Organo-sulfur, organo-phosphorus, and organo-metal compounds

Organic compounds containing sulfur are important in drugs and fundamental research and play a vital role in the metabolic processes that sustain life. The simplest organo-sulfur compounds are thiols, or mercaptans, which are similar to alcohols. The difference between organo-sulfur compounds and alcohols is that in place of the sulfur in a molecule ‘of an organo-sulfur compound, an alcohol molecule has oxygen. One result is that thiols are more acidic and volatile than the corresponding alcohols. They also have disagreeable odors, including the smell of skunk, garlic, and rotten cabbage.

Reactions of thiols

Oxygen and sulfur are both members of Group 6A in the periodic table so they have many reactions in common. However, during certain chemical reactions involving alcohol, water forms as a by-product, while similar reactions involving thiols produce hydrogen sulfide, a toxic gas. Thiols are easier to oxidize than alcohols, becoming disulfides. (A disulfide is a compound containing two linked sulfur atoms combined with another element.) On further oxidation, a disulfide becomes a sulfonic acid, a type of organic acid used in the manufacture of disinfectants and antiseptics, dyes, and drugs. The formation of disulfides also plays an essential role in living organisms. A disulfide “bridge” links different parts of a protein chain, thus affecting the molecule’s three-dimensional structure, which is crucial to its biological function.

Organo-phosphorus compounds

Organo-phosphorus and organo-nitrogen compounds are similar in many respects, because both phosphorus and nitrogen are in Group 5A of the periodic table. Unlike nitrogen, phosphorus does not form strong multiple bonds with carbon, but it forms very strong double bonds with oxygen. Oxygen, in turn, forms strong bonds with carbon, enabling phosphorus to make strong, indirect bonds with carbon. Another important property of organo-phosphorus compounds is their ability to react with acids. Phosphorus is useful and important in a variety of other organic compounds.

Organo-phosphorus compounds play an important part in the transfer of energy within biological organisms. Adenosine, a substance found in tissue (especially muscle tissue) and cells, is important in muscle contraction and the metabolism of sugar. Adenosine triphosphate (ATP), a compound of adenosine and three phosphate groups (organo-phosphorus compounds), is the most important carrier of energy in living cells. The transfer of a phosphate group to another molecule releases large amounts of energy. During this chemical process, the ATP becomes ADP (adenosine diphosphate). ADP is a compound of adenosine and two phosphorus groups. If this chemical reaction takes place in muscle tissue, the energy released is used for the contraction of muscles. If the chemical reaction with ATP occurs inside a cell, the energy released is used to fuel a metabolic process that sustains life by turning food into energy and living tissue.

Essential though phosphorus is in living systems, it can also harm them. The nerve gases Sarin and Tabun are phosphorus compounds that deactivate an enzyme essential in triggering nerve impulses. As a result, the muscles and metabolic activities inside cells can no longer be “turned on” and the organism dies. Phosphorus-based insecticides such as mal-athion and parathion act in essentially the same way.

Four of the simplest types of organo-sulfur compounds are diagramed. Thiols (top left) contain an -SH group. Sulfides (bottom left) have a sulfur atom bonded to two carbon atoms. Sulfoxides (top right) contain a sulfur atom doubly bonded to an oxygen atom. And sulfones (bottom right) have a sulfur atom linked by two double bonds to two oxygen atoms.

Organo-metal compounds

Certain types of organic compounds containing metals have been known for a long time. Grignard reagents (organic compounds that include magnesium) are important in the synthesis of organic compounds. Organic compounds containing lithium are also well established, the best known being butyl lithium, used in making compounds that are important intermediate steps during the long processes sometimes necessary in making synthetic compounds.

Other useful organo-metallics include aluminum trialkyls. These metal compounds, in conjunction with titanium salts, are used to speed up the chemical processes for making complex organic compounds. Another organo-metal compound is tetraethyl lead, an antiknock agent once added to gasoline but now being phased out because lead discharged in exhaust fumes may be harmful to human health.

Ferrocene is probably the best known of the group of organo-metal compounds called sandwich compounds. Structurally, it consists of an iron atom sandwiched between, and bonded to, two parallel cy-clopentadiene rings, as illustrated in the diagram. Ferrocene can be prepared relatively easily. At room temperature, ferrocene is an orange solid and has a camphorlike smell. It is insoluble in water but dissolves in most organic solvents, such as ethanol and benzene.
The odor of skunks is mainly due to the organo-sulfur compound 1-bu-tanethiol, which is secreted by the animal. Thiols are also found in some plants. Onions, for example, give off 1-propanethioI.