12.7 Friedel-Crafts Acylation of Benzene 453 Thus, the industrial preparation of styrene from benzene and ethylene does not involve vinyl chloride but proceeds by way of ethylbenzene + CH,CH HCL 一CH2CH3z0 CH=CH, Dehydrogenation of alkylbenzenes, although useful in the industrial preparation of styrene, is not a general procedure and is not well suited to the laboratory preparation of alkenylbenzenes. In such cases an alkylbenzene is subjected to benzylic bromination (Section 11. 12), and the resulting benzylic bromide is treated with base to effect dehy dehalogenation PROBLEM 12.6 Outline a synthesis of 1-phenylcyclohexene from benzene and cyclohexene 12.7 FRIEDEL-CRAFTS ACYLATION OF BENZENE Another version of the Friedel-Crafts reaction uses acyl halides instead of alkyl halides gen- and AICI CH3CH,CCI t HCl Benzer Propanoyl chloride 1-Phenyl-1-propanone(88%) Hy The electrophile in a Friedel-Crafts acylation reaction is an acyl cation(also referred to as an acylium ion). Acyl cations are stabilized by resonance. The acyl cation derived from propanoyl chloride is represented by the two resonance forms CH3CH2C=O:← CH3CH2C≡O Most stable resonance form oxygen and carbon have octets of electrons Acyl cations form by coordination of an acyl chloride with aluminum chloride, followed by cleavage of the carbon-chlorine bond CH3CH,: →CH3CH2C一C1一ACl3—>CH3CH2C≡O:+AlCl4 Lewis acid-Lewis base Tetrachloro- The electrophilic site of an acyl cation is its acyl carbon. An electrostatic po ( Figure 12.6) illustrates nic concentration of positive charge at the acyl carbon. The mechanism of the reaction between this cation and benzene is analogous to that of other electrophilic reagents(Fig ure12.7) Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteThus, the industrial preparation of styrene from benzene and ethylene does not involve vinyl chloride but proceeds by way of ethylbenzene. Dehydrogenation of alkylbenzenes, although useful in the industrial preparation of styrene, is not a general procedure and is not well suited to the laboratory preparation of alkenylbenzenes. In such cases an alkylbenzene is subjected to benzylic bromination (Section 11.12), and the resulting benzylic bromide is treated with base to effect dehy￾drohalogenation. PROBLEM 12.6 Outline a synthesis of 1-phenylcyclohexene from benzene and cyclohexene. 12.7 FRIEDEL–CRAFTS ACYLATION OF BENZENE Another version of the Friedel–Crafts reaction uses acyl halides instead of alkyl halides and yields acylbenzenes. The electrophile in a Friedel–Crafts acylation reaction is an acyl cation (also referred to as an acylium ion). Acyl cations are stabilized by resonance. The acyl cation derived from propanoyl chloride is represented by the two resonance forms Acyl cations form by coordination of an acyl chloride with aluminum chloride, followed by cleavage of the carbon–chlorine bond. The electrophilic site of an acyl cation is its acyl carbon. An electrostatic poten￾tial map of the acyl cation from propanoyl chloride (Figure 12.6) illustrates nicely the concentration of positive charge at the acyl carbon. The mechanism of the reaction between this cation and benzene is analogous to that of other electrophilic reagents (Fig￾ure 12.7). CH3CH2C O CH3CH2C O Most stable resonance form; oxygen and carbon have octets of electrons H Benzene CH3CH2CCl O Propanoyl chloride AlCl3 carbon disulfide 40°C CCH2CH3 O 1-Phenyl-1-propanone (88%) HCl Hydrogen chloride Benzene CH2 CH2 Ethylene HCl, AlCl3 630°C ZnO CH2CH3 Ethylbenzene CH CH2 Styrene 12.7 Friedel–Crafts Acylation of Benzene 453 CH3CH2C O Cl Propanoyl chloride AlCl3 Aluminum chloride Tetrachloro￾aluminate ion AlCl4  Propanoyl cation CH3CH2C CH3CH2C O O AlCl3  Cl Lewis acid-Lewis base complex An acyl group has the gen￾eral formula RC± O X Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website