1. OF AROMATIC HYDROCARBONS
CHEMICAL REACTIONS
OF AROMATIC HYDROCARBONS
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2. We have noted that aromatic hydrocarbons do not readily undergo the addition reactions which are the characteristics of other unsaturated hydrocarbons
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3. Benzene is highly unresponsive to addition reactions
An addition reaction would require breaking up the delocalized π- bonding present in the benzene ring
What we have found?
Benzene is highly unresponsive to addition reactions
What reaction does it undergo?
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4. Benzene undergoes SUSTITUTION REACTIONS
What is meant by substitution reactions ?
Kind of reactions which are characterized by different atoms or groups of atoms replacing hydrogen atoms in a hydrocarbon molecule
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5. Types of reactions
There are three important types of substitution reactions for benzene for benzene other aromatic hydrocarbons
Alkylation
Halogenation
Nitration
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6. General Mechanism
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7. Mechanism
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8. Bromination of Benzene
Requires a stronger electrophile than Br2.
Use a strong Lewis acid catalyst, FeBr3.
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9. Chlorination and Iodination
Chlorination is similar to bromination. Use AlCl3 as the Lewis acid catalyst.
Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion.
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10. Nitration of Benzene
Use sulfuric acid with nitric acid to form the nitronium ion electrophile.
NO2+ then forms a
sigma complex with
benzene, loses H+ to
form nitrobenzene.
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11. Nitration of Toluene
Toluene reacts 25 times faster than benzene. The methyl group is an Activator.
The product mix contains mostly ortho and para substituted molecules.
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12. Halobenzenes
Halogens are deactivating toward electrophilic substitution, but are ortho, para-directing!
Since halogens are very electronegative, they withdraw electron density from the ring inductively along the sigma bond.
But halogens have lone pairs of electrons that can stabilize the sigma complex by resonance =>
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13. Multiple Substituents
The most strongly activating substituent will determine the position of the next substitution. May have mixtures.
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14. Friedel-Crafts Alkylation
Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl3
Reactions of alkyl halide with Lewis acid produces a carbocation which is the Electrophile.
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15. Friedel-Crafts Acylation
It is done by using acyl chloride and strong Lewis acid catalyst
Acyl chloride is used in place of alkyl chloride.
The acylium ion intermediate is resonance stabilized and does not rearrange like a carbocation.
The product is a phenyl ketone that is less reactive than benzene =>
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16. Mechanism of Acylation
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17. Reduction of ketones or aldehydes By using zinc amalgam and HCl
Clemmensen Reduction
Reduction of ketones or aldehydes
By using zinc amalgam and HCl
Acylbenzenes can be converted to alkyl benzenes by treatment with aqueous HCl and amalgamated zinc.
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19. Gatterman-Koch Formylation
This reaction is used to synthesize benzaldehyde from benzene
Benzaldehyde and many aromatic aldehydes are conveniently synthesized by this reaction
Traces of copper(II) chloride are also needed in this reaction
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20. Formyl chloride is unstable.
Use a high pressure mixture of CO, HCl, and catalyst.
Product is benzaldehyde.
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21. Nucleophilic Aromatic Substitution
A nucleophile replaces a leaving group on the aromatic ring.
Electron-withdrawing substituents activate the ring for nucleophilic substitution =>
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22. Examples of Nucleophilic Substitution
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23. Catalytic Hydrogenation
Elevated heat and pressure is required.
Possible catalysts: Pt, Pd, Ni, Ru, Rh.
Reduction cannot be stopped at an intermediate stage.
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