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A phenol is one of a number of chemically active compounds which are found throughout nature, especially in plants. Their molecules each include a hydroxyl functional group (OH) bonded to the ring of an aromatic compound — a molecule that includes at least one ring of carbon atoms. Phenols exhibit a wide range of properties; some are heralded for their health benefits, while others are deadly poisons. Some have important industrial uses as drugs or food additives. The word phenol may also refer to carbolic acid (C6H5OH), the simplest of this group of chemicals.
Phenol is the simplest member of a family of compounds in which an -OH group is attached directly to a benzene ring. Phenol itself is the only one of the family that you are likely to need to know about for UK A level purposes.
There is an interaction between the delocalized electrons in the benzene ring and one of the lone pairs on the oxygen atom. This has an important effect on both the properties of the ring and of the -OH group.
Pure phenol is a white crystalline solid, smelling of disinfectant. It has to be handled with great care because it causes immediate white blistering to the skin. The crystals are often rather wet and discolored.
Phenol is so inexpensive that it attracts many small-scale uses. It once was widely used as an antiseptic, especially as carbolic soap, from the early 1900s through the 1970s. It is a component of industrial paint strippers used in the aviation industry for the removal of epoxy, polyurethane and other chemically resistant coatings. Phenol derivatives are also used in the preparation of cosmetics including sunscreens, hair colorings, and skin lightening preparations. Concentrated phenol liquids are commonly used in the surgical treatment of ingrown toenails to prevent a section of the toenail from growing back. This process is called penalization.
When phenol donates a hydrogen ion to water, the electron pair remaining on the oxygen atom becomes delocalized, meaning that it becomes redistributed into the phenyl ring and cannot be assigned with certainty to any one pair of atoms. This effect is possible because of the π bonds in the phenyl ring (bonds that form between overlapping unhybridized p-orbitals). In cyclohexanol, on the other hand, all of the carbons in the ring structure share single bonds and there are no π bonds, so the negative charge on the oxygen does not become delocalized if the hydrogen is donated to water.
In a phenol extraction, the acidity of the whole mixture is important to effectively extract the DNA and RNA. Chemists recommend that the phenol should have a pH higher than 7 so that the water layer can successfully gather the DNA. At a pH of 4.5, which is already considered acidic, the RNA can already be gathered by the water phase. To change the phenol’s acidity or alkalinity, some chemicals are added, such as the compound tris or N-ethylmorpholine.