aromatic compound1

8/3/2019 Aromatic Compound1

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Jog your memory !!!

Test 1 week 4 (30th Sept. 2011)

Midterm exam week 9 ( 31stOct. 2011)

Test 2 week 12 (21st Nov. 2011)

Final exam week 16 ( 12th Dec. 2011)

Viel gluck


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Aromatic Compound


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Learning Objectives

1. Determine the nomenclature and the physical propertiesof an aromatic compound

2. Determine the reaction of benzene

3. Propose a mechanism on Chlorination and Bromination,Nitration, Friedel-Crafts alkylation and acylation


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*monosubstituted


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Aniline


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*disubstituted


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*polysubstituted

4-chloro-2-nitrotoluene

2,4,6-tribromophenol

2-bromo-1-ethyl-4nitrobenzene

3,4-dimethoxybenzaldehyde


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Naming the Aromatic Hydrocarbon


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Physical Properties of Benzene

*Colorless liquid with Melting point : 5.5 oC; Boiling point : 80oC

*Benzene burns, and has a toxic vapor

*Immiscible with water but dissolved in organic solution

Sources of Benzene

Benzene is obtained from petroleum oil by fractional distillation followed

by reforming, and also by destructive distillation of coal.

Reactivity of Benzene

The benzene ring is a planar hexagon with a

cloud of delocalised electrons lying aboveand below the ring.

The reactions of benzene usually involve the

attack of an electrophile on the cloud of

electrons


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Reaction of alkenes


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b)Addition Reactions of Benzene

a)Oxidation Reactions

Benzene is unaffected by strong oxidizing agents (H2

CrO4

& KMnO4

)However, side chain methyl group is oxidized to a carboxyl group to give

carboxylic acid

Eg; C6H5CH3 C6H5COOH + H2O

Hydrogenation and Chlorination are undergo the addition reaction of

benzene

Eg; C6H6 + 3H2 C6H12

Eg; ; C6H6 + 3Cl2 C6H5Cl 6

KMnO4, OH-

H3O+,

Ni, 200C

h


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1. Halogenation of benzene and toluene

- Cl2

and Br2

can be substituted in the ring in the presence of a Friedel-Crafts

catalyst which is called a halogen-carrier

C6H6 + X2 C6H5X + HX

Eg; Benzene + Br2 Bromobenzene + HBr

Eg; Toluene + Cl2 o-chlorotoluene + p-chlorotoluene + HCl

! Mechanism

c) Substitution Reactions

Fe/FeX3

Undergoes electrophilic substitution where, electrophile attack electron (at

the aromatic ring) caused substitution of proton


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Mechanism

1. Lewis acid (eg. FeBr3) accepts a pair of electrons from Br atom the

polarised Br2 molecule and enables Br-Br bond to split.

2. A bromonium ion and FeBr4- complex ion are formed.

3. The bromonium ion rapidly loses a proton to form bromobenzene with

a regeneration of the catalyst, FeBr3


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Effects of a Substituent Group on further

substitution

Affect the orientation of new group.

~classified as ortho-para directing or

~classified as meta directing

Affect the rate of further substitution

~classified as activating (caused the rate of second substitution

to be faster than that for benzene itself)or

~classified as deactivating (caused the rate of second

substitution to be slower than that for benzene itself)


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Ortho-Para directing Meta directing

Activating

-NH2. NHR, -NR2, -OH,

-OR,-C6H5, -R

___

Deactivating Halogen (-F, -Cl, -Br, -I)-NO2, -COOH, -CN, -COOR,

-SO3H, -CHO, -COR,

-CONH2, -NH3+, -CF3, -CCl3


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2. Nitration of benzene and toluene

- Substitution of a H atom by a NO2 group

Eg; Benzene + HNO3 Nitrobenzene + H2O

Eg; Toluene + HNO3 o-chlorotoluene + H2O +

p-chlorotoluene

! Mechanism-Formation of nitryl cation (or nitronium ion), NO2

+

H2SO4

H2SO4


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- NO2+ is an electrophile attacks the ( electron) benzene ring to give resonance

stabilized cation intermediate

-This intermediate rapidly loses a proton to form the product


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i. Alkylation

-An alkyl group can be introduced into benzene ring by the reaction of ahalogenoalkane with benzene:

3. Friedel-Craft reaction

An electrophilic aromatic substitution in which a hydrogen of an aromatic

ring is replaced by an alkylor acylgroup


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Mechanism

1. Formation of an ion complex:

2. A carbocation complex attacks benzene to form an intermediate

which loses a proton to form product:


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ii. Acylation

-Introduction of an acyl group :

-Acylation needs acyl halide (RCOCl) + Friedel-Craft catalyst

C

O

R


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Mechanism

1. F-C catalyst + RCO form a complex

2. The elecrophilic complex attacks benzene to form a reactive

intermediate which rapidly converted into the product


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Thank you

aromatic compound1

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