Benzene and its Derivatives new.presentation

Preparation, reaction ,charactristics of benzene

1. Benzene and its Derivatives Pramod B Kumar, Mpharm , SDCP,Mangalore

2. Benzene is a simplest aromatic hydrocarbon with six carbon atom bonded in a hexagonal manner. It was first isolated from an oily film which was deposited from the gas used for lighting in 1825 by Michael Faraday. The molecular formula for benzene is C 6 H 6 , in which all the six carbon atoms and hydrogen arranged in same plane and show planer geometry. There are three pi bonds arranged in alternate manner in hexagonal ring. Although the presence of pi bonds makes the molecule unsaturated which are more reactive for additional reactions like alkene and alkyne, yet benzene is not considered as unsaturated compounds in organic chemistry and generally shows substitution reaction instead of additional reaction. What is Benzene?

3. Hence benzene can easily give substitution reaction with sulfuric acid and bromine to form benzene sulphonic acid and bromobenzene respectively. However alkenes like Cyclohexene can easily form additional product with bromine and sulfuric acid. Similarly Cyclohexene oxidized to adipic acid while benzene shows no reaction with potassium permanganate solution .

4. Proposed structures of benzene must account for its high degree of unsaturation and its lack of reactivity towards electrophilic addition. August Kekulé proposed that benzene was a rapidly equilibrating mixture of two compounds, each containing a six- membered ring with three alternating  bonds. In the Kekul é description, the bond between any two carbon atoms is sometimes a single bond and sometimes a double bond. that shift back and forth so rapidly that the two forms cannot be separated These structures are known as Kekulé structures .

5. 5 Although benzene is still drawn as a six- membered ring with alternating  bonds, in reality there is no equilibrium between the two different kinds of benzene molecules. Current descriptions of benzene are based on resonance and electron delocalization due to orbital overlap. In the nineteenth century, many other compounds having properties similar to those of benzene were isolated from natural sources. Since these compounds possessed strong and characteristic odors, they were called aromatic compounds. It should be noted, however, that it is their chemical properties, and not their odor, that make them special.

6. Any structure for benzene must account for the following facts: It contains a six-membered ring and three additional degrees of unsaturation. It is planar. All C—C bond lengths are equal. It should follow Huckels rule The resonance description of benzene consists of two equivalent Lewis structures, each with three double bonds that alternate with three single bonds. The true structure of benzene is a resonance hybrid of the two Lewis structures, with the dashed lines of the hybrid indicating the position of the  bonds. We will use one of the two Lewis structures and not the hybrid in drawing benzene. This will make it easier to keep track of the electron pairs in the  bonds (the  electrons). Because each  bond has two electrons, benzene has six  electron s.

7. In benzene, the actual bond length (1.39 Å) is intermediate between the carbon—carbon single bond (1.53 Å) and the carbon—carbon double bond (1.34 Å).

8. 8 The Resonance Model of Benzene One of the postulates of resonance theory is that, if we can represent a molecule or ion by two or more contributing structures, then that molecule cannot be adequately represented by any single contributing structure. We represent benzene as a hybrid of two equivalent contributing structures, often referred to as Kekulé structures: Each Kekulé structure makes an equal contribution to the hybrid; thus, the CJC bonds are neither single nor double bonds, but something intermediate-. We recognize that neither of these contributing structures exists (they are merely alternative ways to pair 2p orbitals with no reason to prefer one over the other) and that the actual structure is a superposition of both.

9. 9 The Resonance Energy of Benzene Resonance energy is the difference in energy between a resonance hybrid and its most stable hypothetical contributing structure . One way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of cyclohexene and benzene (benzene can be made to undergo hydrogenation under extreme conditions). In the presence of a transition metal catalyst, hydrogen readily reduces cyclohexene to cyclohexane

10. Stability of Benzene Consider the heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene and benzene, all of which give cyclohexane when treated with excess hydrogen in the presence of a metal catalyst .

11. 11 The catalytic reduction of an alkene is an exothermic reaction. The heat of hydrogenation per double bond varies somewhat with the degree of substitution of the double bond; for cyclohexene -120 kJ/ mol (28.6 kcal/ mol ). If we imagine benzene in which the 2p electrons do not overlap outside of their original C-C double bonds, a hypothetical compound with alternating single and double bonds, we might expect its heat of hydrogenation to be 3 x120 -359 kJ/ mol (85.8 kcal/ mol ). Instead, the heat of hy - drogenation of benzene is only -209 kJ/ mol (-49.8 kcal/ mol ). The difference of 150 kJ/ mol (35.8 kcal/ mol ) between the expected value and the experimentally observed value is the resonance energy of benzene.

12. 12 Figure compares the hypothetical and observed heats of hydrogenation for benzene. The huge difference between the hypothetical and observed heats of hydrogenation for benzene cannot be explained solely on the basis of resonance and conjugation. A comparison between the observed and hypothetical heats of hydrogenation for benzene

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14. 14 The low heat of hydrogenation of benzene means that benzene is especially stable—even more so than conjugated polyenes . This unusual stability is characteristic of aromatic compounds. Benzene’s unusual behavior is not limited to hydrogenation. Benzene does not undergo addition reactions typical of other highly unsaturated compounds, including conjugated dienes . Benzene does not react with Br 2 to yield an addition product. Instead, in the presence of a Lewis acid, bromine substitutes for a hydrogen atom, yielding a product that retains the benzene ring.

15. 15 Four structural criteria must be satisfied for a compound to be aromatic. The Criteria for Aromaticity — H ü ckel’s Rule [1] A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. Have one 2p orbital on each of its atoms.

16. 16 [2] A molecule must be planar. All adjacent p orbitals must be aligned so that the  electron density can be delocalized. Since cyclooctatetraene is non-planar, it is not aromatic, and it undergoes addition reactions just like those of other alkenes.

17. 17 [3] A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom.

18. 18 [4] A molecule must satisfy H ü ckel’s rule , and contain a particular number of  electrons. Benzene is aromatic and especially stable because it contains 6  electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4  electrons. Hückel's rule:

19. 19 Note that H ü ckel’s rule refers to the number of  electrons, not the number of atoms in a particular ring.

20. 20 Aromatic—A cyclic, planar, completely conjugated compound with 4 n + 2  electrons. Antiaromatic —A cyclic, planar, completely conjugated compound with 4 n  electrons. Not aromatic ( nonaromatic )—A compound that lacks one (or more) of the following requirements for aromaticity : being cyclic, planar, and completely conjugated. Considering aromaticity , a compound can be classified in one of three ways:

21. 21 Note the relationship between each compound type and a similar open-chained molecule having the same number of  electrons.

22. 22 1 H NMR spectroscopy readily indicates whether a compound is aromatic. The protons on sp 2 hybridized carbons in aromatic hydrocarbons are highly deshielded and absorb at 6.5-8 ppm, whereas hydrocarbons that are not aromatic absorb at 4.5-6 ppm.

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24. Molecular Structure of Benzene Resonance of Benzene The molecular structure of benzene can be explained by using valence bond theory which based on resonance and hybridization. Benzene is a type of hydrocarbon composed of carbon (1s 2 2s 2 2p 1 2py 1 ) and hydrogen (1s 1 ). In cyclic structure of benzene, each carbon is bonded with two carbon atoms and one hydrogen atom through sigma bond. Hence each carbon required minimum three unpaired electrons to form three sigma bonds and for that one of the 2s 2  electron has to promote to empty 2pz orbital.

25. This excitation of electron makes four unpaired electrons in each carbon atom. Out of these four electrons, only three electrons involve in hybridization to form three sigma bonds and give sp 2  hybridization Three sp 2  hybrid orbitals get arranged in trigonal planer geometry at 120° bond angle and overlap with two carbon atoms and one hydrogen atom to form three sigma bonds. The un-hybridized p-orbital oriented at right angle to hybridized orbital and involve in pi bond formation.

26. The side way overlapping of these unhybridized p-orbitals form pi bonds which are delocalized over all carbon atom of ring, as there is equal probability of each p-orbital to get overlap with any one of the neighbour p-orbital. Overall benzene molecule is a planer hexagon with three pi bonds arranged in alternate manner with the carbon- carbon bond length 1.39 Ã…. The pi electron clouds distributed above and below the plane of ring and perpendicular to plane of ring

27. 27 Bond distances and Bond Angles of Benzene

28. 28 The Basis of H ü ckel’s Rule Why does the number of  electrons determine whether a compound is aromatic? The basis of aromaticity can be better understood by considering orbitals and bonding.

29. 29 Thus far, we have used “valence bond theory” to explain how bonds between atoms are formed. Valence bond theory is inadequate for describing systems with many adjacent p orbitals that overlap, as is the case in aromatic compounds. Molecular orbital (MO) theory permits a better explanation of bonding in aromatic systems. MO theory describes bonds as the mathematical combination of atomic orbitals that form a new set of orbitals called molecular orbitals (MOs). A molecular orbital occupies a region of space in a molecule where electrons are likely to be found.

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33. 33 When forming molecular orbitals from atomic orbitals, keep in mind that a set of n atomic orbitals forms n molecular orbitals. If two atomic orbitals combine, two molecular orbitals are formed. Recall that aromaticity is based on p orbital overlap. Also note that the two lobes of each p orbital are opposite in phase, with a node of electron density at the nucleus. When two p orbitals combine, two molecular orbitals should form .

34. 34 The combination of two p orbitals can be constructive —that is, with like phases interacting—or destructive , that is, with opposite phases interacting. When two p orbitals of similar phase overlap side-by-side, a  bonding molecular orbital results. When two p orbitals of opposite phase overlap side-by-side, a * antibonding orbital results.

35. 35 Combination of two p orbitals to form π and π * molecular orbitals

36. 36 When forming molecular orbitals from atomic orbitals, keep in mind that a set of n atomic orbitals forms n molecular orbitals. If two atomic orbitals combine, two molecular orbitals are formed. Recall that aromaticity is based on p orbital overlap. Also note that the two lobes of each p orbital are opposite in phase, with a node of electron density at the nucleus. When two p orbitals combine, two molecular orbitals should form .

37. 37 The combination of two p orbitals can be constructive —that is, with like phases interacting—or destructive , that is, with opposite phases interacting. When two p orbitals of similar phase overlap side-by-side, a  bonding molecular orbital results. When two p orbitals of opposite phase overlap side-by-side, a * antibonding orbital results.

38. 38 Combination of two p orbitals to form π and π * molecular orbitals

39. 39 The molecular orbital description of benzene is much more complex than the two. Since each of the six carbon atoms of benzene has a p orbital, six atomic p orbitals combine to form six  molecular orbitals The six MOs are labeled  1 -  6 , with  1 being the lowest energy and  6 being the highest. The most important features of the six benzene MOs are as follows: The larger the number of bonding interactions, the lower in energy the MO. The larger the number of nodes, the higher in energy the MO.

40. 40 The most important features of the six benzene MOs (continued): The larger the number of bonding interactions, the lower in energy the MO. The larger the number of nodes, the higher in energy the MO. Three MOs are lower in energy than the starting p orbitals, making them bonding MOs, whereas three MOs are higher in energy than the starting p orbitals, making them antibonding MOs. Two pairs of MOs with the same energy are called degenerate orbitals. The highest energy orbital that contains electrons is called the highest occupied molecular orbital (HOMO) . The lowest energy orbital that does not contain electrons is called the lowest unoccupied molecular orbital (LUMO) .

41. 41 Consider benzene. Since each of the six carbon atoms in benzene has a p orbital, six atomic p orbitals combine to form six  MOs. To fill the MOs, the six electrons are added, two to an orbital. The six electrons completely fill the bonding MOs, leaving the anti-bonding MOs empty. All bonding MOs (and HOMOs) are completely filled in aromatic compounds. No  electrons occupy antibonding MOs. The six molecular orbitals of benzene

42. Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2  electrons (n is 0,1,2,3,4 ) For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized Planar, cyclic molecules with 4 n  electrons are much less stable than expected ( antiaromatic ) They will distort out of plane and behave like ordinary alkenes 4- and 8-electron compounds are not delocalized (single and double bonds) HUCKEL ‘s RULE

43. Cyclobutadiene is so unstable that it dimerizes by a self-Diels-Alder reaction at low temperature Cyclooctatetraene has four double bonds, reacting with Br 2 , KMnO 4 , and HCl as if it were four alkene

44. Aromatic Ions The 4 n + 2 rule applies to ions as well as neutral species Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic The key feature of both is that they contain 6  electrons in a ring of continuous p orbitals

45. Aromaticity of the Cyclopentadienyl Anion 1,3-Cyclopentadiene contains conjugated double bonds joined by a CH 2 that blocks delocalization but Removal of H + at the CH 2 produces a cyclic 6-electron system, which is stable Cycloheptatriene Cycloheptatriene has 3 conjugated double bonds joined by a CH 2 Removal of “H - ” leaves the cation The cation has 6 electrons and is aromatic

46. Properties of Benzene The various properties of benzene are mentioned below: Benzene is immiscible in water but soluble in organic solvents. It is a colorless liquid and has an aromatic odor . It has a density of 0.87g cm-3. It is lighter than water. Benzene has a moderate boiling point and a high melting point. (Boiling point: 80.5°C, Melting point: 5.5°C) Benzene shows resonance. It is highly inflammable and burns with a sooty flame.

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49. 49 To name a benzene ring with one substituent, name the substituent and add the word benzene. Nomenclature of Benzene Derivatives Many monosubstituted benzenes have common names which you must also learn.

50. 50 Ortho, para , meta positions of benzene When hydrogen atom or atoms in the benzene ring are replaced by another atom(s) or group (X = Cl , Br, NO 2 , OH, NH 2   etc ), we name positions of carbon atoms of benzene ring as ortho , meta and para related to the position of X. Electrons density of those ortho para meta positions are different. Electrons density of some positions will increase and in some positions decrease. Therefore position of attaching another group to carbon ring(substituting instead of H) is changed according to the position of carbon atom. As a example, phenol, aniline increases electrons density in ortho , para locations. Electrons density of ortho para positions in nitrobenzene is less than meta position. This phenomenon can be explained by resonance effect. This ortho , para , meta positions are important when we study about substituting reactions about benzene. Phenol and aniline are ortho para directors. That means substitution mainly occurs in ortho para positions in the benzene ring.

51. 51 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 . 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

52. 52 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.

53. 53 For three or more substituents on a benzene ring: Number to give the lowest possible numbers around the ring. Alphabetize the substituent names. 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.

54. 54 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 .

55. 55 The benzyl group , another common substituent that contains a benzene ring, differs from a phenyl group . Substituents derived from other substituted aromatic rings are collectively known as aryl groups .

56. 56 Benzene can be prepared by several ways. But we have to be careful when benzene is being produced due to it's toxicity. From benzoic acid Prepare benzene from Phenol from diazonium salts from acetylene Preparation of Benzene

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60. 60 Electrophilic substitution of benzene The delocalized π electrons in benzene give it a special stability. This means that addition reactions, which would lead to loss of the stable arene ring, are generally not favored as the products would be of higher energy than the reactant. Instead, substitution reactions, in which one (or more) of the hydrogen atoms is replaced by an incoming group, occur more readily as these lead to products in which the arene ring is conserved. As the delocalized ring of π electrons represents an area of electron density, benzene is susceptible to attack by electrophiles. Therefore, most typically, the arenes undergo electrophilic substitution reactions.

61. 61 These electrophilic substitution reactions of benzene have high activation energies and so proceed rather slowly. This is because the first step in the mechanism, in which an electron pair from benzene is attracted to the electrophile a disruption of the symmetry of the delocalized π system. The unstable carbocation intermediate that forms has both the entering atom or group and the leaving hydrogen temporarily bonded to the ring. The incomplete circle inside the ring shows its loss of symmetry, with the positive charge distributed over the bulk of the molecule. Loss of a hydrogen ion, H+, from this intermediate leads to the electrically neutral substitution product as two electrons from the C−H bond move to regenerate the aromatic ring.

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63. 63 A variety of substituents can take part in these reactions, so they can be used to introduce different functional groups into the ring. In the four descriptions that follow Niitration , Sulphonation Halogination , Alkylation Acylation, much of the discussion of the best conditions for a reaction is based on generating an electrophile which is able to attract electrons from benzene sufficiently strongly to disrupt the ring for a substitution product to form. Reactions of Benzene .

64. 64 Nitration of benzene The nitration of benzene is the substitution of -H by -NO2 to form nitrobenzene, C6H5NO2 The electrophile for the reaction is NO2+ , the nitronium ion. This is generated by using a nitrating mixture: a mixture of concentrated nitric and concentrated sulfuric acids at 50°C. As the stronger of the two acids, sulfuric acid protonates the nitric acid, which then loses a molecule of water to produce NO2+:

65. 65 NO2+ is a strong electrophile and reacts with the π electrons of the benzene ring to form the carbocation intermediate. Loss of a proton from this leads to re-formation of the arene ring in the product nitrobenzene, which appears as a yellow oil. The hydrogen ion released reacts with the base HSO4− to re-form sulfuric acid, H2SO4:

66. 66 Chlorination of benzene The chlorination of benzene involves the substitution of -H by - Cl to form chlorobenzene , C6H5CI: The electrophile for the reaction is CI+. This is produced by use of a halogen carrier catalyst, such as AlCl3. Because this is an electron-deficient species, it acts as a Lewis acid, accepting a lone pair of electrons from the halogen and so inducing polarity in the molecule:

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68. 68 The positively charged end of the halogen molecule is now electrophilic and attacks the benzene ring. Heterolytic fission of the chlorine molecule forms Cl + which bonds to a carbon atom in the ring, forming the carbocation. This then deprotonates to form chlorobenzene . The hydrogen ion released reacts with Cl − to form hydrogen chloride and regenerate the AlCl3 catalyst: The reaction is carried out in anhydrous conditions using dry ether, as aluminium chloride reacts violently with water. Substitution with halogens by this method works well for both bromine and chlorine. The reaction goes with difficulty with iodine and too violently with fluorine to be of any use. Halogen carrier catalysts also include FeBr3 or Fe, which reacts with the halogen to form the iron (III) halide during the reaction: 2 Fe + 3 Br2 → 2 FeBr3

69. 69 Alkylation of benzene This type of reaction, where an arene reacts with reagents that can give rise to a positively charged carbon atom, is an example of a Friedel -Crafts reaction, named for the two chemists who developed the process in 1877. Alkylation of benzene involves the substitution of -H by an alkyl group R (for example, -CH3), to form the alkyl benzene compound, The electrophile is R+, which is generated using the halogenoalkane and a catalyst of AlCl3 in dry ether (anhydrous conditions). As with the substitution of halogens, the catalyst acts to accept an electron pair, so helping the halogenoalkane to split heterolytically and generate a positive ion, a carbocation:

70. 70 This carbocation then acts as the electrophile, attacking benzene Proceeding by a similar mechanism to those shown above where the delocalized π ring is temporarily disrupted Then reformed as a proton is lost:

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74. 74 Acylation of benzene Acylation means substitution of an RCO- acyl group into the benzene ring. This is another example of a Friedel -Crafts reaction where the electrophile is a carbocation. In this case, it is an acyl cation , RCO+.

75. 75 The acyl cation then attacks the benzene ring, proceeding with a similar mechanism to the alkylation reaction:

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77. 77 The acylation reaction, unlike the alkylation reaction, usually stops after the first substitution. This is because the acylated benzene product has a deactivated ring that makes further substitution more difficult

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110. 110 Structure and Uses OF BHC: gamma - hexachlorocyclohexane  ( γ- HCH),  gammaxene ,  Gammallin   sometimes incorrectly called  benzene hexachloride  (BHC), Is an  organochlorine  chemical and an isomer of  hexachlorocyclohexane  that has been used both as an agricultural  insecticide  and as a  pharmaceutical  treatment for  lice  and  scabies .

111. 111 Saccharin, or 2H,2-benzothiazol-1,1,3-trione, Saccharin is a molecule that has found extensive use as an artificial sweetener it also found utility as a preservative and was often prescribed as an all-purpose medicine instead of sugar. Sodium Saccharin E954 is widely used as a substitute for sugar in foods, beverages, soft drinks, table top sweeteners, baked goods, chewing gum, canned fruits, dessert toppings and dressings.

112. 112 Structure and Uses of Chloramine-T : Tosylchloramide  or  N - chloro tosylamide , sodium salt, sold as  chloramine-T , is a  N - chlorinated  and  N - deprotonated   sulfonamide  used as a  biocide  and a mild  disinfectant .  Chloramines   (also known as secondary disinfection) are disinfectants  used  to treat drinking water Are most commonly formed when ammonia is added to chlorine to treat drinking water. Provide longer-lasting disinfection as the water moves through pipes to consumers

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