Benzene-structure, properties, reactions,

1. Organic Chemistry II
Benzene and Its Derivates
DR. S. S. HARAK
ASST. PROF. PHARM. CHEM.
GOKHALE EDUCATION SOCIETY’S
SIR DR. M. S. GOSAVI COLLEGE OF PHARMACEUTICAL EDUCATION & RESEARCH,
NASHIK-5

2. Outline
 Aromaticity
 Huckel’s rule
 The Reactions (Electrophilic
Substitution)
 Halogenation
 Friedel-Craft’s Reaction
 Alkylation and acylation
 Nitration and sulphonation
 Oxidation and reduction
of benzene derivates
 Disubstitution (Ortho, meta,
para directing groups)
 Phenol and aniline
 The relative acidity of phenol
 The relative basicity of aniline
 Diazoniums compounds

4. Kekulé’s Model of Benzene
 The first structure for benzene, proposed by August Kekulé in
1872
 consisted of a six membered ring with alternating single and
double bonds and with one hydrogen bondedto each carbon.
 Kekulé further proposed that the ring contains three double bonds
that shift back and forth so rapidly that the two forms cannot be
separated.
 Each structure has become known as a Kekulé structure.

5. Kekulé’s Model of Benzene
His proposal also accounted for the fact that the bromination of bromobenzene gives
three (and only three) isomeric dibromobenzenes.
If benzene contains three double bonds,, his critics asked,
why doesn’t it show the reactions typical of alkenes?
Why doesn’t it add three moles of bromine to form 1,2,3,4,5,6-hexabromocyclohexane?
Why, instead, does benzene react by substitution rather than addition?

6. The Main Features
 The bond length is between C – C and C=C (1.38A)
 Due to delocalised electron (resonance structure)

7. The Main Features
 The structure is planar
 Each carbon has p orbital that forms π bonding
 Maximum bonding benzene should planar

8. pi Cloud Formation in Benzene

9. Aromaticity (Hückel’s Rules)
 Huckel’s rules define the classification of aromatic and non-aromatic
molecule.
 The criteria of aromatic molecule:
 All the atoms are sp2 hybridised and in planar cyclicarrangement.
All atoms are sp2 but not acyclic.
Hence, non-aromatic
There is non-sp2 atom.
Hence, non-aromatic
All atoms are sp2 and acyclic.
Hence, could be aromatic

10. Huckel’s rules
 Huckel’s rule
 Number of π-electrons is (4n+2),
 How to calculate π-electrons? 
 based on the structure, p-orbitals in sp2 arrangement has 1electron
Has 6 π-electrons (4n+2, n=1)
Hence, aromatic
Has 4 π-electrons (4n, n=1)
Hence, anti-aromatic

11. Huckel’s Rule (summary)
Does the molecule have
non-sp2 atoms?
NO YES
How many π-
electrons in the
molecule?
4n+2 Not 4n+2
aromatic Anti-
aromatic
non-aromatic

12. Huckel’s rules
The Porphyrin ring in the heme is an aromatic

13. Huckel’s rule
 Which molecules are aromatic?
6 π-electrons
 Is this molecule aromatic?
2 π-electrons

14. The reactions
 Benzene undergoes electrophilic substitution
 Doesn’t undergo electrophilicaddition
 The consequence of aromatic properties

15. The reactions - Halogenation
 Halogenation. E.g. chlorination
 Via:
 The presence of Lewis acid (e.g. AlCl3) helps benzene to react with Cl2

16. The Reactions – Friedel-Crafts
Reaction
 Friedel-Crafts Reaction (Alkylation)
 To substitute with hydrocarbon chain
 Via:
Electrophilic
generation

17. Friedel-Crafts Reaction
 There is a problem for this reaction when longer alkyl halide isused
 Rearrangement of the electrophile (carbocation)
Trying to find the most stable carbocation

18. Friedel-Crafts Reaction (Acylation)
 To substitute with R-CO –
 Via:
 Electrophilic generation  acylium ion
 stabilised by resonance. Both structures are valid.

19. Friedel-Crafts Reaction (Acylation)
 Acylation can be used to get around the ‘messy’ long chain alkylation.

20. Nitration
 The nitration (conc. H2SO4 as catalyst)
 Via:

21. Sulphonation
 Via:

22. Sulphonation
 Sulphonation
 Producing strong sulphonic acid

23. Oxidation
 The Oxidation of toluene
R
OH
O
1) KMnO4, OH-
, Heat
2) H3O+
 Where R is alkyl group
 The Reduction ofAniline
NO2 NH2
Fe
HCl
aniline

24. Diazonium formation
 Diazonium salts is a good precursor compound for:
 Halogenation
 formation of phenol
 deamination
 coupling reaction of arenediazonium salts
 Formation of Diazonium salts
NH2 N
+ N
Cl
-
NaNO2, HCl
0 - 15 o
C, H2O

25. The Reactions

26. Halogenation
The bromination of benzene: All electrophilic aromatic substitution reactions share a common
mechanism.
This mechanism consists of a series of steps.
1. An electrophile — an electron‐seeking reagent — is generated.
2. For the bromination of benzene reaction, the electrophile is the Br+ ion generated by the
reaction of the bromine molecule with ferric bromide, a Lewis acid.

27. BROMINATION OF BENZENE
This mechanism consists of a series of steps.
1. An electrophile — an electron‐seeking reagent — is generated.
 2. The electrophile attacks the π electron system of the benzene ring to form a
nonaromatic carbocation.

28. BROMINATION OF BENZENE
 3. The positive charge on the carbocation that is formed is delocalized throughout the
molecule.
 4. The aromaticity is restored by the loss of a proton from the atom to which the bromine
atom (the electrophile) has bonded.
 5. Finally, the proton reacts with the FeBr 4
− to regenerate the FeBr 3 catalyst and form the
product HBr.

29. Summarization of Halogenation

30. The nitration of benzene
 In another example of an electrophilic aromatic substitution reaction, benzene
reacts with a mixture of concentrated nitric and sulfuric acids to create
nitrobenzene.

31. The mechanism for the nitrobenzene reaction occurs in six steps.
1. Sulfuric acid ionizes to produce a proton.
2. Nitric acid accepts the proton in an acid‐base reaction.
3. The protonated nitric acid dissociates to form a nitronium ion ( +NO 2).

32.  4. The nitronium ion acts as an electrophile and is attracted to the π electron
system of the benzene ring.
 5. The nonaromatic carbocation that forms has its charge delocalized around the
ring.

33.  6. The aromaticity of the ring is reestablished by the loss of a proton from the
carbon to which the nitro group is attached.

34.  The reaction of benzene with concentrated sulfuric acid at room temperature
produces benzenesulfonic acid.
The mechanism for the reaction that produces benzenesulfonic acid occurs in the
following steps:
1. The sulfuric acid reacts with itself to form sulfur trioxide, the electrophile.
This reaction takes place via a three‐step process:
The sulfonation of benzene

35. FORMATION OF ELECTROPHILE – SO3
This reaction takes place via a three‐step process:
 A.
 B.

36. FORMATION OF BENZENE SULPHONIC
ACID
 C.
 2. The sulfur trioxide is attracted to the π electron system of the benzene
molecule.
 3. Formation of benzene sulphonic acid

37. Monosubstituted Benzenes
 Monosubstituted alkylbenzenes are named as derivatives of benzene; an
example is ethylbenzene.
 The IUPAC system retains certain common names for several of the simpler
 monosubstituted alkylbenzenes. Examples are toluene (rather than
methylbenzene) and styrene (rather than phenylethylene):

38.  The common names phenol, aniline, benzaldehyde, benzoic acid, and anisole are also
retained by the IUPAC system:
 The physical properties of substituted benzenes vary depending on the nature of the
substituent.
 Alkylbenzenes, like other hydrocarbons, are nonpolar and thus have lower boiling points
than benzenes with polar substituents such as phenol, aniline, and benzoic acid.
 The melting points of substituted benzenes depend on whether or not their molecules can
be packed close together.
 Benzene, which has no substituents and is flat, can pack its molecules very closely, giving it
a considerably higher melting point than many substituted benzenes.

39.  the substituent group derived by the loss of an H from benzene is a phenyl group (Ph);
 that derived by the loss of an H from the methyl group of toluene is a benzyl group (Bn):

40. In molecules containing other functional groups, phenyl groups and benzyl
groups are often named as substituents:

41. Test your Knowledge
 Write a structural formula for the product formed by Friedel–Crafts alkylation or
acylation of benzene with

42. Strategy
 Utilize the fact that the halogenated reagent in Friedel–Crafts reactions will normally
form a bond with benzene at the carbon bonded to the halogen (Br or Cl).
 Therefore, to predict the product of a Friedel–Crafts reaction, replace the halogen in
the haloalkane or acyl halide with the benzene ring.
 One thing to be wary of, however, is the possibility of rearrangement once the
carbocation is formed.

43. Solution

44. Solution

45. Substituted
Benzenes

46. Substituted Benzenes
 When two substituents occur on a benzene ring, three constitutional isomers are
possible.
 We locate substituents either by numbering the atoms of the ring or by using the
locators
 ortho, meta, and para.
 The numbers 1,2- are equivalent to ortho (Greek: straight);
 1,3- to meta (Greek: after); and
 1,4- to para (Greek: beyond).

47. Substituted Benzene Nomenclature
 When one of the two substituents on the ring imparts a special name to the
compound, as, for example, toluene, phenol, and aniline, then we name the
compound as a derivative of that parent molecule.
 In this case, the special substituent occupies ring position number 1.
 The IUPAC system retains the common name xylene for the three isomeric
dimethylbenzenes.
 When neither group imparts a special name, we locate the two substituents and
list them in alphabetical order before the ending -benzene.
 The carbon of the benzene ring with the substituent of lower alphabetical ranking
is numbered C-1.

48. Substituted Benzenes Nomenclature

49. Polysubstituted Benzenes
 When three or more substituents are present on a ring, we specify their locations by numbers.
 If one of the substituents imparts a special name, then the molecule is named as a derivative
of that parent molecule.
 If none of the substituents imparts a special name, we number them to give the smallest set of
numbers and list them in alphabetical order before the ending -benzene.
 In the following examples, the first compound is a derivative of toluene, and the second is a
derivative of phenol.
 Because there is no special name for the third compound, we list its three substituents in
alphabetical order, followed by the word benzene:

50. Test your Knowledge
 Write names for these compounds:

51. Strategy
 First, determine whether one of the substituents imparts a special name to the benzene
compound (e.g., toluene, phenol, aniline).
 Identify all substituents and list them in alphabetical order.
 Use numbers to indicate relative position.
 The locators ortho, meta, or para can be used for disubstituted benzenes.

52. Solution
 Write names for these compounds:
 (a) 3-Iodotoluene or m-iodotoluene
 (b) 3,5-Dibromobenzoic acid
 (c) 1-Chloro-2,4-dinitrobenzene
 (d) 3-Phenylpropene

53. TYPES OF SUBSTITUTION IN AROMATIC RINGS
 Substituted rings are divided into two groups based on the type of the substituent
that the ring carries:
 Activated rings: the substituents on the ring are groups that donate electrons.
 Deactivated rings: the substituents on the ring are groups that withdraw
electrons.

54. ACTIVATING GROUPS
 Examples of activating groups in the relative order from the most activating group to the
least activating:
-NH2, -NR2 > -OH, -OR> -NHCOR> -CH3 and other alkyl groups
with R as alkyl groups (CnH2n+1)
 Examples of deactivating groups in the relative order from the most deactivating to the
least deactivating:
-NO2, -CF3> -COR, -CN, -CO2R, -SO3H > Halogens
with R as alkyl groups (CnH2n+1)
 The order of reactivity among Halogens from the more reactive (least deactivating
substituent) to the least reactive (most deactivating substituent) halogen is:
F> Cl > Br > I

55. HALOGENATION ORDER OF REACTIVITY
 The order of reactivity of the benzene rings toward the electrophilic substitution
when it is substituted with a halogen groups, follows the order of
electronegativity.
 The ring that is substituted with the most electronegative halogen is the most
reactive ring ( less deactivating substituent ) and the ring that is substituted with
the least electronegativity halogen is the least reactive ring ( more deactivating
substituent), when we compare rings with halogen substituents.
 Also the size of the halogen effects the reactivity of the benzene ring that the
halogen is attached to. As the size of the halogen increase, the reactivity of the
ring decreases.

56. Direction of the reaction
 The activating group directs the reaction to the ortho or para position,
 The electrophile substitute the hydrogen that is on carbon 2 or carbon 4.
 The deactivating group directs the reaction to the meta position,
 i.e. the electrophile substitute the hydrogen that is on carbon 3 with the exception
of the halogens that is a deactivating group but directs the ortho or para
substitution.

57. Substituents determine the reaction direction
 RESONANCE
Resonance effect is the conjugation between the ring and the substituent, which means
the delocalizing of the π electrons between the ring and the substituent.
 INDUCTIVE EFFECT
Inductive effect is the withdraw of the sigma ( the single bond ) electrons away from the
ring toward the substituent, due to the higher electronegativity of the substituent
compared to the carbon of the ring

58. Activating groups (ortho/para directors)
 When the substituents like -OH have an unshared pair of electrons, the resonance effect is stronger than the inductive effect
which make these substituents stronger activators, since this resonance effect direct the electron toward the ring.
 In cases where the substituents is esters or amides, they are less activating because they form r
 By looking at the mechanism above, we can see how groups donating electron direct the ortho, para electrophilic substitution.
 Since the electrons locating transfer between the ortho and para carbons, then the electrophile prefer attacking the carbon
that has the free electron.
 Inductive effect of alkyl groups activates the direction of the ortho or para substitution, which is when s electrons gets pushed
toward the ring.
 Resonance structure that pull the electron density away from the ring.

59. Substitution Summary
 The reaction of a substituted ring with an activating group is faster than benzene.
 On the other hand, a substituted ring with a deactivated group is slower than benzene.
 Activating groups speed up the reaction because of the resonance effect.
 The presence of the unpaired electrons that can be donated to the ring, stabilize the carbocation in
the transition state.
 Thus; stabilizing the intermediate step, speeds up the reaction; and this is due to the decrease of the
activating energy.
 On the other hand, the deactivating groups, withdraw the electrons away from the carbocation
formed in the intermediate step, thus;
 the activation energy is increased which slows down the reaction.

60. Deactivating group (meta directors)
 The deactivating groups deactivate the ring by the inductive effect in the presence of an electronegative
atom that withdraws the electrons away from the ring.
 we can see from the mechanism above that when there is an electron withdraw from the ring, that leaves
the carbons at the ortho, para positions with a positive charge which is unfavorable for the electrophile, so
the electrophile attacks the carbon at the meta positions.
 Halogens are an exception of the deactivating group that directs the ortho or para substitution.
The halogens deactivate the ring by inductive effect not by the resonance even though they have an
unpaired pair of electrons. The unpaired pair of electrons gets donated to the ring, but the inductive effect
pulls away the s electrons from the ring by the electronegativity of the halogens.

61. RESONANCE STRUCTURE OF ANILINE

62. RESONANCE STRUCTURE OF PHENOL

63. RESONANCE STRUCTURE OF NITROBENZENE

64. RESONANCE STRUCTURE OF BENZALDEHYDE

65. RESONANCE STRUCTURE OF BENZOIC ACID

66. Halogenation of Phenols
 Due to a highly activating effect of the hydroxyl group in phenols, they undergo halogenation even in
the absence of Lewis acids. When phenols are treated with bromine in the presence of a solvent of low
polarity like CHCl3 at low temperatures, monobromophenols are formed.
 When phenol is treated with bromine water, a white precipitate of 2, 4, 6-tribromophenol is formed.