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Introduction • Hydrocarbon that contain rings which stabilised by electron delocalisation are called aromatic hydrocarbon or arenes • The simplest yet most important member of the aromatic hydrocarbons is benzene
Naming Aromatic Hydrocarbon • Monosubstituted alkylbenzene is named as derivatives of benzene. For example:
The IUPAC system retains certain common names for some simpler alkylbenzenes. For example: Aniline, styrene, toluene. • When two or more substituents on a benzene ring, three structural isomers are possible. • Location of two substituents may be indicated by the prefixsortho, meta or para • 1,2 is equivalent to ortho, o- • 1,3 is equivalent to meta, m- • 1,4 is equivalent to para, p- • The order of decreasing priorities of common substituents is: COOH > SO3H> CHO> CN > C=O> OH> NH2> R> NO2> X Where R= alkyl group ; X= halogen
When three or more groups are on the benzene ring, a numbering system must be used to locate them. For example,
In more complex molecules, the benzene ring is named as a substituent on a parent chain. In this case, the C6H5- group is called a phenyl group. For example
Exercise • Name the following aromatic compounds
Reaction of benzene • Nitration • Nitration is the substitution of a nitro group, -NO2, for one of the hydrogen atoms on the benzene ring • Benzene reacts with a mixture of concentrated nitric acid and concentrated sulphuric acid • The product is nitrobenzene, a pale yellow liquid
Halogenation • This reaction involves substitution of a halogen atom for a hydrogen atom on the aromatic ring • In the presence of a catalyst or halogen carrier such as aluminium chloride, AlCl3, benzene is substituted by chlorine or bromine at room temperature.
The catalyst aluminium chloride, AlCl3, an electron-deficient compound which has only six electrons in its outer shell, accepts a lone pair of electrons from one of the chlorine atoms, which induces polarisation in the chlorine molecule. • The positively charged end of the chlorine molecule is now an electrophile and attacks the benzene ring.
Alkylation: Friedel-Crafts reaction • This reaction substitutes an alkyl group for a hydrogen atom on an aromatic ring • When benzene is warmed with chloromethane, CH3Cl and aluminium chloride (catalyst) under anhydrous conditions, a substitution reaction occurs and methylbenzene is formed
AlCl3 polarise the CH3 Cl molecule • The positively charged methyl group attacks the benzene ring and electrophilic substitution occurs
Acylation • Involves substitution of an acyl group, RCO, for a hydrogen atom on the aromatic ring. • When benzene is added to ethanoyl chloride and AlCl3, under anhydrous conditions, a substitution reaction occurs and phenylethanone is formed.
Exercise • Write the structural formula for the producta formed from Friedal Crafts alkylation or acylation of benzene with (a) CH3CH2CH2Cl (b) CH3CH2CH2COCl
Sulphonation • Involves substitution of an –SO3H group for a hydrogen atom on the aromatic ring. • If benzene and [H2SO4] are reflux together for several hours, benzenesulphonic acid is formed:
Reaction of methylbenzene • Reaction with chlorine • When chlorine, Cl2, is bubbled through boiling methylbenzene in strong sunlight or uv light, substitution takes place on the methyl group:
If Cl2 is bubbled through methylbenzene in the absence of uv light but in the presence of AlCl3, the ring is substitutes. A mixture of two isomers is obtained. • White fumes of HCl are given off • Electron density of the benzene ring more strongly at the 2- and 4- positions
Reaction with HNO3 • When methyl benzene is added to HNO3 and concentrated H2SO4, at room temperature, a mixture of 2- and 4-nitromethylbenzene
If the temperature is raised, two or three nitro groups, NO2 are introduced
Reaction with CH3I (Friedal craft alkylation) • When methlbenzene is added to CH3I in AlCl3 at 20 oC, a mixture of 1,2-dimethylbenzene and 1,4-dimethylbenzene is formed
Reaction with CH3COCl (FriedalCraft Acylation) • The reaction of an acyl chloride with an aromatic hydrocarbon is known as FriedalCraft acylation. • Involves substitution of an acyl group, RCO-, for a hydrogen atom on the aromatic ring in the presence of AlCl3.
Exercise • Write the structural formula for the products formed by Friedal Crafts acylation or alkylation of methylbenzene with • C6H5COCl (b) CH3CH2Br
Reaction with SO3 • When methylbenzene is added to fuming H2SO4 containing SO3 at 0oC, a mixture of 2- and 4-methylbenzenesulphonic is obtained.
Oxidation of alkylbenzene • Alkylbenzenes are oxidised to benzoic acid on prolonged reflux with KMnO4 or K2Cr2O7
Exercise • Predict the products from treatment of each of the following compounds with KMnO4 / H2SO4 under reflux. (c) (a) (b)
Exercise • Name compound P, Q, R and S below
Exercise • Draw the structural formulae of the organic products formed when methylbenzene reacts with • Concentrated nitric acid and concentrated sulphuric acid at room temp • Chlorine in the presence of anhydrous aluminium chloride • Chloromethane in the presence of aluminium chloride