Benzene & Its Derivatives-Aromaticity.pptx

Study about Benezene and it's derivatives

1. Benzene and its derivatives 6.1 Introduction 6.2 nomenclature 6.3 chemical properties

2. aromaticity An aromatic compound is a compound with at least one benzene ring or having other ring structure that resembles benzene in its chemical properties. Benzene Toluene Naphthalene

3. The molecular formula of benzene is C 6 H 6 The benzene molecule is expected to have a high degree of unsaturation and it should be reactive. Benzene was found to be quite stable towards several reactions and is not keen to undergo addition reactions like alkenes. Benzene undergoes substitution reactions even though it is highly unsaturated Aromaticity is a property associated with the extra or unusual stability of π system in aromatic compounds It also describes the stabilisation associated with a resonance delocalised structure found in aromatic compounds

4. Four Criteria for aromaticity: Conjugated π system – system of atoms covalently bonded with alternating single and double bonds in a molecule. Cyclic Planar geometry Obey Huckel’s rule which contains (4n+2) π electrons All carbon atoms are sp2 hybridised . A cyclic, planar and conjugated characteristic contribute to the delocalisation of π electrons in the unhybridized p orbitals of aromatic compounds.

5. Aromatic compounds with (4n+2) π electrons, where n= 0, 1, 2, 3 and so on, have extra stability according to Huckel’s rule Benzene and naphthalene have 6 and 10 π electrons respectively. Both of the structure show aromaticity n 1 2 3 π electrons 2 6 10 14 n=2 4n+2 4(2) + 2 = 10 n=1 4n+2 4(1) + 2 = 6

6. Quiz 1 Determine whether the following molecule or ion possess aromaticity as in benzene by calculating the number of π electrons using Huckel’s rule Cyclobutadiene Cyclopenta-1,3-diene Cyclohepta-1,3,5-triene

7. Kekul È structure August Kekule put forward the structure of benzene as two forms of 1,3,5-cyclohexatriene as shown Kekule suggested that the six carbon atoms are arranged in a hexagon plane and there are alternating double and single bonds between them Each carbon atom has hydrogen attached to it. The structure is satisfactory in the sense that all six hydrogen atoms are equivalent, and all carbon atoms obey the octet rule. However, the Kekule structure still cannot explain the chemical properties of benzene at all well. It would be expected to undergo addition reactions due to the presence of double bonds, but this is not how benzene reacts

8. Resonance structure The concepts of atomic orbital hybridisation and resonance introduced in 1930s by Linus Pauling were used to explain the benzene structure more satisfactorily The six carbon atoms skeleton form a hexagon with all the bond angles at 120º. Each carbon atom uses sp2 hybrid orbital to bond with each other. Each sp2 hybridised carbon atom has an unhybridized p orbital perpendicular to the ring. The unhybridized p orbitals overlap to form π electron cloud above and below the plane of the molecule.

9. According to the resonance theory, the real structure of benzene molecule is the resonance hybrid of the two Kekule’s structures and can be represented as follows: The structure of benzene is neither structure 1 nor structure 2 but is an intermediate between both structures. It can be presented as structure 3 and is termed as a resonance hybrid. The resonance hybrid is more stable than the resonance structures 1 and 2. Both Kekule’s structures contribute equally towards this hybrid structure. This can be seen from C-C bond lengths, which are all equal in benzene molecule. That is, the bonds are neither single nor double but are intermediate between these two. The π electrons are said to be delocalised above and below the plane of the molecule. These delocalised electrons stabilise the benzene ring.

10. 6.2 NOMENCLATURE Monosubstituted Benzene The substituent is indicated as the prefix of benzene Such common names such as toluene, phenol, aniline, benzaldehyde and benzoic acid are accepted by the IUPAC

11. Disubstituted Benzene Benzene derivatives with two substituents are given names based on the IUPAC nomenclature by using numbers of prefixs ortho-, meta-, para- ( o- , m- , p- ) to indicate their relative positions. The relative positions on benzene ring: a) ortho-, (o-) : 1,2- position b) meta-, (m-) : 1, 3- position c) para-, (p-) : 1,4 – position The prefixes such as ortho, meta and para are common names by the IUPAC

12. Tri and Tetrasubstituted Benzene When there are more than two substituents on the benzene ring, their positions are shown by numbers. The ring carbon atoms are given numbers in such a way that the substituents at the lowest positions possible. 1,2,4- trichlorobenzene (not 1,3,4-trichlorobenzene) 1,2,3-tribromobenzene (not 1,2,6-tribromobenzene)

13. When there are more than two different substituents, the substituents are written according to their alphabetical order . For examples: 2-bromo-1-chloro-3-iodobenzene // 2-bromo-3-chloro-1-iodobenzene 1-bromo-4-chloro-2-nitrobenzene

14. Whenever a substituent can give a new parent name with the benzene ring, the carbon atom that carries the substituents is now recognised as at position 1 3-bromo-5-methylaniline 3,4-dibromophenol 2,4,6-trichlorobenzoic acid

15. Note that the prefixes of ortho, meta and para cannot be used in naming the tri- and tetrasubstituted benzene 4-bromo-1,2-dimethylbenzene (not 4-bromo-o-dimethylbenzene)

16. Phenyl and benzyl as substituents When a benzene ring is attached to a carbon chain containing other functional group or attached to a chain containing six or more carbon atoms, the benzene ring is then considered as a substituent, named phenyl The group with the structure C6H5CH2-is called benzyl 4-phenyl-1-butene 3-phenylheptane 2-phenylethanol Benzyl chloride

17. Quiz 2 Give the IUPAC name for each of the following compounds

18. Quiz 3 Draw the structural formula for each of the following compound 2-bromo-1-chloro-3-nitrobenzene 4-chlorobenzoic acid 1-bromo-3-ethyl-4-phenylhexane 4-chloro-2-isopropyltoluene 3-benzylheptane 1,4-diphenyl-1,3-butadiene

19. 6.3 Chemical properties Electrophilic Aromatic Substitution Reaction Electrophilic aromatic substitution reaction is the characteristic reaction of benzene The electrophilic attacks the π electrons of the benzene ring and causes a proton to be substituted

20. a) Nitration Reagents used for nitration of benzene are concentrated nitric acid and concentrated sulphuric acid as catalyst. The reaction mixture is heated to yield nitrobenzene. The electrophile for this substitution is nitronium ion (NO 2 + ) The general equation for the reaction is + H 2 O

21. Mechanism for nitration reaction Step 1: Formation of nitronium ion (NO 2 + ) HNO 3 + H 2 SO 4 H 2 NO 3 + + HSO 4 - H 2 NO 3 + NO 2 + + H 2 O Step 2: Formation of arenium ion The nitronium ion then attacks the π electronsof the benzene ring and a reaction intermediate arenium ion is formed

22. Step 3: Deprotonation Deprotonation (loss proton) occurs to retain the aromaticity of benzene ring. Nitrobenzene is formed

23. B) HALOGENATION The halogenation reaction occurs easily in the presence of Lewis acid catalyst such as FeX 3 where X is Cl or Br The electrophile for this substitution reaction is bromonium (Br + ) or chloronium (Cl + ) ion The general equation for the reaction is:

24. Mechanism for halogenation reaction Step 1: Formation of bromonium ion (Br + ) Br Br + FeBr 3 Br Br + - FeBr 3 Br+ + FeBr 4 - The halogen molecule is polarised by catalyst FeBr 3 which dissociates to form bromonium ion (Br + ) Step 2: Formation of arenium ion The bromonium ion then attacks the π electrons of benzene ring and a reaction intermediate, arenium ion is formed

25. Step 3: Deprotonation Deprotonation (loss of proton) occurs to retain the aromaticity of benzene ring. Bromobenzene is formed

26. C) FRIEDAL-CRAFTS ALKYLATION Alkylbenzenes can be prepared from benzene via Friedal – Crafts’ method. The reagents used are haloalkanes and AlCl 3 as Lewis acid catalyst In Friedal -Crafts alkylation reaction, a hydrogen atom on the benzene ring is substituted by an alkyl (-R) group. The electrophile for this substitution reaction is carbocation The general equation for the reaction is:

27. Mechanism for Friedal -Crafts alkylation: Step 1: Formation of electrophile The haloalkane molecule is polarised by catalyst (AlCl 3 ) to form an electrophile

28. Step 2: Formation of arenium ion The electrophile then attacks the π electrons of the benzene ring and a reaction intermediate, arenium ion is formed Step 3: Deprotonation Deprotonation(loss of proton) occurs to retain the aromaticity of benzene ring. An alkylbenzene (isopropyl benzene is formed)

29. D) FRIEDAL-CRAFTS AcYLATION The Friedal -Crafts acylation reaction is an effective method of introducing an acyl group, RC=O into benzene ring. Thus is a good method for preparing aromatic ketones The reagents used are acyl chloride ( RCOCl ) is the presence of aluminium chloride AlCl 3 as catalyst The electrophile for this substitution reaction is acylium ion The general equation for the reaction is:

30. Influence of ortho-para and meta directing substituents towards electrophilic aromatic substitution reaction: The substituent that is initially present on the benzene ring can influence the further electrophilic aromatic substitution reaction in two ways namely orientation and reactivity In terms of orientation, the substituent that is iniatially present on the benzene ring tends to direct the second substituent goes either to (a) ortho and para position or (b) meta position

31. Step 2: Formation of arenium ion The electrophile then attacks the π electrons of the benzene ring and a reaction intermediate, arenium ion is formed Step 3: Deprotonation Deprotonation(loss of proton) occurs to retain the aromaticity of benzene ring. An alkylbenzene (isopropyl benzene is formed)

32. Influence Of ortho-para and meta directing substituents towards electrophilic aromatic substitution reaction The substituent that is initially present on the benzene ring can influence the further electrophilic aromatic substitution reaction in two ways namely orientation and reactivity In terms of orientation, the substituent that is initially present on the benzene ring tends to direct the second substituents goes either to ortho and para positions or meta position The substituent that direct the second substituent to ortho and para position on the benzene ring is called ortho-para director

33. The substituent that direct the second substituent to the meta position on the benzene ring is called meta director

34. a) Ortho-para director All activating substituents are ortho-para directors. They are electron donating groups that tend to release electrons to the ring A substituent activates if it causes the ring to be more reactive than benzene itself. Therefore, the activated ring undergoes electrophilic aromatic substitution reaction faster than benzene. For examples, toluene reacts 25 times faster than benzene in nitration because methyl group is said to activate the ring by making it more reactive towards electrophile.

35. The methyl group that releases electrons will effectively increase the electron density to the ortho and para positions on the benzene ring. Therefore, the second substituent electrophile will be oriented to the ortho and para positions. In general, the activating groups which are ortho-para directing substituents have the following characteristics: saturated groups with only single bonds between the atoms such as alkyl group (-R) Atom adjacent to the benzene ring has lone pair of electrons i ) –OR where R is H, alkyl, aryl, acyl -OH, -OR ii) –NR2 where R is H, alkyl, aryl, acyl or any combination thereof - NH 2 , -NHR, -NR 2 , -NHCOR

36. b) Meta director The deactivating substituents are meta directors. They are electron withdrawing groups that tend to withdraw electrons from the ring A substituent deactivates if it causes the ring to be less reactive than benzene itself. Therefore, the deactivated ring undergoes electrophilic aromatic substitution reaction slower than benzene For example, nitrobenzene reacts slower than benzene in nitrationbecause nitro group is said to deactivate the ring by making it less reactive towards electrophile.

37. The nitro group that withdraw electrons will effectively decrease the electron density to the ortho and para positions on the benzene ring. Therefore, the second substituent electrophile will be orientated to the meta position which has high electron density In general, the deactivating groups which are meta directing substituents has the following characteristic: -Unsaturated groups with multiple bonds i ) –NO 2 ii) –NR 3 where R is H, alkyl, aryl or combination thereof iii) -CN iv)-SO 3 H v) O where R is H, alkyl, aryl, hydroxy, alkoxy, phenoxy or any combination thereof -C-R

38. However, halogens are exceptional, in which they are deactivating groups but ortho-para directing substituents. Halogens have lone pairs of electrons that enable them to release electrons to the ring via resonance donation. It increases the electron to the ortho and para positions on the benzene ring. Therefore, the second substituent electrophile will be orientated to the ortho and para position Halogens are electronegative elements that tend to withdraw electrons from the benzene ring inductively through sigma bond. As such, halogens will weakly deactivate the benzene ring

39. Predict the product of electrophilic aromatic substitution of Monosubstituted Benzene Example 1: Draw the structural formula of the product(s) formed Cl 2 , FeCl 3

40. Predict the product of electrophilic aromatic substitution of Monosubstituted Benzene Example 2: Draw the structural formula of the product(s) formed AlCl 3 + CH 3 Cl

41. Predict the product of electrophilic aromatic substitution of Monosubstituted Benzene Example 3: Write the synthetic routes for the following conversion

42. Reactions of alkylbenzene Oxidation with hot, acidified KMnO 4 or K 2 Cr 2 O 7 Benzene does not react with oxidising agents such as potassium permanganate or potassium dichromate However, the alkyl group of alkylbenzene could be oxidised to carboxyl group The oxidation product is always benzoic acid The oxidising agents used are hot, acidified solution of potassium permanganate (KMnO 4 ) or potassium dichromate (K 2 Cr 2 O 7 )

43. If the alkyl group is other than the methyl group, only the carbon atom directly bonded to the benzene ring is oxidised to carboxyl group. This carbon atom benzylic carbon must be possess at least one hydrogen atom (benzylic hydrogen) Tert-butylbenzene is not oxidised because the benzylic carbon does not have hydrogen atoms bonded to it.

44. b) Halogenation The reaction of alkylbenzene with halogen such as bromine and chlorine in the presence of sunlight is a free-radical substitution reaction The substitution of hydrogen atoms in the alkyl group by halogen atom such as chlorine and bromine is occurred at the side chain not at benzene ring. If bromine (Br 2 ) is used in the reaction, the substitution yields only one major product due to selectivity of bromine.

45. However , if chlorine (Cl2) is used in the reaction, the substitution gives a mixture of product because all hydrogen atoms in the alkyl group can be substituted by chlorine atoms