Chemistry Form 6 Sem 3 03

September 26, 2017 | Author: Ng Swee Loong Steven | Category: Toluene, Chemical Reactions, Benzene, Chlorine, Acid
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Organic Chemistry_Aromatic Compounds...

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Chemistry form 6 organic chemistry chapter 3 : benzene and its compound

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 name Structural formula

Molecular formula

Common name

IUPAC name

C6H6

Benzene

Benzene

C7H8

Toluene

Methylbenzene

C8H10

Ortho-xylene

1,2-dimethylbenzene

C6H5OH

Phenol

Phenol

Structural formula

Molecular formula

Common name

IUPAC name

C6H5NO2

Nitrobenzene

Nitrobenzene

C6H5COOH

Benzoic acid

Benzenecarboxylic acid

C6H5COH

Benzaldehyde

Phenylmethanal

C6H5NH2

Aniline

Phenylamine

C10H8

Naphthalene

Naphthalene

3.1    

Nomenclature of aromatic compounds For simple aromatic compound, it is as describe in the table above Benzene can also be considered as a branched group. Branched benzene is called as phenyl (C6H5–) When there are 2 or more substituents on benzene ring, 3 structural isomers are possible. The substituents may be located by numbering the atoms of the ring, or may be indicates by prefixes of ortho, meta, or para

Position of the 2 substituents in benzene ring 1,2-position [ortho (o)] 1,3-position [meta (m)] 1,4-position [para (p)]

1,2 – dichlorobenzene ortho-dichlorobenzene

1,3 – dichlorobenzene meta-dichlorobenzene

1,4 – dichlorobenzene para-dichlorobenzene

1,2-dinitrobenzene o-dinitrobenzene

1,3-dinitrobenzene m-dinitrobenzene

1,4-dinitrobenzene p-dinitrobenzene

2-nitrophenol

3-nitrophenol

4-nitrophenol

2-bromotoluene

3-hydroxybenzoic acid

4-methylbenzaldehyde

When 3 or more groups are on benzene ring, a numbering system must 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 base on alphabetical order.





2,3-dichlorotoluene

5-bromo-3-nitrotoluene

4-chloro-2-ethylphenol

Br

Br

NO2

Br

2-hydroxy-5-methylbenzoic 2,4,6-tribromonitrobenzene 3-chloro-2-phenylbutane acid

3.2 Reaction of Benzene  Even though in benzene contain 3 double bonds, but as explained in Kekule’s structure, it give an extra stability due to delocalised π – 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 3. Acylation 4. Nitration 5. Sulphonation Name of reaction

Reagent used and condition

Halogenation

Chlorine gas, Cl2 with AlCl3 as halogen carrier (catalyst) ----------------Bromine gas, Br2 with FeBr3 as halogen carrier (catalyst)

Equation

benzene

halogen

halobenzene

Name of reaction Friedel – Crafts Alkylation

Friedel – Crafts Acylation

Nitration

Sulphonation

Reagent used and condition Haloalkane (R – X) with AlCl3 as halogen carrier (catalyst) Acyl chloride with AlCl3 as halogen carrier (catalyst) Concentrated Nitric acid (HNO3) catalysed by concentrated sulphuric acid and reflux at 55oC

Equation

benzene

haloalkane

benzene

acyl chloride

benzene nitric acid

Concentrated sulphuric acid (H2SO4) and heat at 55oC under benzene sulphuric acid reflux

alkylbenzene

nitrobenzene

benzenesulphonic acid

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 catalyst

iron (III) bromide and some hear

 The mechanism of halogenation of benzene  Step 1 : Formation of halogen ion (X+) as electrophile using

heterolytic fission reaction. In chlorine, aluminium chloride (electron deficient compound) is readily to receive lone pair electron (act as Lewis acid) from chlorine

 Step 2 : Electrophilic attack on benzene ring to form a

carbocation. Cl+ ion attack the benzene ring and the delocalise πelectron form a C–Cl bond in benzene. This will result a carbocation formed as intermediate and disturb the ring (cause benzene ring become unstable)

 Step 3 : Proton lost from carbocation. Carbocation transfers a

proton to [AlCl4]− and the benzene ring is stabilised back. This results in the formation of chlorobenzene and HCl.



[As extra note, benzene also react with chlorine in the presence of UV and some heat to form 1,2,3,4,5,6-hexachlorocyclohexane (addition reaction)]

Friedel–Crafts reaction  Similar to halogenation, Friedel – Crafts reaction also required a halogen carrier to act as catalyst  Depending on the type of haloalkane used, the halogen carrier is also different.  If chloroalkane (R–Cl) is used, the halogen carrier will be aluminium chloride (AlCl3)  If bromoalkaane (R–Br) is used, the halogen carrier will be iron (III) bromide (FeBr3) 3.2.2 Alkylation of Benzene  When chloroethane (CH3CH2Cl) react with benzene with the presence of AlCl3, ethylbenzene is produced (C6H5–CH2CH3) under room temperature

The mechanism of alkylation is very similar in ways of how halogenation 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.

3.2.3 Acylation of Benzene  When ethanoyl chloride (CH3COCl) reacts with benzene under the presence of AlCl3, phenylethanone is produced (C6H5–COCH3) at 80oC.

 The mechanism of acylation

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 O O CCH3 CCH3 Cl AlCl3 + H carbocation

+ +

HCl AlCl3

 For nitration and sulphonation of benzene, halogen carrier is not

used, 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, HNO3 will only react with benzene under 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 base where it accept a proton donated by sulphuric acid

Step 2 : Electrophile attacked benzene ring to form carbocation. NO2+ ion attack the benzene ring and delocalise πelectron form a C–NO2 bond in benzene. This will result a carbocation formed as intermediate and disturb the ring (cause benzene ring become unstable)

Step 3 : Proton lost from carbocation. Carbocation transfers a proton to HSO4− and the benzene ring is stabilised back. This results in the formation of nitrobenzene and H2SO4 (catalyst)

 When nitration is carried out at higher temperature

(above 200oC), a 1,3,5-trinitrobenzene can be formed where : O2 N + 3 HNO3

NO2 + 3 H2O

200oC

O2N

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, sulphonation does not required a catalyst as the reagent used, sulphuric acid (H2SO4) act as a catalyst itself

 Step 1 : Formation of electrophile from sulphuric acid. The

protonation of sulphuric acid when it received one H+ (BronstedLowry base) from another sulphuric acid

Step 2 : Electrophile attacked benzene ring to form carbocation.

Step 3 : Proton lost from carbocation

Other chemical reaction of benzene  Unlike alkene, benzene is stabilised by the delocalised π electrons. So, it does not react easily as in alkene. For example, if benzene react with acidified potassium manganate (VII), KMnO4 (H2SO4)

 When react with hydrogen gas with presence of nickel as catalyst at

180oC, 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 250oC + CH2CH CH2

H C

CH3 CH3

3.3 

Influence of Substitution Group on Reactivity and Orientation of Substituted Benzene When benzene ring contained a substituents M, the reaction of C6H5–M may be faster / slower compare to benzene

Group of M Effect of groups

Examples

Ring activating groups (ortho, para directing) Cause ring more reactive ( increase rate)

Ring deactivating groups (meta directing) Cause ring less reactive ( decrease rate)

– CH3

– NH2

– OH

– NO2

– COOH

– COH

– CH2CH3

– NH2R

– OR

– SO3H

– COR

–X (Cl, Br)

Type of director ortho director

para director

meta director



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.





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 activated when a 30 carbocation formed, it increase the rate of reaction

 



Properties of ring deactivate group Electron withdrawing groups have negative inductive effect (–I) δ+ δ− Under (–I) effect, C – M, carbon had already bear partial positive charge δ+







Unlike electron donating group, when the cation is placed at the directing group of electron withdrawing group, it will tend to become unstable So attacking at meta position is more stable than in ortho / para position. Still, since in react much slower than in benzene, so electron withdrawing group is to say deactivate benzene ring and cause the rate of reaction decrease.

3.4 Reaction of methylbenzene  Methylbenzene resemble with benzene in many ways. As methylbenzene is less toxic, is often used as reagent instead of benzene. Moreover, methyl (CH3–) is ring activate group, it react faster and required lesser effort (lower temperature, concentration electrophile) compare to benzene.  Unlike benzene, methylbenzene contain an aliphatic (CH3–) and aromatic (C6H6). In other words, methylbenzene undergoes 2 distinctive type of reaction : ⇒ reaction of the methyl group ⇒ reaction of the benzene ring 3.4.1 Reaction of the methyl group in methylbenzene

Name of reaction

Oxidation of methylbenzene

Reagent used and condition

Equation

Acidified potassium manganate (VII) KMnO4 / H2SO4

*Observation : (1) purple colour of potassium manganate (VII) decolourised when react with toluene

Acidified potassium dichromate (VI) K2Cr2O7 / H2SO4

+ H2 *Observation : Green colour of potassium dichromate (VI) changed to orange colour

Chlorination Chlorine gas of under UV light methylbenz at room ene temperature * side product of reaction is HCl (g)

 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 method to 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.

3.4.2

Reaction of methylbenzene in the benzene ring

Name of reaction

Reagent used And condition

Halogenation

Cl2 / AlCl3 or Br2 / FeBr3

Equation

o-chlorotoluene p-chlorotoluene

Friedel – Crafts Alkylation

CH3Cl / AlCl3 o-xylene

Friedel – Crafts Acylation

p-xylene

CH3COCl / AlCl3 o-ethanoyltoluene p-ethanoyltoluene

Nitration

Conc. HNO3 + conc. H2SO4 o-nitrotoluene

Sulphonation

p-nitrotoluene

Concentrated H2SO4 o / p - methylbenzenesulphonic acid

Other types of alkylbenzene synthesis and reaction  Formation of phenol

 Formation of aniline

 Practice : Suggest the methods of how to synthesis these products

from benzene. 1.

+ HNO3

NO2 H3C

2.

HO3S

CH3

3.

H3C

NO2

H2SO4

4.

O

CCH3

5.

6.

+ HNO3

H2SO4

7. + CH3CH=CHCH3

8. NH2

AlCl3

Step 1 :H2SO4 + HNO3  NO2+ + HSO4-- + H2O [1]

 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]

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 won’t Equation :

HNO3 catalysed by H2SO4 under reflux Acidified KMnO4 under reflux HCl under Sn as catalyst

Step 1 :H2SO4 + HNO3  NO2+ + HSO4-- + H2O [1]

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 :

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