What is Phenol?
You wake up with a sore and itchy throat. The first thing you reach for is a bottle of throat spray to relieve those nasty symptoms. But, have you ever wondered what, exactly, is in that throat spray? One active ingredient is phenol, a group of organic compounds whose aromatic ring is bonded to an alcohol group. Now, this family, the phenols, doesn’t mind being called a different name: carbolic acid or benzenol. The molecular formula of phenol is C6H5OH.
The suffix ending -ol in phenol provides a great clue as to what functional group this organic compound belongs to. In chemistry nomenclature, this suffix refers to compounds that contain alcohol (-OH) groups. Phenol is no exception to this rule, as seen by its molecular formula; an alcohol group is present in its structure.
Because phenol contains an alcohol group, it is not only able to form a very strong hydrogen bond with other molecules, but this bond contributes to phenol’s affinity towards polar substances. As a polar molecule, phenol is soluble in water. In other words, phenol and water are friends.
Other physical properties of phenol include its physical state of a colorless liquid and its high boiling point. Keep in mind that those strong hydrogen bonds contribute to phenol’s higher boiling point. At room temperature, the solid form of phenol is white in color. It is classified as having a sweet odor.
What does the structure of phenol look like? Shown in diagram 1, you will notice the presence of the alcohol group (-OH) we discussed earlier, as well as an aromatic ring. Aromatic compoundsare compounds that form a cyclic ring containing double and single bonds. In phenol, this aromatic compound is referred to as benzene. It is worth noting benzene in phenol’s structure because the synthesis of phenol most often begins with the starting use of a benzene molecule.
For example, as shown in diagram 2, benzene utilizes two intermediates, chlorobenzene and sodium phenoxide, to make our friend phenol. Luckily, there is no need to remember the synthesis of phenol. But, it is very useful for us to learn a little bit about phenol’s historical origins in organic chemistry. Although phenol is commonly made in the laboratory using reactions, as shown in diagram 2, sources of phenol are commonly found in nature as well.
Many of the proteins in our body contain phenol in their chemical structure. The hormone serotonin, which gives us a happy feeling, and epinephrine, what gets our adrenaline pumping, both contain phenols. Even our favorite natural sweetener vanilla bean, derived from a compound called vanillin, contains a phenol group. Diagram 3 shows the structure of both hormones as well as the structure of vanillin. Test your knowledge to see if you can locate the phenol group.
Physical properties of Phenols
- Phenols are colourless liquids or crystalline solids but become coloured due to slow oxidation with air.
- Due to the presence of strong intermolecular hydrogen bonding, phenols have a higher boiling point than the corresponding hydrocarbon or aryl halides.
- Due to their ability to form hydrogen bonds with water, phenols are moderately soluble in H2O.
- The phenols are acidic in nature and stronger acids than alcohols. This is due to the fact that the sp2 hybridised carbon of phenol to which −OH is attached, is highly electronegative which causes a decrease in electron density on oxygen. This Increases the polarity of O−H bond and results in an increase in ionisatlon of phenols than that of alcohols.
Moreover, the phenoxide ion so produced is stabilised by the delocalization of charge in phenol.
Note: The presence of electron withdrawing group like NO2 group, increases the acidic strength whereas the electron donating groups like an alkyl group decreases the acidic strength. Therefore, the acidic strength order is