The sulfur atom lies below oxygen in the periodic table. It has an electronic configuration [Ne]3s2-3p4, which is similar to that of oxygen. Oxygen can form a maximum of two bonds with an additional 2 lone pairs.
In contrast, sulfur, due to the existence of the low lying empty 3d orbital, can form an extended octet which can accomodate up to six bonds. A hybridization of the sulfur orbitals of one 3s orbital and three 3p orbitals and two 3d orbitals give a hybridization of d2sp3. This includes six orbitals with identical energies.
Due to the lack of d orbitals on oxygen, it is unable to form an extended octet. Due to the fact that sulfur is able to form an extended octet, it can form a variety of compounds with different oxidation states.
Sulfur forms sulfides with the metals, which are negatively charged sulfur species. Sulfur is a non-metal that has good reactivity. It has many uses in organic chemistry as well as a biological role in the body.
Unlike oxygen, which exists as a diatomic molecule, sulfur exists as a molecule that is composed of eight sulfur atoms or S8. Sulfur participates and finds many uses in organic chemistry.
DMSO or dimethyl sulfoxide is an organic compound which contains sulfur. It is used as a polar solvent for organic reactions. It is a polar solvent due to the polarization of the sulfur-oxygen double bond.
Another compound of sulfur which is an inorganic reagent but finds uses in organic chemistry is thionyl chloride. This is a chlorinating reagent for alcohols and organic acids. It is used to convert the bad leaving group hydroxyl to the better leaving group the chloride ion.
In addition, thionyl chloride is used as a dehydrating agent which removes water from hydrated organometallic complexes that react with thionyl chloride and remove the moisture from the compound.
Sulfur compounds have an important role in biology. Biologically speaking, a sulfur atom is found in two important amino acids. These are cysteine and methionine. These amino acids are incorporated into proteins and enzymes and determine their quaternary structure.
Sulfur-sulfur bonds are important in protein biochemistry as they can form S-S bonds between units of polypeptides. Thus contributing to the quaternary structure of the proteins.
In addition, sulfur is incorporated in the structure of the cytochromes molecules of the respiratory chain. They are incorporated there as sulfur-iron clusters. They participate in the oxidation-reduction reactions of the oxidative phosphorlyation process.
Sulfoneamides are compounds of sulfur which are important from the pharmacological point of view. They have similar structure to PABA or para amino benzoic acid. PABA is an essential component in the metabolism of bacteria.
Sulfoneamides compete with PABA for the active site of an important enzyme that is necessary for building folic acid for the bacteria. Thus leading to the death of the bacteria. Bacterial resistance emerged against sulfoneamides and so they are not effective anymore.
An application of an organosulfur compound that is used in chemical warfare is bis-(chloroethylsulfide) which is called sulfur mustard. In this compound sulfur has two bonds in addition to two electron pairs. The lone pair on the sulfur exerts neighboring group participation and thus activating the compound for electrophilic attack by bases of the DNA. This kills the cell.
One important compound of sulfur is sulfuric acid. It is prepared industrially from the oxidation of elemental sulfur to give sulfur trioxide, which upon treatment with water gives sulfuric acid.
Sulfur is found naturally as sulfides. Elemental sulfur is obtained from the earth crust by the Frasch process. It includes superheating sulfur by steam then ejecting the sulfur to the surface by air compression.
An interesting compound of sulfur is a polymer with nitrogen which is called thiazyl. This compound is formed by alternating bonds between sulfur and nitrogen. In contrast to the atoms which are nonmetals, this polymer behaves as a semiconductor.