulfur bond. The diverse valency is the major feature that can be listed under the important characteristics of the sulfur atom. Due to their diverse characteristics, organic sulfur compounds play important roles in organic synthesis. Sulfur containing heterocycles generally appear to have the unique characteristic functions such as biologically active materials applied in the fields of pharmacology and agriculture. Therefore, R&D studies are vigorously taking place in order to introduce sulfur atoms into cyclic structures.
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