Chemistry Form 6 Sem 3 Chapter 3
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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
2-nitrophenol
2-bromotoluene
1,3-dinitrobenzene m-dinitrobenzene
3-nitrophenol
3-hydroxybenzoic acid
1,4-dinitrobenzene p-dinitrobenzene
4-nitrophenol
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
2,4,6-tribromonitrobenzene
5-bromo-3-nitrotoluene
4-chloro-2-ethylphenol
2-hydroxy-5-methylbenzoic 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 reflux benzene sulphuric acid
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 π-electron 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
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 π-electron 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 :
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+ (Bronsted-Lowry 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
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
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)
Examples
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
Reagent used and condition
Equation
Acidified potassium manganate (VII) *Observation : (1) purple colour of potassium manganate KMnO4 / H2SO4 (VII) decolourised when react with toluene Oxidation of methylbenzene Acidified potassium dichromate (VI) + H2 K2Cr2O7 / *Observation : Green colour of potassium dichromate (VI) H2SO4 changed to orange colour
Chlorination of methylbenze ne
Chlorine gas under UV light at room 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.
2.
3.
4.
5.
6.
7.
8.
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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