Aromatic compounds

aromatic compounds

1. By Dr KHALED ALGARIRI CAMS- QASSIM UNIVERSITY October 2020 BENZENE AND ITS DERIVATIVES

2. INTRODUCTION More than 150 years ago, organic chemists realized that yet another class of hydrocarbons exists, one whose properties are quite different from those of aliphatic hydrocarbons. Because some of these new hydrocarbons have pleasant odors , they were called aromatic compounds. Today we know that not all aromatic compounds share this characteristic. Some do have pleasant odors , but some have no odor at all, and others have downright unpleasant odors .

3. Arene A compound containing one or more benzene-like rings. Aryl group A group derived from an arene by removal of a H atom from an arene and given the symbol Ar Benzene is an important compound in both the chemical industry and the laboratory, but it must be handled carefully. Not only is it poisonous if ingested in liquid form, but its vapor is also toxic and can be absorbed either by breathing or through the skin. Long-term inhalation can cause liver damage and cancer.

4. CHEMSITRY OF BENZENE COMPUND Benzene, the simplest aromatic hydrocarbon, was discovered in 1825 by Michael Faraday (1791–1867). Its structure presented an immediate problem to chemists of the day. Benzene has the molecular formula C6H6, and a compound with so few hydrogens for its six carbons. chemists argued, should be unsaturated.But benzene does not behave like an alkene (the only class of unsaturated hydrocarbons known at that time). Whereas 1-hexene, for example, reacts instantly with Br2 ,benzene does not react at all with this reagent. Nor does benzene react with HBr , H 2 O/H 2 SO 4 , or H 2 , all reagents that normally add to carbon–carbon double bonds

5. CHEMSITRY OF BENZENE COMPUND Kekulé’s Structure of Benzene The first structure for benzene was proposed by Friedrich August Kekulé in 1872 and consisted of a six-membered ring with alternating single and double bonds, with one hydrogen bonded to each carbon. The major objection was its failure to account for the unusual chemical behavior of benzene. If benzene contains three double bonds, Kekulé’s critics asked, why doesn’t it undergo reactions typical of alkenes? Resonance Structure of Benzene

6. NAMES OF AROMATIC COPMPOUNDS One Substituent Monosubstituted alkylbenzenes are named as derivatives of benzene — for example, ethylbenzene . The IUPAC system retains certain common names for several of the simpler monosubstituted alkylbenzenes , including toluene and styrene The substituent group derived by loss of an H from benzene is called a phenyl group, C 6 H 5 - , the common symbol for which is Ph -

7. B. Two Substituents When two substituents occur on a benzene ring, three isomers are possible. We locate the substituents either by numbering the atoms of the ring or by using the locators ortho ( o ) , meta ( m ), and para ( p ). When one of the two substituents on the ring imparts a special name to the compound (for example, -CH3, - OH, -NH2, or - COOH), we name the compound as a derivative of that parent molecule and assume that the substituent occupies ring position number 1. NOMENCLATURE OF AROMATIC COPMPOUNDS

8. C. Three or More Substituents When three or more substituents are present on a benzene ring, specify their locations by numbers. If one of the substituents imparts a special name, then name the molecule as a derivative of that parent molecule. If none of the substituents imparts a special name, then locate the substituents, number them to give the smallest set of numbers, and list them in alphabetical order before the ending “benzene.”

9. Write names for these compounds . Strategy First check to see if one of the substituents on the benzene ring imparts a special name. If one of them does, then name the compound as a derivative of that parent molecule. Solution (a) The parent is toluene, and the compound is 3-iodotoluene or m- iodotoluene . (b) The parent is benzoic acid, and the compound is 3,5-dibromobenzoic acid. (c) The parent is aniline, and the compound is 4-chloroaniline or p- chloroaniline .

10. Characteristic Reactions of Benzene and Its Derivatives ? Halogenation In the presence of an iron catalyst, chlorine reacts rapidly with benzene to give chlorobenzene and HCl : Treatment of benzene with bromine in the presence of FeCl3 results in formation of bromobenzene and HBr .

11. B. Nitration When we heat benzene or one of its derivatives with a mixture of concentrated nitric and sulfuric acids, a nitro -NO2 group replaces one of the hydrogen atoms bonded to the ring. C. Sulfonation Heating an aromatic compound with concentrated sulfuric acid results in formation of an arenesulfonic acid, all of which are strong acids, comparable in strength to sulfuric acid.

12. PHENOLS A phenol is an organic compound in which an -OH group is attached to a carbon atom that is part of an aromatic carbon ring system .

13. Nomenclature for Phenols The IUPAC rules for naming phenols are simply extensions of the rules used to name benzene derivatives with hydrocarbon or halogen substituents .The parent name is phenol. Ring numbering always begins with the hydroxyl group and proceeds in the direction that gives the lower number to the next carbon atom bearing a substituent. The numerical position of the hydroxyl group is not specified in the name because it is 1by definition.

14. Methyl and hydroxy derivatives of phenol have IUPAC-accepted common names.Methylphenols are called cresols . The name cresol applies to all three isomeric methylphenols . For hydroxyphenols , each of the three isomers has a different common name

15. Physical and Chemical Properties of Phenols Phenols are generally low-melting solids or oily liquids at room temperature. Most of them are only slightly soluble in water. Many phenols have antiseptic and disinfectant properties.

16. The similarities and differences between the Phenols and Alcohols 1. Both alcohols and phenols are flammable. 2. Dehydration is a reaction of alcohols but not of phenols; phenols cannot be dehydrated. 3. Both 1° and 2° alcohols are oxidized by mild oxidizing agents. Tertiary (3°) alcoholsand phenols do not react with the oxidizing agents that cause 1° and 2° alcohol oxidation.Phenols can be oxidized by stronger oxidizing agents. 4. Both alcohols and phenols undergo halogenation in which the hydroxyl group is replaced by a halogen atom in a substitution reaction

17. Acidity of Phenols Phenols are weak acids, with p Ka values of approximately 10 . Most phenols are insoluble in water, but they react with strong bases, such as NaOH and KOH, to form water-soluble salts. Most phenols are such weak acids that they do not react with weak bases such as sodium bicarbonate; that is, they do not dissolve in aqueous sodium bicarbonate .

18. Phenols and Chemistry of life Dilute (2%) solutions of phenol have long been used as antiseptics. Concentrated phenol solutions, however, can cause severe skin burns. Today, phenol has been largely replaced by more effective phenol derivatives such as 4-hexylresorcinol. The compound 4-hexylresorcinol is an ingredient in many mouthwashes and throat lozenges. The phenol derivatives o- phenylphenol and 2-benzyl-4-chlorophenol are the active ingredients in Lysol, a disinfectant for walls, floors, and furniture in homes and hospitals. A number of phenols found in plants are used as flavoring agents and/or antibacterials . Included among these phenols a

19. A number of phenols possess antioxidant activity . An antioxidant is a substance that protects other substances from being oxidized by being oxidized itself in preference to the other substances. An antioxidant has a greater affinity for a particular oxidizing agent than do the substances the antioxidant is “protecting”; the antioxidant, therefore, reacts with the oxidizing agent first. Many foods sensitive to air are protected from oxidation through the use of phenolic antioxidants. Two commercial phenolic antioxidant food additives are BHA (butylated hydroxy anisole) and BHT (butylated hydroxy toluene ) Phenols and Chemistry of life

20. Vanillin, which gives vanilla its flavor , is extracted from the dried seed pods of the vanilla orchid. Natural supplies of vanillin are inadequate to meet demand for this flavoring agent. Synthetic vanillin is produced by oxidation of eugenol . Phenols and Chemistry of life

21. Thymol , obtained from the herb thyme, possesses both flavorant and antibacterial properties. It is used as an ingredient in several mouthwash formulations. Eugenol is responsible for the flavor of cloves. Dentists traditionally used clove oil as an antiseptic because of eugenol’s presence; they use it to a limited extent even today. Isoeugenol , which differs in structure from eugenol only in the location of the double bond in the hydrocarbon side chain, is responsible for the odor associated with nutmeg Phenols and Chemistry of life

22. Capsaicin, for Those Who Like It Hot Capsaicin, the pungent principle from the fruit of various species of peppers ( Capsicum and Solanaceae ), was isolated in 1876, and its structure was determined in 1919 . Capsaicin contains both a phenol and a phenol ether.

23. The inflammatory properties of capsaicin are well known; the human tongue can detect as little as one drop in 5 L of water. We all know of the burning sensation in the mouth and sudden tearing in the eyes caused by a good dose of hot chili peppers. For this reason, capsaicin containing extracts from these flaming foods are used in sprays to ward off dogs or other animals that might nip at your heels while you are running or cycling .

24. Home work Discuss the following 1-Hybridization of carbon; properties of sp3 , sp2 , sp . 2-Carbon as basic element of life: Electronic structure of carbon and valency 3- Covalent bonds of carbon; sigma bonds and pi bonds . 4-Hybridization of carbon in simple organic compounds; CH4, C2H4,C2H2 . 5- Delocalization and pi bonds; C 6 H 6 Deadline is this Thrusday 29 October 2020

25. Thank you for your attention!