Synthesis of Alkylbenzenes by Acylation-Reduction

Succinic anhydride, the structure of which is shown, is a cyclic anhydride often used in Friedei-Crafts acyiations. Give the structure of the product obtained when benzene is acyiated with succinic anhydride in the presence of aiuminum chioride. 

Because acylation of an aromatic ring can be accomplished without rearrangement, it is frequently used as the first step in a procedure for the alkylation of aromatic compounds by acylation-reduction. As we saw in Section 12.6, Friedei-Crafts alkylation of benzene with primary alkyl halides normally yields products having rearranged alkyl groups. When preparing a compound of the type ArCH2R, a two-step sequence is used in which the first step is a Friedei-Crafts acylation. 

The second step is a reduction of the carbonyl group (C=0) to a methylene group (CH2). 

The most commonly used method for reducing an aryl ketone to an alkylbenzene employs a zinc-mercury amalgam in concentrated hydrochloric acid and is called the Clemmensen reduction. Zinc is the reducing agent. 

The synthesis of butylbenzene illustrates the acylation-reduction sequence. 

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

An important difference between Friedel-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 conceivably arise from it by a hydride or alkyl group shift. 

Acetophenone is one of the commonly encountered benzene derivatives listed in Table 11.1. 

More stable cation all atoms have octets of electrons 

An amalgam is a mixture or alloy of mercury with another metal. For many years silver amalgams were used in dental fillings. 

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Representative Electrophilic Aromatic Substitution Reactions of Benzene 457 Mechanistic Principles of Electrophilic Aromatic Substitution 458 Nitration of Benzene 459 Sulfonation of Benzene 461 Halogenation of Benzene 462 Biosynthetic Halogenation 464 Friedel-Crafts Alkylation of Benzene Friedel-Crafts Acylation of Benzene Synthesis of Alkylbenzenes by Acylation-Reduction 469 Rate and Regioselectivity in Electrophilic Aromatic Substitution 470 Rate and Regioselectivity in the Nitration ofToluene 472... 

Unlike alkylation, acylation is controlled easily to give monosubstitution, because once an acyl group is attached to a benzene ring, it is not possible to introduce a second acyl group into the same ring. Because of this, a convenient synthesis of alkylbenzenes starts with acylation, followed by reduction of the carbonyl group with zinc and hydrochloric acid (Section 16-6). For example, propylbenzene is prepared best by this two-step route because, as we have noted, the direct alkylation of benzene with propyl chloride produces considerable amounts of isopropylbenzene and polysubstitution products ... 

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