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Aromatic Hydrocarbons Nomenclature of the Derivatives of Benzene Physical Properties of Aromatic Hydrocarbons Preparation of Benzene Reactions of Benzene.

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Presentation on theme: "Aromatic Hydrocarbons Nomenclature of the Derivatives of Benzene Physical Properties of Aromatic Hydrocarbons Preparation of Benzene Reactions of Benzene."— Presentation transcript:

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2 Aromatic Hydrocarbons Nomenclature of the Derivatives of Benzene Physical Properties of Aromatic Hydrocarbons Preparation of Benzene Reactions of Benzene Alkylbenzenes

3 Nomenclature of the Derivatives of Benzene 1.Monosubstituted benzenes (a)For certain compounds, benzene is the parent name and the substituent is simply indicated by a prefix Nomenclature of Benzene Derivatives

4 (b)For other compounds, the substituent and the benzene ring taken together may form a new parent name Nomenclature of the Derivatives of Benzene

5 4 There are three different ways that two groups can be attached to a benzene ring, so a prefix—ortho, meta, or para—can be used to designate the relative position of the two substituents. Nomenclature of Benzene Derivatives ortho-dibromobenzene or o-dibromobenzene or 1,2-dibromobenzene meta-dibromobenzene or m-dibromobenzene or 1,3-dibromobenzene para-dibromobenzene or p-dibromobenzene or 1,4-dibromobenzene

6 5 If the two groups on the benzene ring are different, alphabetize the names of the substituents preceding the word benzene. If one substituent is part of a common root, name the molecule as a derivative of that monosubstituted benzene. Nomenclature of Benzene Derivatives

7 6 For three or more substituents on a benzene ring: 1.Number to give the lowest possible numbers around the ring. 2.Alphabetize the substituent names. 3.When substituents are part of common roots, name the molecule as a derivative of that monosubstituted benzene. The substituent that comprises the common root is located at C1. Nomenclature of Benzene Derivatives

8 Nomenclature of the Derivatives of Benzene

9 When a substituent is one that when taken together with the benzene ring gives a new parent name, that substituent is assumed to be in position 1 and the new parent name is used

10 9 A benzene substituent is called a phenyl group, and it can be abbreviated in a structure as “Ph-”. Therefore, benzene can be represented as PhH, and phenol would be PhOH. Nomenclature of Benzene Derivatives

11 Example 31-1 Draw the structural formula for each of the following compounds: (a)1,3,5-Trichlorobenzene (b)2,5-Dibromophenol (c)2,4-Dinitrobenzoic acid Answer Nomenclature of the Derivatives of Benzene Solution: (a) (b) (c)

12 Check Point 31-1 Give the IUPAC name for each of the following compounds: (a) (b) (c) (d) 31.2 Nomenclature of the Derivatives of Benzene (SB p.150) Answer (a)1,2-Dimethylbenzene (b)1-Methyl-2-nitrobenzene or 2-nitrotoluene (c)3-Bromo-5-chlorobenzoic acid (d)4-Bromo-2,6-dinitrophenol

13 12 Give the IUPAC name for each compound. PhCH(CH 3 ) 2 isopropylbenzene m-butylphenol 2-bromo-5-chlorotoluene

14 13 Which structure matches the given name? o-dichlorobenzene

15 14 4-chloro-1,2-diethylbenzene

16 Physical properties of aromatic hydrocarbons: Physical Properties of Aromatic Hydrocarbons have a fragrant smell generally less dense than water at 20°C usually immiscible with water soluble in organic solvents

17 Physical Properties of Aromatic Hydrocarbons NameFormula Boiling point (°C) Melting point (°C) Density at 20°C (g cm –3 ) Benzene Methylbenzene111– Ethylbenzene136–

18 Physical Properties of Aromatic Hydrocarbons NameFormula Boiling point (°C) Melting point (°C) Density at 20°C (g cm –3 ) 1,2- Dimethylbenzene 144– ,3- Dimethylbenzene 139– ,4- Dimethylbenzene

19 Preparation of Benzene Catalytic reforming converts alkanes and cycloalkanes into aromatic hydrocarbons e.g. Industrial Preparation Catalytic Reforming of Alkanes Pt C 6 H 14  C 6 H 6 + 4H 2 500°C, 10 – 20 atm

20 31.5 Preparation of Benzene (SB p.157) Heating coal in the absence of air gives out coal gas, ammoniacal liquor, coal tar (قطران الفحم) and coke (فحم الكوك) Coal tar is a mixture of many organic compounds, mainly aromatic ones Benzene and methylbenzene can be obtained Destructive Distillation of Coal

21 Preparation of Benzene When sodium benzoate is fused with sodium hydroxide, the carboxylate group is removed and benzene is formed Decarboxylation of Sodium Salt of Benzoic Acid Laboratory Synthesis

22 Preparation of Benzene Phenol vapour is passed slowly over heated zinc dust to produce benzene and zinc(II) oxide Reduction of Phenol

23 Reactions of Benzene Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon) Action of Br 2 in CH 3 Cl 3 (in dark) No reactionBr 2 decolourized and no HBr evolved No reaction with Br 2 alone In the presence of FeBr 3, Br 2 decolourized and HBr fumes evolved Action of H 2 (with Ni catalyst) No reaction1 mole of cyclohexene reacts with 1 mole of H 2 at room temperature 1 mole of methylbenzene reacts 3 moles of H 2 at high temperature and pressure

24 Reactions of Benzene Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon) Action of acidified KMnO 4 No reactionKMnO 4 decolourized No reaction Action of conc. HNO 3 and conc. H 2 SO 4 No reactionCyclohexene oxidized and colour darkens A yellow liquid is formed

25 Methylbenzene is highly unsaturated, but it is resistant to oxidation and addition reactions Reactions of Benzene The resistance of oxidation and addition reactions of aromatic compounds is used to distinguish from unsaturated alkenes Methylbenzene reacts with Br 2 in the presence of FeBr 3. It is through substitution reaction

26 Reactions of Benzene Electrophilic Aromatic Substitution Reactions The electrophiles attack the benzene ring, replacing one of the hydrogen atoms in the reaction Electrophiles are either a positive ion (E + ) or some other electron-deficient species with a partial positive charge (  +) Most characteristic reaction of aromatic compounds: Electrophilic substitution reactions

27 Reactions of Benzene Conc. H 2 SO 4 increases the rate of reaction by increasing the concentration of the electrophile, NO 2 + (nitronium ion) Nitration

28 Reactions of Benzene Benzene reacts with fuming sulphuric(VI) acid at room temperature to give benzenesulphonic acid Sulphonation Heating aqueous solution of benzenesulphonic acid above 100°C, benzene and sulphuric(VI) acid are formed

29 Reactions of Benzene Benzene reacts with chlorine and bromine in the presence of catalysts such as AlCl 3, FeCl 3, FeBr 3, to give chlorobenzene and bromobenzene respectively Halogenation

30 Reactions of Benzene When benzene is warmed with a haloalkane in the presence of catalysts such as AlCl 3, an alkylbenzene is formed Alkylation Important step in chemical industry to produce polystyrene, phenol and detergents

31 Example 31-2 Complete each of the following by supplying the missing reactant or product as indicated by the question mark: (a) (b) (c) Answer Reactions of Benzene Solution: (a) (b)conc. H 2 SO 4, conc. HNO 3 (c)fuming H 2 SO 4

32 Check Point 31-2 (a)One mole of benzene reacts with three moles of chlorine under special conditions. What is the reaction condition required for the reaction? (b)Draw the structure of the reaction product in (a). Answer Reactions of Benzene (a)UV radiation or diffuse sunlight must be present for the free radical addition reaction to take place. (b)

33 Alkylbenzenes Alkylbenzenes are a group of aromatic hydrocarbons in which an alkyl group is bonded directly to a benzene ring also known as arenes e.g.

34 Alkylbenzenes Alkylbenzenes are oxidized to benzoic acid by strong oxidizing agents such as hot alkaline potassium manganate(VII) This type of oxidation is limited to those molecules with

35 Alkylbenzenes Examples:

36 Alkylbenzenes The C = C double bond and acyl groups in the side chain are oxidized by hot alkaline potassium manganate(VII) e.g.

37 Example 31-3 State the conditions under which methylbenzene can be converted in the laboratory to (a)C 6 H 5 CH 2 Cl (b)C 6 H 5 COOH Answer Solution: (a)Reagent: Cl 2 Condition: in the presence of light (b)Reagent: (1) KMnO 4 –, OH –, (2) H 3 O + Condition: heating under reflux Alkylbenzenes

38 Check Point 31-3 Methylbenzene undergoes two different types of chlorination reaction by different mechanisms. Compare the two different types of chlorination reaction in terms of reaction conditions as well as the products formed. Answer Alkylbenzenes

39 Two different types of chlorination reaction of methylbenzene are: Type I: free radical substitution reaction Type II: electrophilic aromatic substitution reaction 31.7 Alkylbenzenes (SB p.164)

40 Orientation effects of substituents in electrophilic aromatic substitution reactions of monosubstituted Benzenes  Alkyl groups and groups with lone pairs (electron donating groups) direct new groups to ortho-, para-positions and speed-up the reaction (i.e. o & p directors and activating groups).  Halogens direct new groups to ortho-, para- positionsthey slow down the  Halogens direct new groups to ortho-, para- positions but they slow down the reaction (i.e. halogens are o & p directors and deactivating groups). reaction (i.e. halogens are o & p directors and deactivating groups).  Electron withdrawing groups such as nitro, nitrile, and carbonyl direct new groups to the meta-position and slow the reaction down (i.e. i.e. m directors deactivating groups). groups to the meta-position and slow the reaction down (i.e. i.e. m directors and deactivating groups).  Thus the order of reactivity of benzene and monosubstituted benzene derivatives in E.Ar.sub. is as in the following chart Substituted benzene with o,p directors > Benzene > Halobenzene derivatives > Substituted benzene with m- directors Substituted benzene with o,p directors > Benzene > Halobenzene derivatives > Substituted benzene with m- directors

41 Orientation effects of substituents in electrophilic aromatic substitution reactions of monosubstituted Benzenes Meta directorsOrtho, para directors -NO2 -SO3H -COOH, -COOR -CHO, -COR -CN -OH, -OR -NH2, -NHR, -NR2 -C6H5 -CH3, -R (alkyl) -F, -Cl, -Br, -I

42 Q1: What are the major products of the following reaction? Q2: What is the final product of the following reaction? a) o-chlorobenzaldehyde b) m-chlorobenzaldehyde c) p-chlorobenzaldehyde d) a,c Q3:Which one of the following compounds has aromatic character?

43 The END


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