Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Friedel-Crafts limitations

Memorization Task 19.11 Other of Friedel-Crafts limitations... [Pg.358]

This is the infamous Friedel-Crafts method and works in a manner similar to the previously mentioned method where P2P was made by merging benzene and chloroacetone using AICI3. This method is for speed makers only and is not recommended for conversion of 1,3-benzodioxole. However, this should work in a limited way on catechol. The conversion factor is very low but that isn t a major concern of speed chemists because cheap old benzene is the precursor and all of that benzene that isn t converted can be run back through this simple little process over and over again. Before she knows it, the chemist will have amassed an enormous amount of allylbenzene [139, 140]. [Pg.243]

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, "free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. [Pg.15]

Other limitations to Friedel-Crafts reactions will be encountered in this chap ter and are summarized in Table 12 4 (page 511)... [Pg.482]

Neither Friedel-Crafts acylation nor alkylation reactions can be earned out on mtroben zene The presence of a strongly deactivating substituent such as a nitro group on an aromatic ring so depresses its reactivity that Friedel-Crafts reactions do not take place Nitrobenzene is so unreactive that it is sometimes used as a solvent m Friedel-Crafts reactions The practical limit for Friedel-Crafts alkylation and acylation reactions is effectively a monohalobenzene An aromatic ring more deactivated than a mono halobenzene cannot be alkylated or acylated under Friedel-Crafts conditions... [Pg.505]

It IS sometimes difficult to limit Friedel-Crafts alkylation to monoalkylation... [Pg.511]

Other typical electrophilic aromatic substitution reactions—nitration (second entry) sul fonation (fourth entry) and Friedel-Crafts alkylation and acylation (fifth and sixth entnes)—take place readily and are synthetically useful Phenols also undergo elec trophilic substitution reactions that are limited to only the most active aromatic com pounds these include mtrosation (third entry) and coupling with diazomum salts (sev enth entry)... [Pg.1002]

Solid Superacids. Most large-scale petrochemical and chemical industrial processes ate preferably done, whenever possible, over soHd catalysts. SoHd acid systems have been developed with considerably higher acidity than those of acidic oxides. Graphite-intercalated AlCl is an effective sohd Friedel-Crafts catalyst but loses catalytic activity because of partial hydrolysis and leaching of the Lewis acid halide from the graphite. Aluminum chloride can also be complexed to sulfonate polystyrene resins but again the stabiUty of the catalyst is limited. [Pg.565]

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. [Pg.245]

Zirconium monochloride reacts with sodium ethoxide to form additional adducts which hydrolyze in water. The monochloride does not react with benzene in a Friedel-Crafts reaction, and does not enter into intercalation reactions similar to those of zirconium disulfide. Both monohaUdes add hydrogen reversibly up to a limiting composition of ZrXH (131). [Pg.436]

It has also been stated that the 5-position of selenazoles is more reactive toward electrophilic substitution than that of thiazoles. Such reactivity is still further increased by substituents in the 2-position of the selenazole ring, which can have an —E-effect. Simultaneously, however, an increasing tendency toward ring fission was observed by Haginiwa. Reactions of the selenazole ring are thus limited mainly to the 5-position which, specially in the 2-amino-and the 2-hydrazino-selenazoles, is easily substituted by electrophilic reagents. However, all attempts to synthesize selenazole derivatives by the Gattermann and by the Friedel-Crafts methods... [Pg.354]

The applicability of the Gattermann synthesis is limited to electron-rich aromatic substrates, such as phenols and phenolic ethers. The introduction of the formyl group occurs preferentially para to the activating substituent (compare Friedel-Crafts acylation). If the /jara-position is already substituted, then the ort/zo-derivative will be formed. [Pg.134]

The synthetic importance of the Houben-Hoesch reaction is even more limited by the fact that aryl ketones are also available by application of the Friedel-Crafts acylation reaction. [Pg.134]

While the Friedel-Crafts acylation is a general method for the preparation of aryl ketones, and of wide scope, there is no equivalently versatile reaction for the preparation of aryl aldehydes. There are various formylation procedures known, each of limited scope. In addition to the reactions outlined above, there is the Vdsmeier reaction, the Reimer-Tiemann reaction, and the Rieche formylation reaction The latter is the reaction of aromatic compounds with 1,1-dichloromethyl ether as formylating agent in the presence of a Lewis acid catalyst. This procedure has recently gained much importance. [Pg.135]

Despite its utility, the Friedel-Crafts alkylation has several limitations. For one thing, only alkyl halides can be used. Aromatic (atyl) halides and vinylic halides do not react because aryl and vinylic carbocations are too high in energy to form under Friedel-Crafts conditions. [Pg.555]

Another limitation is that Friedel-Crafts reactions don t succeed on aromatic rings that are substituted either by a strongly electron-withdrawing group... [Pg.555]

Figure 16.8 Limitations on the aromatic substrate in Friedel-Crafts reactions. No reaction occurs if the substrate has either an electron-withdrawing substituent or an amino group. Figure 16.8 Limitations on the aromatic substrate in Friedel-Crafts reactions. No reaction occurs if the substrate has either an electron-withdrawing substituent or an amino group.
Yet a final limitation to the Friedel-Crafts reaction is that a skeletal rearrangement of the alkyl carbocation electrophile sometimes occurs during reaction, particularly when a primary alkyl halide is used. Treatment of benzene with 1-chlorobutane at 0 °C, for instance, gives an approximately 2 1 ratio of rearranged (sec-butyl) to unrearranged (butyl) products. [Pg.556]

Many variations of the reaction can be carried out, including halogenation, nitration, and sulfonation. Friedel-Crafts alkylation and acylation reactions, which involve reaction of an aromatic ling with carbocation electrophiles, are particularly useful. They are limited, however, by the fact that the aromatic ring must be at least as reactive as a halobenzene. In addition, polyalkylation and carbocation rearrangements often occur in Friedel-Crafts alkylation. [Pg.587]

Aromatic hydrocarbons substituted by alkyl groups other than methyl are notorious for their tendency to disproportionate in Friedel-Crafts reactions. This tendency has previously limited the application of the isomerization of para- or ortho-) m ky -benzenes to the corresponding meta compounds. At the lower temperature of the present modification, disproportionation can be minimized. [Pg.43]

The Friedel-Crafts alkylation reaction does not proceed successfully with aromatic reactants having EWG substituents. Another limitation is that each alkyl group that is introduced increases the reactivity of the ring toward further substitution, so polyalkylation can be a problem. Polyalkylation can be minimized by using the aromatic... [Pg.1015]

On account of the great reactivity of the substances taking part in the reaction, the choice of a solvent in the Friedel-Crafts synthesis is limited. The most important are carbon bisulphide, well purified petrol ether, chlorobenzene, and nitrobenzene. [Pg.349]

Two main limitations of CHAOS are i) it does not recognise stereochemical features and ii) it does not deal with typical aromatic electrophilic substitution (only Friedel-Craft-type disconnections are performed). [Pg.427]


See other pages where Friedel-Crafts limitations is mentioned: [Pg.252]    [Pg.511]    [Pg.551]    [Pg.552]    [Pg.425]    [Pg.722]    [Pg.511]    [Pg.123]    [Pg.556]    [Pg.557]    [Pg.709]    [Pg.145]    [Pg.158]    [Pg.437]    [Pg.99]    [Pg.131]    [Pg.46]    [Pg.535]    [Pg.51]    [Pg.107]   
See also in sourсe #XX -- [ Pg.358 ]




SEARCH



Aromatic compounds Friedel-Crafts reactions, limitations

Friedel-Crafts acylation limitations

Friedel-Crafts alkylation limitations

Friedel-Crafts alkylation reaction limitations

Friedel-Crafts reaction, acylation limitations

Friedel-Crafts reactions, limitations

Limitations of Friedel-Crafts Alkylations

Limitations of Friedel-Crafts Reactions

Limitations of Friedel-Crafts alkylation

© 2024 chempedia.info