Aromatic Hydrocarbons in Organic Chemistry

1.  Welcome

2. Mahatma Phule A.S.C. College, Panvel Prof. KokilDhananjayNarayan Dept. of Chemistry Class : S.Y.B.Sc. Paper III {Organic Chemistry}

3. Aromatic Hydrocarbons Aromatic hydrocarbons are the aromatic compounds containing carbon and hydrogen. The following are the basic aromatic hydrocarbons. Benzene [C6 H6] The kekule’s structure of benzene are

4. 2) Naphthalene [C10 H8] The resonating structure of naphthalene are Resonance hybrid

5. Acenes:The aromatic bydrocarbons having more than two benzne rings fused together are called as acenes. i) Linear acenes :- When benzene ring fused linearly called as linear acene. e. g. Anthracene Resonance hybrid

6. ii) Angular acenes :- When benzene rings are fused non- linearly called as angular acenes.

7. Arenes :-The aromatic hydrocarbons are also called as arenes. When one of hydrogen of benzene ring is replaced by alkyl, alkenyl, or alkynylgroup, its derivatives are formed. e. g. Areneshaving one benzene ring:- Alkyl arenes :- Examples

8. 2) Alkyl arenes : Examples -CH = CH2 Vinyl group -CH2-CH=CH2 Ally group

9. 3) Alkynylarenes :- B) Arene having two benzene ring :-

10. Preparation of alkyl arenes :- Catlytic Reforming :- When light naphtha fraction i.e. aliphatic hydrocarbons having C6 to C8 carbon of petroleum oil is passed over platinum alumina catulyst at 500ºc and at 25-35 atmospheric pressure, undergoes reactions like cracking, cyclisation and aromatisation to form alkyl arenes. This reaction is called as catalytic reforming.

11. i.e. i) ii)

12. II) Polymerisation :- Acetylene when passed through red hot brick surface at 1500-2000ºc undergoes polymerisation reaction to form benzene.

13. III) High temp Cracking :- Kerusene i.e. middle fraction of petroleum oil when treated with metal oxide (Cr2O3) catalyst at 650ºCundergoes cracking, cyclisation and aromatisation to form arenes

14. IV) By friedel-craft’s reaction :- When benzene is treated with alkyl halide in presence of anhydrous AlCl3a vigorous reaction takes place to form alkyl benzene. This reaction is known as freidelcraftis reaction.

15. Mechanism :- It consists of three steps : Step-i) Alkyl halide reacts with lewis acid to form alkyl carbonation. Step-ii)Alkyl carbocation acts as electrophile attacks π electron of benzene ring to form benzenonium ion which is stabilised by resonance.

16. Step-iii)Benzenoniumion eliminates a proton to give alkyl benzene. [Alkylated product] The reactivity of alkyl halides it benzyl> tertiary> secondary > primary

17. Reactions of alkyl arenes :- Side chain oxidation:- When alkyl arenes are oxidised by oxidising agent KMnO4/NaOH or K2 Cr2 O7/ H2 SO4, then entire side chain gets oxidised to – COOH group .

18. Oxidiation of o,m,p- xylene give corresponding dicarboxylic acids.

19. II) Side chain halogenation of alkyl arenes : These reactions undergoes by free radical mechanism.

20. Mechanism : e.g. Halogenation of toluene It involves 3 steps Step i) Initiation :- [[[ (Free radical)

21. Stepii)Propagation (Attack of Cl. to tolucne)

22. Step iii) Termination :

23. III) Ring halogenation of alkyl arenes:- Addition of halogen to arenes (benzene) by free radical mechanism Mechanizm :

24. 2) Electrophilicsubstitution of halogen in arenes :- Benzene when heated with bromine in presence of metallic iron forms bromobenzene

25. In this reaction Fe (iron) increases the electrophilic character of bromine FeBr3 is act as a lewis acid

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