OF AROMATIC HYDROCARBONS CHEMICAL REACTIONS OF AROMATIC HYDROCARBONS 4/14/2019 Chem-114

We have noted that aromatic hydrocarbons do not readily undergo the addition reactions which are the characteristics of other unsaturated hydrocarbons 4/14/2019 Chem-114

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? 4/14/2019 Chem-114

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 4/14/2019 Chem-114

Types of reactions There are three important types of substitution reactions for benzene for benzene other aromatic hydrocarbons Alkylation Halogenation Nitration 4/14/2019 Chem-114

General Mechanism 4/14/2019 Chem-114

Mechanism => 4/14/2019 Chem-114

Bromination of Benzene Requires a stronger electrophile than Br2. Use a strong Lewis acid catalyst, FeBr3. => 4/14/2019 Chem-114

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. 4/14/2019 Chem-114

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. 4/14/2019 Chem-114

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. => 4/14/2019 Chem-114

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. => 4/14/2019 Chem-114

Multiple Substituents The most strongly activating substituent will determine the position of the next substitution. May have mixtures. => 4/14/2019 Chem-114

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. 4/14/2019 Chem-114

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. => 4/14/2019 Chem-114

Mechanism of Acylation => 4/14/2019 Chem-114

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. 4/14/2019 Chem-114

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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 4/14/2019 Chem-114

Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst. Product is benzaldehyde. => 4/14/2019 Chem-114

Nucleophilic Aromatic Substitution A nucleophile replaces a leaving group on the aromatic ring. Electron-withdrawing substituents activate the ring for nucleophilic substitution. => 4/14/2019 Chem-114

Examples of Nucleophilic Substitution => 4/14/2019 Chem-114

Catalytic Hydrogenation Elevated heat and pressure is required. Possible catalysts: Pt, Pd, Ni, Ru, Rh. Reduction cannot be stopped at an intermediate stage. => 4/14/2019 Chem-114