Natural sources of phenols

Phenols are common in nature; examples include tyrosine, one of the standard amino acids found in most proteins; epinephrine (adrenaline), a stimulant hormone produced by the adrenal medulla; serotonin, a neurotransmitter in the brain; and urushiol, an irritant secreted by poison ivy to prevent animals from eating its leaves. Many of the more complex phenols used as flavourings and aromas are obtained from essential oils of plants. For example, vanillin, the principal flavouring in vanilla, is isolated from vanilla beans, and mehyl salicytate, which has a characteristic minty taste and odour, is isolated from wintergreen. Other phenols obtained from plants include thymol, isolated from thyme, and eugenol, isolated from cloves.

Revision exercises

Ex.1. Answer the following questions:

1. What is the definition of phenols? 2. What is the common feature between phenols and alcohols? 3. What are the physical properties of phenols? 4. Where are phenols used in industry and household utilities production? 5. What are the natural sources of phenols?

Ex.2. Match the words with their definitions:

1. caustic	a. any of a large class of volatile odoriferous oils of vegetable origin that give plants their characteristic odours and often other properties from different parts of plants (flowers, leaves, or bark) by steam distillation, expression, or extraction that are usually mixtures of compounds, and that are often used in the form of essences in perfumes, flavorings, and pharmaceutical preparations;
2. dye	b. to stop from happening and existing;
3. generic	c. joined together, especially in pairs; acting or operating as if joined;
4. essential oils	d. able to destroy or burn something by chemical action;
5. prevent	e. related or having the rank of a biological genus;
6. conjugate	f. a soluble or insoluble coloring material.

Ex.3. Say whether the following statements are true or false:

1.The ability of phenols to form strong hydrogen bonds also enhances their solubility in water. 2. Many of the more complex phenols used as flavourings. 3. Like other alcohols, phenols undergo dehydration. 4. Benzene is derived from chlorobenzene by a variety of methods. 5. Phenols are rarely used in household products and as intermediates for industrial synthesis. 6. Phenols occur either as colourless liquids or white solids at room temperature.

Ex.4. Insert the necessary word:

1. Phenols are compounds that have an OH group directly attached to an aromatic ring. 2. Therefore, the oxygen is sp3 hybridised and the aryl carbon is sp2 …. 3. Although phenols share some characteristics with alcohols, they have distinct properties and reactions that set them apart from that functional group. 4. Phenols can participate in intermolecular hydrogen … that means that they have moderate water … and have higher boiling points than aromatic compounds lacking the phenolic group. 5. Phenols are weakly …, and in aqueous solution an equilibrium exists between the phenol and the phenoxide ion. 6. When treated with a base, the phenol gets converted to the phenoxide ion. The phenoxide ion is stabilised by resonance and delocalisation of the negative charge into the ring, therefore phenoxide ions are weaker bases than alkoxide ions. 7. This means that phenols are more acidic than alcohols, but less acidic than carboxylic acids. 8. They are useful reagents in organic synthesis. 9. The ability of phenols to form strong hydrogen bonds also enhances their … in water. 10. Phenol … to give a 9.3 percent solution in water, compared with a 3.6 percent solution for cyclohexanol in water. 11. The association between water and phenol is unusually strong; when crystalline phenol is left out in a humid environment, it picks up enough water from the air to form liquid droplets. 12. Synthesis of phenols: most of the phenol used today is produced from benzene, through either hydrolysis of chlorobenzene or oxidation of isopropylbenzene (cumene). 13. Hydrolysis of chlorobenzene (the Dow process).: benzene is easily converted to chlorobenzene by a variety of methods, one of which is the Dow process. 14. Chlorobenzene is hydrolyzed by a strong base at high … to give a phenoxide salt, which is acidified to phenol. 15. Oxidation of isopropylbenzene: benzene is converted to isopropylbenzene (cumene) by … with propylene and an acidic catalyst. 16. … yields a cumene hydroperoxide which undergoes acid-catalyzed rearrangement to phenol and acetone. 17. Although this process seems more complicated than the Dow process, it is advantageous because it produces two … industrial products: phenol and acetone. (valuable, hybridized, temperatures, solubility×2, acidic, dissolves, charge, bonding, treatment, oxidation).

Ex.5. Read the text and summarize the chemical properties of phenols: