Benzene: acylation

A dramatic acceleration of benzene acylation is also observed by using a similar catalyst consisting of a combination of hafnium triflate and triflic acid. While in the benzoylation of benzene with BC, low yields of benzo-phenone (BP) (5%-10%) are obtained in the presence of hafnium triflate (5% mol) or triflic acid (5% mol), a 77% yield is obtained when combining hafnium triflate (5% mol) with triflic acid (5% mol). The yield is improved to 82% when hafnium triflate and triflic acid are used in higher quantity (10% mol) a further increase of the catalyst amount does not improve the... 

Thallium oxide supported on differenf materials was studied as a catalyst in benzene acylation with In general, TIO deposited on... 

Ram Reddy, R, Subrahmanyam, M., and Durga Kumari, V. 1999. Vapor-phase synthesis of acetophenone in benzene acylation over CeHZSM-5(30) zeolite. Catal. Lett. 61 207-211. 

Besides avoiding carbocation rearrangements, another advantage to preparing alkyl-substituted benzenes by acylation-reduction rather than by direct alkylation is that a large excess of benzene does not have to be used (Section 19.8). Unlike alkyl-substituted benzenes, which are more reactive than benzene, acyl-substituted benzenes are less reactive than benzene, so they will not undergo a second Friedel-Crafts reaction (Section 19.14). 

The nitration, sulphonation and Friedel-Crafts acylation of aromatic compounds (e.g. benzene) are typical examples of electrophilic aromatic substitution. 

TTie true ketones, in which the >CO group is in the side chain, the most common examples being acetophenone or methyl phenyl ketone, C HjCOCH, and benzophenone or diphenyl ketone, C HjCOC(Hj. These ketones are usually prepared by a modification of the Friedel-Crafts reaction, an aromatic hydrocarbon being treated with an acyl chloride (either aliphatic or aromatic) in the presence of aluminium chloride. Thus benzene reacts with acetyl chloride... 

Upon warming with 10-20 per cent, sodium or potassium hydroxide solution, no ammonia is evolved (distinction from primary amides). The base, however, is usually liberated upon fusion with soda lime (see experimental details in Section IV,175) and at the same time the acyl group yields a hydrocarbon. Thus benz-p-toluidide affords p-tolu-idine and benzene. 

One of the two possible disconnections a is better as it gives us an acyl rather than an alkyl halide and an activated benzene ring. 

Indene derivatives 264a and 264b are formed by the intramolecular reaction of 3-methyl-3-phenyl-l-butene (263a) and 3,3,3-triphenylpropylene (263b) [237]. Two phenyl groups are introduced into the /3-substituted -methylstyrene 265 to form the /3-substituted /3-diphenylmethylstyrene 267 via 266 in one step[238]. Allyl acetate reacts with benzene to give 3-phenylcinnamaldehyde (269) by acyl—O bond fission. The primary product 268 was obtained in a trace amount[239]. 

Selenazolidines are acylated on carbon without ring rupture when the reaction is carried out in benzene in the presence of pyridine (73),... 

Friedel-Crafts acylation An analogous reaction occurs when acyl halides react with benzene in the presence of alumi num chloride The products are acylben zenes... 

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 8) illustrates nicely the concentration of positive charge at the acyl carbon as shown by the blue color The mechanism of the reaction between this cation and benzene is analogous to that of other electrophilic reagents (Figure 12 9)... 

Carboxylic acid anhydrides compounds of the type RCOCR can also serve as sources of acyl cations and m the presence of aluminum chloride acylate benzene One acyl unit of an acid anhydride becomes attached to the benzene ring and the other becomes part of a carboxylic acid... 

An important difference between Fnedel-Crafts alkylations and acylations is that acyl cations do not rearrange The acyl group of the acyl chloride or acid anhydride is transferred to the benzene ring unchanged The reason for this is that an acyl cation is so strongly stabilized by resonance that it is more stable than any ion that could con ceivably arise from it by a hydride or alkyl group shift... 

The two step acylation-reduction sequence is required Acylation of benzene puts the side chain on the ring with the correct carbon skeleton Clemmensen reduc tion converts the carbonyl group to a methylene group... 

Because the position of electrophilic attack on an aromatic nng is controlled by the direct ing effects of substituents already present the preparation of disubstituted aromatic com pounds requires that careful thought be given to the order of introduction of the two groups Compare the independent preparations of m bromoacetophenone and p bromoace tophenone from benzene Both syntheses require a Friedel-Crafts acylation step and a bromination step but the major product is determined by the order m which the two steps are carried out When the meta directing acetyl group is introduced first the final product IS m bromoacetophenone... 

Friedel-Crafts acylation of benzene with benzoyl chloride (CgH5CCl) (j) Nitration of the product from part (1)... 

Partial rate factors may be used to estimate product distributions in disubstituted benzene derivatives The reactivity of a particular position in o bromotoluene for example is given by the product of the partial rate factors for the corresponding position in toluene and bromobenzene On the basis of the partial rate factor data given here for Fnedel-Crafts acylation predict the major product of the reaction of o bromotoluene with acetyl chlonde and aluminum chloride... 

The suffix phenone indicates that the acyl group is attached to a benzene ring)... 

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