Phenyl Group

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A phenyl group is the functional group C₆H₅. It is the portion of an organic molecule that is derived from a benzene molecule, C₆H₆, by removal of a hydrogen atom. The term phenyl is used when a benzene ring is connected to a chain of six or more carbonatoms. If there are fewer than six carbon atoms in the chain, the compound is named as a substituted benzene. The phenyl group can be abbreviated in chemical structures as -Ph or sometimes as the Greek letter phi, -ϕ.

Benzene is a cyclic compound containing six carbon atoms and six hydrogen atoms. The molecular formula for benzene, C₆H₆, was determined soon after it was isolated by Michael Faraday in 1825 from the oily deposits removed from London’s gas pipes. Later in 1834, benzene was found by Mitscherlich to be the product obtained from various chemical reactions involving gum benzoin, a fragrant medicinal ointment. In the manuscript describing his experiments, Mitscherlich suggested the compound be called benzin. Liebig, who edited the paper, renamed the compound benzol based on the German word for oil, öl. English and French chemists eventually changed the -ol ending to -ene, resulting in the name benzene. The reasoning was that the -ol suffix indicates an alcohol group whereas the -ene is used for compounds that contain double bonds. The term pheno, based on the Greek word, phainein, meaning “to shine” was proposed by Auguste Laurent in 1837. This suggestion was never

accepted, but it resulted in the term phenyl being commonly used when referring to the -C₆H₅ group.

During the early nineteenth century, benzene was well established in a number of industrial processes, but its chemical structure had not been determined. August Kekulé in 1858 had developed the theory, which later proved true, that carbon atoms could be connected by bonds to form chains like those that make up alkanes. He then directed his attention to the structure of benzene. As the story goes, in 1865, Kekul´ was writing at his desk one night and could not concentrate on the problem. He started to gaze into the fire in his fire place, when he eventually fell asleep. August had a dream of carbon atoms dancing before him, slowly forming chains and the chains turning into snakes. One snake’s head grabbed its own tail and formed a ring. Kekul´ quickly woke up and spent the rest of the night developing his proposal that carbon atoms can be connected in a manner that forms rings. He combined this idea with the molecular formula for benzene, C₆H₆, and suggested the structure. Kekul´ also knew that benzene does not undergo the same types of reactions as simple chains of carbon and hydrogen atoms and that it has a greater chemical stability. In 1872, he proposed that the double bonds in benzene are not situated between any two carbon atoms but move around the ring. The best way to represent benzene is by what are called the Kekul´ resonance forms or a hexagon with a circle in it (Figure 1).

In the left-hand representations, the benzene molecule is drawn as a hexagon with alternating single and double bonds. This is a shorthand method of drawing compounds used by chemists. A carbon atom is represented by an intersection of two straight lines or at the end of a line and the correct number of hydrogen atoms to give each carbon atom four bonds is implied but not drawn. The two equivalent Kekul´ resonance forms indicate that the bond between any two carbon atoms in benzene is a combination of a single bond and a double bond. The hexagon with a circle means

KEY TERMS

Arene— A compound that has a chain of carbon atoms connected to a benzene ring.

Phenyl group— The name given to the portion of an organic molecule that is derived from a benzene ring by removal of a hydrogen atom (-C₆H₅) and is used when a benzene ring is connected to a chain of six or more carbon atoms.

that the “real” structure for benzene is a combination of the two Kekul´ resonance forms. This combined bond form is called resonance. The use of hexagon with a circle to represent benzene, was first suggested by Johannes Thiele in 1899. Today, it is the representation most preferred by chemists. Benzene and other cyclic compounds that have alternating single and double bonds which oscillate, such as naphthalene and anthracene, are called aromatic hydrocarbons.

Those compounds that have a chain of carbon atoms connected to a phenyl group are called arenes. Arenes are named by two different methods depending on the length and complexity of the carbon atom chain. If the chain of carbon atoms contains six or fewer carbon atoms, then the arene is named as a benzene ring with the appropriate alkyl group. For example, a benzene ring connected to a chain of two carbon atoms is named ethylbenzene. However, if the carbon atom chain is longer than six carbon atoms or is rather complex, then the arene is designated as a phenyl group attached to the alkane chain. The compound, 3-phenyl-octane, consists of a chain of eight carbon atoms with the third carbon atom bonded to a benzene ring. No matter what the length of the carbon atom chains; if the compound contains two or more benzene rings, it is named as a phenyl-alkane. A central carbon atom bonded to two benzene rings and to two hydrogen atoms would be called diphenylmethane.

The phenyl group is an important structural unit in many natural and synthetic or man-made chemicals. It is an integral part of the molecular framework of many drugs, herbicides, dyes, plastics, perfumes, and food flavorings. Phenylephrine is used in the treatment of asthma, as well as in conjunction with various anesthetics to increase their time of activity. The melon flavored compound 2-phenyl propionaldehyde contains a benzene ring in its molecular framework. Phenylethyl alcohol is used routinely in the perfume industry because of its rose fragrance. Various pesticides such as the phenylureas and phenylcarba-mates contain phenyl rings and many of the preparations of indigo dye, a dye used in making blue jeans, use phenyl containing compounds such as N-phenylglycine.