Benzene and its Derivatives along with its structure and uses

Benzene ppt

1. BENZENE AND ITS DERIVATIVES PREPARED BY : Ms . Kadam A. J. Dept: Pharmaceutical Chemistry PRES’s Institute of Pharmacy, Loni 1

2. UNIT I Benzene and its derivatives Analytical, synthetic and other evidences in the derivation of structure of benzene, Orbital picture, resonance in benzene, aromatic characters, Huckel’s rule Reactions of benzene - nitration, sulphonation, halogenation reactivity, Friedelcrafts alkylation- reactivity, limitations, Friedelcrafts acylation. Substituents, effect of substituents on reactivity and orientation of mono substituted benzene compounds towards electrophilic substitution reaction Structure and uses of DDT, Saccharin, BHC and Chloramine

3. Content: Physicai Properties Of Benzene N o m e n c l a t u re Structure Of Benzene Molecular Formula Resonance Hybrid Structure Of Benzene Huckel’s Rule And Aromaticity Reactions Of Benzene Friedal Craft Acylation Friedal Craft Alkylation

4. Physicai properties of Benzene Benzene is a colourless or light yellow It is liquid at room temperature. It has a sweet odour and is highly flammable. Natural sources of benzene include volcanoes and forest fires. Benzene is also a natural part of crude oil, gasoline, and cigarette smoke. IUPAC name Benzene Cyclohexa-1,3,5-triene;1,3,5- Cyclohexatriene 2 Space-filling model Ball and stick model

5. Benzene is an organic chemical compound consisting of carbon and hydrogen atoms with alternating double bonds. As it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon. The chemical formula of benzene is C 6 H 6 , so it consists of six carbon atoms and six hydrogen atoms. joined in a ring with one hydrogen atom attached to each.

6. N O M E N C L A T U RE Monosubstituted alkylbenzenes are named as derivatives of benzene. For example, ethylbenzene. The IUPAC system retains certain common names for several of the simpler monosubstituted alkyl benzenes CH 2 CH 3 CH 3 CH=CH 2 T o l u e n e E t h y l b e n z e n e S t y r e n e

7. Common names for some monosubstituted benzenes are as follows P h e n o l A n i s o l e A n ili n e B e n z a l d e h yd e B e n z o i c a c i d Phenyl group (C 6 H 5 - or Ph-): Derived by loss of an H from benzene, OCH 3 O C - O H NH 2 O H O C - H C H 6 5 1 - P h e n y l c y c l o h e x e n e 4 - P h e n y l - 1 - b u t e n e P h e n y l g r o u p 1 2 4 3

8. W hen t w o substi t ue n ts occur o n a be n z e ne ring, thr e e isomers are possible, they may be located by numbering the atoms of the ring or using the locators ortho (o), meta (m), and para (p) C OO H B r CH 3 CH 3 C H 2 C H 3 1 2 - B r o m o b e n z o i c a c i d ( o - B r o m o b e n z o i c a c i d ) 1 , 3 - D i m e t h y l b e n z e n e ( m - X y l e n e ) 1 - C h l o r o - 4 - e t h y l b e n z e n e ( p - C h l o r o e t h y l b e n z e n e ) 1 2 2 2 3 3 4 1 C l

9. STRUCTURE OF BENZENE Facts that supported Kekul e Formula 1) Molecular formula 2) Open Chain Structure not accepted 3) Evidences in favor of Ring Structure Catalytic Hydrogenation of benzene yields cyclohexane Benzene yields one mono substitution product: Benzene forms 3-di and trisubstituted product which can be explained on basic of ring structure. 4) Resonance hybrid Structure 5) Molecular Orbital Structure 8

10. MOLECULAR FORMULA 1) Benzene has molecular formula- C 6 H 6 . From its elemental composition and molecular weight, benzene was known to contain 6-C and 6-H atoms. Benzene has molecular formula C 6 H 6 as compared to hexane C 6 H 12 which confirms that benzene is highly unsaturated than hexane.From this, it was concluded that 6-C atoms in the benzene were linked by double or triple bonds so as to form a straight chain or closed ring as proposed by Kekulè. C C C C C H H C H H H H A Ke k u lŭ stru c tu re sh o w in g all ato m s A Ke k u lŭ stru ctu re a s a li n e - a n g l e f o r m u l a

11. 2) OPEN CHAIN STRUCTURE NOT ACCEPTED 16 The possible open chain structure for benzene could have been CH 2 = CH − C ≡ C − CH = CH 2 CH 3 − C ≡ C − C ≡ C − CH 3 CH ≡ C − CH 2 − CH 2 − C ≡ CH structure I structure II - structure III All this structures were ruled out because benzene didn’t give the usual reactions of alkenes, alkynes

12. Fo r example 18 when abo v e open chain struct u r e d compounds containing double and triple bonds are added to tetra c hl o r i de s o l u t i o n o f b r o m i n e in which is red in colour c a r b on beco m es colourless but benzene do not give this reactions benzene can not have open chain structure .

13. 3) EVIDENCE IN FAVOR OF RING STRUCTURE 19 a) Catalytic Hydrogenation of benzene yields Cyclohexane + 3H 2 Since, hydrogenation is not bringing about any major structural change in carbon frame. It proves the presence of closed ring of 6-C atoms in Benzene molecule. Pt/Ni Δ Ben z ene Cyclohexane

14. b) Benzene yields one mono substitution product: If bromination is done, only one bromobenzene is obtained because 1-H atom is replaced by bromine. It is same in the case of Chlorobenzene and Nitrobenzene. This proves that each Hydrogen must be exactly equidistant to other hydrogen since the replacement of any Hydrogen gives the same product and suppose we consider the open chain structure it would yield the isomeric monoderivetives, as all the Hydrogen atoms are not equal as shown in struters I,II & III This could be possible only if 6 carbons in benzene are joined to each other to form a closed ring and one hydrogen atom is attached to each carbon. 20

15. C ) Benzene Forms 3-di And 3-trisubstituted Products Which Can Be Explained On Basic Of Ring Structure. Positions 2 And 6 Are Same (Ortho Product) Positions 3 And 5 Are Same (Meta Product) Position 4 i s Para Position 21

16. Benzene forms 3-di and trisubstituted product which can be explained on basic of ring structure. + 3Br 2 FeBr 3 Br Positions 2 and 6 are same (ortho product) Positions 3 and 5 are same (meta product) therefore only three dibromobenzene are possible . Straight Chain structure will give more than 3 di or trisubstituted products. Br Br Br Br Br

17. RESONANCE HYBRID STRUCTURE OF BENZENE The above Kekulè structures of benzene differ in position of electrons.Benzene is a hybrid of structures I & II are exactly equivalent and have same stability and they make equal contribution to the hybrid structure . The resonance stabilization energy which is responsible for the unusual stability of benzene can be calculated from the measurement of heat of combustion or heat of hydrogenation 16 H C C C C C C H H H C C C C C H C H H H H H H H

18. STABILITY OF BENZENE Stability of benzene can be explained in the following way Heat of Hydrogenation is the quantity of heat evolved when mole of an unsaturated compound is hydrogenated.Addition of Hydrogen to a double bond is an exothermic reaction. As heat is given out in hydrogenation means the product in each case is more stable than the original one. + H 2 + 28.6 kcal/mol + 2H 2 + 55 kcal/mol + 3H 2 + 50 kcal/mol 17 Cyclohexane C y c l ohex a d i ene Cyclohexane Cyclohexene Cyclohexane Ben z ene

19. M OLECULAR O RBITAL S TRUCTURE By the X ray diffraction mechanism ,it is proved that benzene consists of planar hexagon of 6-c atoms havinh all c-c bonds equal in length i.e. 1.4 Angstron c-c-c bond angle is 120 degrees . Therefore, it can be proved that each of this 6-c in benzene ring is in the state of trigonal hybridization.The ring system is costructed from six sp 2 -hybridized carbons by the overlapping of two hybrid orbitals each to form sigma bonds . 25

20. The structure of benzene molecule is best described in terms of molecular orbital treatment theory. According to this theory, all the C-atoms in benzene are sp 2 -hybridized. Remaining six sp 2 -orbital of six C-atoms overlap with 1s orbital of six H-atoms individually to form six sigma bonds. 26

21. HUCKEL’S RULE AND AROMATICITY 27 The complete delocalization of π electrons caused by side-side overlapping gives benzene aromatic character.An aromatic compound must contain 4n+2 electrons (n=0, 1, 2, and so forth). Cyclic, planar and completely conjugated compounds that contain 4n π electrons are especially unstable, and are said to be antiaromatic. Benzene is aromatic and especially stable because it contains 6 π electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4π electrons.

22. Characteristic reaction of benzene involves substitution in which resonance stabilized ring is preserved.This reaction is called Aromatic Substitution. Compared to sigma bonds pi electrons are loosely held and are available to the reagent i.e. seeking electrons.So typical reaction of benzene is electrophilic substitution reaction, where hydrogen of aromatic ring is substituted by an atom or group A r - H  A r - X Reactions of Benzene 28

23. G ENERAL MECHANISM OF ELECTROPHILIC SUBSTITUTION REACTIONS . Aromatic substitution reactions are initiated by the attack of electrophile on the ring followed by the elimination of proton.Such reactions are called electrophilic substitution reactions. Benzene with its pi electrons behaves as an electron rich system.Electrons in the pi clouds are readily available to form a new bond with electron deficient species,i.e. the electrophile.Electrophilic substitution reactions folows the following mechanism 29

24. Step-1 :Generation of electrophile either by spontaneous dissociation of reagent or by acid catalyzed dissociation E-Nu  E + + Nu E-Nu +A -  E-Nu + - A -  E + + Nu- A - Step-2:Formation of the  -complex first then  -complex  -complex forms by association of electrophile with the aromatic ring .In this  -complex,an electrophile is not attached to any specific position of ring but later it rearranges to give  -complex.  -complex is a resonance hybrid of stabilized carbonium (arenium) ion produced by the attack of electrophile on benzene ring Step-3: A proton H + is then eliminated from  -complex by the base to yield the final substitution product. 30

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26. HALOGENATION NITRATION AND SULFONATION 32 Nitration and sulfonation of benzene are two examples of electrophilic aromatic substitution. The nitronium ion (NO 2 + ) and sulfur trioxide (SO 3 ) are the electrophiles and individually react with benzene to give nitrobenzene and benzenesulfonic acid respectively.

27. NITRATION OF BENZENE Mechanism Step- 1 - G e nera t ion o f e lect r op hil e HN O 3 is activ a t ed with sulfuric acid through protonation which then causes the loss of a water m ole c ule and for m s a ni t ronium ion,whi c h i s a str o ng e r electrophile,. H H N O 3 H 2 S O 4 N O 2 + H 2 O N i t r o b e n z e n e +

28. Step-2 & Step-3- Formation of the  -complex and then e li m ina t ion of prot o n H + fr o m  -co m p le x t a k e s p l a c e to get the final product

29. S ULFONATION Sulfonation is a reversible reaction that produces benzenesulfonic acid by adding sulfur trioxide and fuming sulfuric acid. The reaction is reversed by adding hot aqueous acid to benzenesulfonic acid to produce benzene. 35

30. MECHANISM OF SULFONATION 36 Step-1-Generation of electrophile The sulfur in sulfur trioxide is electrophilic because the oxygens pull electrons away from it as oxygen is very electronegative.To produce benzenesulfonic acid from benzene, fuming sulfuric acid and sulfur trioxide are added. Fuming sulfuric acid, also refered to as oleum , is a concentrated solution of dissolved sulfur trioxide in sulfuric acid.

31. THE SUL F UR A N D S TE P -2 & S TE P - 3 - T HE SUBSEQUENT PROTON 37 BENZENE TRAN S F E R S ATTACKS OCCUR TO PRODUCE BENZENESULFONIC ACID . I . E . F ORMATION OF THE  - COMPLEX AND THEN ELIMINATION OF PROTON H + FROM  - COMPLEX TAKES PLACE TO GET THE FINAL PRODUCT

32. F RIEDEL C RAFTS A CYLATION This reaction involves the introduction of RCO (acyl) group in the aromatic ring in presence of AlCl3,BF3, FeCl3 etc 38

33. M ECHANISM Step 1- The very first step involves the generation of the electrophile i.e. the acylium ion 39

34. Step 2- The second step involves the attack of the acylium ion on benzene as a new electrophile to form one complex: 40

35. Step 3- The third step involves the departure of the proton in order for aromaticity to return to benzene 41

36. FRIEDEL CRAFTS ALKYLATION 42 This rea c ti o n invo l v es t h e at t ack of an alkyl group i n ben z ene in pre se n ce of anhyd r ous alu m inum chlorid e . This rea c tion is c al l ed Overall the F r ie d e l ‐ Cra f ts a l k y l a t i on r e actio n . transformation: Ar - H to Ar - R The mechanism for this reaction begins with the generation of a methyl carbocation from methylbromide. The carbocation then reacts with the π electron system of the benzene to form a nonaromatic carbocation that loses a proton to reestablish the aromaticity of the system.

37. 43 1. Step 1- The first step involves the generation of the electrophile methyl carbocation by the reaction of methylchloride with aluminum chloride. 2. Step 2- T h e ele c t r op h ile a tt a cks t h e π e l e ctr o n sys tem o f the benzene ring to form a nonaromatic carbocation.

38. 44 The positive charge on the carbocation that is formed is delocalized throughout the molecule. Step 3- The aromaticity is restored by the loss of a proton from the atom to which the methyl group has bonded.

39. Step 4- Finally, the proton reacts with the AlCl 4 − to regenerate the AlCl 3 catalyst and form the product HCl. Carbocations can rearrange during the Friedel ‐ Crafts alkylation reaction, leading to the formation of unpredicted products. One example is the formation of isopropyl benzene by the reaction of propyl chloride with benzene.

40. Structure and uses of Following compounds : DDT Saccharin BHC Chloramine

41. DDT: Dichlorodiphenyltrichloroethane DDT (Dichlorodiphenyltrichloroethane) is a synthetic organic compound with the chemical formula C14H9Cl5 . Its molecular structure consists of: 1. Two benzene rings (phenyl groups) connected by a central carbon atom 2. Three chlorine atoms attached to the central carbon atom 3. Two additional chlorine atoms attached to each benzene ring

42. Method of Preparation : DDT is prepared by heating chlorobenzene with Chloral in presence of conc. H₂ SO4

43. Uses of DDT DDT was widely used in the past for: Insecticide : DDT was used to control mosquitoes, flies, and other insects that spread diseases like malaria, typhus, and yellow fever. Agriculture : DDT was used to control pests that damaged crops, such as insects, mites, and ticks. Public health : DDT was used to control disease-carrying insects in homes, schools, and public areas. Veterinary medicine : DDT was used to control external parasites on animals, such as ticks and lice. However, due to its environmental and health risks, the use of DDT has been largely banned or restricted in many countries since the 1970s. Environmental and Health Risks Persistence in the environment Bioaccumulation in wildlife Toxicity to aquatic life Potential human health risks, including cancer and neurological effects

44. Saccharin: Saccharin is an artificial sweetener with the chemical formula C7H5NO3S. Its molecular structure consists of: 1. A benzene ring with a sulfonamide group (-SO2NH2) attached to it 2. A nitrogen atom bonded to the sulfonamide group 3. A carboxyl group (-COOH) attached to the nitrogen atom

45. Method of Preparation : It is prepared by  sulfonation of toluene, followed by reaction with ammonia to form sulphonamide and then, oxidation of sulphonamide to form saccharin. - The para isomer formed after the sulfonation of toluene is not considered since it cannot form saccharin.

46. Uses of Saccharin Saccharin is commonly used as a: Artificial sweetener : Saccharin is 300-400 times sweeter than sugar and is used in: Diet foods and beverages Sugar-free gum and candy Low-calorie desserts Food additive : Saccharin is used as a sweetening agent in: Baked goods Soft drinks Fruit juices Canned goods Pharmaceutical applications : Saccharin is used as a: Sweetening agent in cough drops and syrups Excipient in tablet and capsule formulations Research applications : Saccharin is used in: Biochemical research as a sweetening agent Toxicological studies as a control substance

47. BHC: Benzene Hexachloride : BHC, also known as Lindane , is an organochlorine compound with the chemical formula C6H6Cl6. Its molecular structure consists of: 1. A benzene ring with six chlorine atoms attached to it 2. The chlorine atoms are arranged in a specific geometric pattern, resulting in different isomers (alpha, beta, gamma, delta, epsilon, and zeta)

48. Preparation of Benzene hexachloride Chlorine combines with benzene, in the presence of UVlight and in the absence of oxygen as well as substitution catalysts, to form hexachlorocyclohexane .

49. Uses of BHC BHC is used as: Insecticide : BHC is used to control insects, such as: Lice and scabies in humans Fleas and ticks in animals Agricultural pests, like wheat and rice borers Fungicide : BHC is used to control fungal diseases in: Crops, like wheat, barley, and oats Seeds, like wheat and rice Pharmaceutical applications : BHC is used in: Topical creams and lotions for skin conditions, like eczema and dermatitis Shampoos for lice and dandruff Veterinary medicine : BHC is used to control: External parasites, like ticks and lice, in animals Fungal infections in animals Other uses BHC is used in some industrial applications, like: Wood preservation Leather treatment

50. Chloramine : Chloramine is a chemical compound with the formula NH2Cl. Its molecular structure consists of: 1. A nitrogen atom bonded to two hydrogen atoms and one chlorine atom 2. The nitrogen atom has a lone pair of electrons, making chloramine a polar

51. Method of preparation: chloramine is prepared by the reaction of ammonia with sodium hypochlorite: NH 3  + NaOCl → NH 2 Cl + NaOH . Gaseous chloramine can be obtained from the reaction of gaseous ammonia with chlorine gas (diluted with nitrogen gas): 2  NH 3  + Cl 2  ⇌ NH 2 Cl +  NH 4 Cl Pure chloramine can be prepared by passing  fluoroamine through  calcium chloride: 2  NH 2 F + CaCl 2  → 2 NH 2 Cl + CaF 2

52. Uses of Chloramine Chloramine is used as: Disinfectant : Chloramine is used to disinfect: Water treatment plants Swimming pools Wastewater treatment plants Bleaching agent : Chloramine is used as a bleaching agent in: Textile industry Paper industry Pharmaceutical applications : Chloramine is used as: An antiseptic in wound care products A preservative in eye drops and contact lens solutions Laboratory reagent : Chloramine is used as a reagent in: Organic synthesis Analytical chemistry Other uses Chloramine is used in some industrial applications, like: Cleaning and sanitizing surfaces Controlling algae growth in cooling towers