Chemistry Form 6 Sem 3 03

8/13/2019 Chemistry Form 6 Sem 3 03

1/39

Chemistry form 6organic chemistry

chapter 3 :benzene and its compound


2/39

3.0 Introduction

Organic compounds which contain benzene are categorise as

aromatic compounds (arene)

For most of simple aromatic compounds, it will end with benzene.

There are basic type of aromatic compounds, structural formula,

common name and IUPAC nameStructural formula Molecular formula Common name IUPAC name

Toluene Methylbenzene

Ortho-xylene 1,2-dimethylbenzene

Phenol Phenol

C7H8

C8H10

C6H5OH


3/39

Structural formula Molecular formula Common name IUPAC name

Nitrobenzene Nitrobenzene

Benzoic acidBenzenecarboxylic

acid

C6H5NO2

C6H5COOH

Benzaldehyde Phenylmethanal

Aniline Phenylamine

Naphthalene Naphthalene

C6H5COH

C6H5NH2

C10H8


4/39


5/39

1,2-dinitrobenzene

o-dinitrobenzene

1,3-dinitrobenzenem-dinitrobenzene

1,4-dinitrobenzene

p-dinitrobenzene

2-nitrophenol 3-nitrophenol 4-nitrophenol

2-bromotoluene 3-hydroxybenzoic acid 4-methylbenzaldehyde


6/39

When 3 or more groups are on benzene ring, a numbering systemmust be used to name them. Usually a smaller number of groups

will be C1 and the other will be numbered accordingly. If there are 3 different groups, the one which have a common name

will be given priority. The other 2 will be name and numbered baseon alphabetical order.

NO2

Br

Br

Br

2,3-dichlorotoluene 5-bromo-3-nitrotoluene 4-chloro-2-ethylphenol

2,4,6-tribromonitrobenzene2-hydroxy-5-methylbenzoicacid 3-chloro-2-phenylbutane


7/39

3.2 Reaction of Benzene

Even though in benzene contain 3 double bonds, but as explained in

Kekules structure, it give an extra stabilitydue to delocalised electrons in the ring and the resonance structure.

Thus, benzene usually undergoes substitution reaction instead of

addition reaction. The substitution reactions of benzene with an electrophilic reaction

include : 1. Halogenation 2. Alkyation

. Ac lation . Nitration . Sul honation

Name of reactionReagent used

and conditionEquation

Halogenation

Chlorine gas, Cl2with AlCl3 as

halogen carrier(catalyst)

-----------------

Bromine gas, Br2with FeBr3 as

halogen carrier(catalyst)

benzene halogen halobenzene


8/39

Name of

reaction

Reagent used


Friedel Crafts

Alkylation

Haloalkane (R

X) with AlCl3 as

halogen carrier

(catalyst)benzene haloalkane alkylbenzene

Friedel Crafts

Acylation

Acyl chloridewith AlCl3 as

halogen carrier

(catalyst) benzene acyl chloride

Nitration

ConcentratedNitric acid (HNO3)

catalysed by

concentrated

sulphuric acid and

reflux at 55oC

benzene nitric acid nitrobenzene

Sulphonation

Concentrated

sulphuric acid

(H2SO4) and heat

at 55oC underreflux

benzene sulphuric acid benzenesulphonic acid


9/39

3.2.1 Halogenation

Chlorine react with benzene under aluminium chloride as catalyst

under room condition

Bromine reacts with benzene only under the presence of catalystiron (III) bromide and some hear

The mechanism of halogenation of benzene

Step 1 : Formation of halogen ion (X+) as electrophile usingheterolytic fission reaction. In chlorine, aluminium chloride

(electron deficient compound) is readily to receive lone pairelectron (act as Lewis acid) from chlorine


10/39


11/39


12/39


13/39

The mechanism of alkylation is very similar in ways of howhalogenation occur.

Step 1 : Formation of electrophile by heterolytic fission

Step 2 : Electrophile attacking the benzene ring to form carbocation

Step 3 : Proton lost from the unstable carbocation formed earlier.


14/39

3.2.3 Acylation of Benzene

When ethanoyl chloride (CH3COCl) reacts with benzene under the presence

of AlCl3, phenylethanone is produced (C6H5COCH3) at 80o

C.

The mechanism of acylation

Step 1 : Formation of electrophile by heterolytic fission

Step 2 : E ectrop e attac ng t e enzene r ng to orm car ocat on

Step 3 : Proton lost from the unstable carbocation formed earlier

AlCl3Cl+CCH3

H

carbocation

CCH3+ HCl

+ AlCl3

OO


15/39

For nitration and sulphonation of benzene, halogen carrier is notused, as the reagent used for the reaction is an acid. The mechanism

of nitration and sulphonation are also nearly similar to each other.3.2.4 Nitration of benzene

Concentrated nitric (V) acid, HNO3will only react with benzeneunder the presence of a little concentrated sulphuric acid (H2SO4) at

55oC heated under reflux, to produce nitrobenzene

The mechanisms of nitration are explained below

Step 1 : Production of nitronium ion, NO2+. In nitration of

benzene, nitric (V) acid act as Bronsted-Lowry basewhere it

accept a proton donated by sulphuric acid


16/39

Step 2 : Electrophile attacked benzene ring to formcarbocation. NO2

+ ion attack the benzene ring and delocalise -

electron form a CNO2bond in benzene. This will result a carbocationformed as intermediate and disturb the ring (cause benzene ringbecome unstable)

Step 3 : Proton lost from carbocation. Carbocation transfers aproton to HSO4

and the benzene ring is stabilised back. This results inthe formation of nitrobenzene and H2SO4 (catalyst)


17/39

When nitration is carried out at higher temperature(above 200oC), a 1,3,5-trinitrobenzene can be formed

where :

+ 3 HNO3 NO2 +200oC3 H2O

O2N

O2N


18/39

3.2.5 Sulphonation of benzene

The mechanisms occur for sulphonation of benzene is more or less

the same with nitration of benzene. Unlike nitration, sulphonationdoes not required a catalyst as the reagent used, sulphuric acid(H2SO4) act as a catalyst itself

Ste 1 : Formation of electro hile from sul huric acid. The

protonation of sulphuric acid when it received one H+ (Bronsted-Lowry base) from another sulphuric acid


19/39

Step 2 : Electrophile attacked benzene ring to formcarbocation.

Step 3 : Proton lost from carbocation


20/39

Other chemical reaction of benzene

Unlike alkene, benzene is stabilised by the delocalised electrons.

So, it does not react easily as in alkene. For example, if benzene reactwith acidified potassium manganate (VII), KMnO4 (H2SO4)

When react with hydrogen gas with presence of nickel as catalyst at180oC, it form cyclohexane. The reaction is an additional reaction.

benzene cyclohexane

Benzene also reacts with propene to give isopropylbenzene (well

known as cumene) which is a starting material to synthesis phenol.Concentrated H3PO4 serve at catalyst under 250

oC

+ CH2CH CH2

H

C

CH3

CH3


21/39

3.3 Influence of Substitution Group on Reactivity and

Orientation of Substituted Benzene

When benzene ring contained a substituents M, the reaction ofC6H5M may be faster / slower compare to benzene

Group of MRing activating groups

(ortho, para directing)

Ring deactivating groups

(meta directing)

Effect of

groups

Cause ring more reactive (

increase rate)

Cause ring less reactive (

decrease rate)

Examples

3 2 2

CH2CH3 NH2R OR SO3H COR

X (Cl,

Br)

Type of

director

ortho director para director meta director


22/39

Properties of ring activate group

Electron donating groups have positive inductive effect (+I)

When electrophile attacked the benzene ring, carbocation is formed. Since a more stable carbocation form faster than a less stable one,

when electrophile attacked at ortho &para position.


23/39

As discussed earlier, 3o carbocation is more stable than 2o

carbocation. Using resonance, it is possible for cation to reside at 3o

carbon.

Since ortho / para position are more activatedwhen a 30

carbocation formed, it increase the rate of reaction


24/39

Properties of ring deactivate group

Electron withdrawing groups have negative inductive effect (I)

+ Under (I) effect, C M, carbon had already bear partial positive

charge +


25/39

Unlike electron donating group, when the cation is placed at the

directing group of electron withdrawing group, it will tend tobecome unstable

So attacking at meta position is more stable than in ortho / paraposition.

Still, since in react much slower than in benzene, so electronwithdrawing group is to say deactivate benzene ring and causethe rate of reaction decrease.


26/39

3.4 Reaction of methylbenzene

Methylbenzene resemble with benzene in many ways. As

methylbenzene is less toxic, is often used as reagent instead ofbenzene. Moreover, methyl (CH3) is ring activate group, it reactfaster and required lesser effort (lower temperature, concentrationelectrophile) compare to benzene.

Unlike benzene, methylbenzene contain an aliphatic (CH3) andaromatic (C6H6). In other words, methylbenzene undergoes 2distinctive type of reaction :

react on o t e met y group react on o t e enzene r ng

3.4.1 Reaction of the methyl group in methylbenzene


27/39

Name of

reaction

Reagent used


Oxidation ofmethyl-

benzene

Acidified

potassium

manganate

(VII)

KMnO4 /

H2SO4

*Observation : (1) purple colour of potassium manganate

(VII) decolourised when react with toluene

Acidified

potassium

(VI)K2Cr2O7 /

H2SO4

+ H2*Observation : Green colour of potassium dichromate

(VI) changed to orange colour

Chlorination

of

methylbenz

ene

Chlorine gas

under UV light

at room

temperature

* side product of reaction is HCl (g)


28/39

Methylbenzene reacts with strong oxidising agent such as acidified

potassium manganate (VII) [KMnO4 / H+] or acidified potassium

dichromate (VI) [K2Cr2O7 / H

+

] to form benzoic acid. This is a methodto distinguish between benzene and methylbenzene.

Under room temp, only H in methyl is substituted by Cl atom.

Step 1 : Initiation Formation of Cl (radical)

Step 2 : Propagation Radical attack methylbenzene to form multiple form

of radical

Step 3 : Termination chlorine radical react and methylbenzene radical

If temperature increases to 200oC, then, even the H inside benzene ring

may be substituted by Cl.


29/39

3.4.2 Reaction of methylbenzene in the benzene ring

Name of

reaction

Reagent used

And condition

Equation

Halogenation

Cl2 / AlCl3or

Br2 / FeBr3o-chlorotoluene p-chlorotoluene

CraftsAlkylation

CH3Cl / AlCl3

o-xylene p-xylene

Friedel

Crafts

Acylation

CH3COCl /

AlCl3

o-ethanoyltoluene p-ethanoyltoluene


30/39

NitrationConc. HNO3 +

conc. H2SO4

o-nitrotoluene p-nitrotoluene

Sulpho-nation

ConcentratedH2SO4

o / p - methylbenzenesulphonic acid

Formation of phenol

Formation of aniline


31/39

Practice : Suggest the methods of how to synthesis these productsfrombenzene.

1.

2.

NO2

H3C

CHHO S

+ HNO3H2SO4

3.

NO2H3C

4 O


32/39

4.

5.

CCH3

O

6. + HNO3 H2SO4


33/39

7.+ CH3CH=CHCH3

AlCl3

8.

NH2


34/39

Step 1 :H2SO4 + HNO3 NO2+ + HSO4

-- + H2O [1]


35/39

Reaction I is oxidation [1], where acidified potassium manganate (VII)

[1] under reflux [1]

Reaction II is free radical substitution reaction [1], where bromine gas

[1] under the presence of sunlight [1] is required Reaction III is electrophilic aromatic substitution reaction [1], where

bromine gas react under the presence of iron (III) bromide [1]


36/39

A : chlorine gas under the presence of AlCl3 as catalyst

B : chlorine gas under the presence of UV

Reagent : Using acidified potassium manganate (VII)

Observation : A will decolourised purple colour of acidified KMnO4, while B wont

Equation :


37/39


38/39

HNO3 catalysed by H2SO4 under reflux

Acidified KMnO4 under reflux

HCl under Sn as catalyst

Step 1 :H2SO4 + HNO3 NO2+ + HSO4

-- + H2O [1]


39/39

Reagent : Using acidified potassium manganate (VII)

Observation : methylbenzene will decolourised purple colour of acidified KMnO4,

while benzene will not.

Equation :

Reagent : Using nitric acid catalysed by concentrated sulphuric acid under reflux

Observation : benzene will turn from colourless to yellow liquid while cycloalkane

will remain colourless

Equation :

Date post:	04-Jun-2018
Category:	Documents
Upload:	ng-swee-loong-steven
View:	241 times
Download:	0 times

Chemistry Form 6 Sem 3 03

Documents