The Friedel -Crafts alkylation is a name reaction in organic chemistry and named after their discoverers Charles Friedel and James Mason Crafts. Under the catalytic action of a Lewis acid (eg, FeCl3, AlCl3, H2SO4, H3PO4, HF, HgSO4 ) reacting an aromatic compound with an alkyl halide, alcohol, alkene or alkyne in the Friedel -Crafts alkylation. This results in alkyl aromatics.
The alkylation may also occur intramolecularly. An example is the synthesis of tetralin.
The mechanism will be explained in the following section of the reaction of an alkyl chloride with benzene. The Friedel -Crafts alkylation is initiated by that the Lewis acid coordinated to the chlorine of the alkyl halide and thereby the adjacent carbon 1 further positivized (2). The complex thus formed can act as an electrophilic reagent. For the case that the alkyl radical is in a position to form a stable carbonium ion 3 (for example tert-butyl cation) can also this the aromatic system (in this case, benzene) electrophilic attack. The aromaticity of the ring is canceled temporarily. After the loss of a proton, the preliminary final rearomatized 5
As an alternative alkylating reagents ( reactants, precursors ), alcohols or alkenes are used in the Friedel -Crafts alkylation. However, these must be first activated by catalysts. In addition to aluminum chloride, inter alia, iron (III ) chloride and bromide, as well as phosphoric acid, sulfuric acid, hydrogen fluoride and mercury ( II) sulfate may be used as catalysts. Also suitable are substituted aromatics can be used.
An already existing alkyl group on the aromatic ring activates the ring towards further substitution reactions. The reason for this is its positive inductive effect. This has the consequence that in alkylation often polysubstituted products can be isolated. In order to obtain mono-substituted products must be used an excess of the aromatic why the Friedel -Crafts alkylation is not suitable for expensive benzene derivatives. Polyalkylaromatics can be obtained by an excess of alkylating reagent. In the case of benzene is to note that the equivalence of the positions lost comes through the introduction of an alkyl group in an unsubstituted ring on the ring. Substituents have called a directing influence on the position of the subsequent substitution reaction. One speaks of ortho -, meta- and para-directing substituents (see: Constitution, Konstitutionsisomerie ). Multiple substitution is usually undesirable because due to the small inductive effect of the alkyl group is often a mixture of products is obtained from several isomers. Because of the increased by each alkyl group newly added affinity of aromatics towards electrophiles, for example it is possible to introduce a seventh methyl group in hexamethylbenzene. The reaction then ends with a relatively stable Phenoniumion.
Problem by rearrangement
Problems arise especially in the reaction of primary alkyl halides and alcohols. The formed therefrom by action of the catalyst primary carbenium ions tends to stabilize by rearrangement. Mostly this occurs the Wagner -Meerwein rearrangement. The therefrom resulting isomeric carbonium ion can serve as electrophile now also resulting in the formation of undesirable by- products.
Disadvantages in the alkylation
Especially in the case of preparative methodology, a major drawback turns out: the resulting benzene in the alkylation monoalkylation, C6H5 -R, is defined by the I effect of the alkyl substituent electron-rich aromatic ring and more reactive than benzene itself and becomes lighter as an electrophilic reagent attacked as benzene. Therefore, the reaction does not stop at the level of mono-substitution, and caused it - usually unwanted - Mehrfachalkylierungsprodukte. Synthetically useful is the Friedel -Crafts alkylation, when the steric bulk of the alkyl group introduced limits the degree of substitution, for example, with the introduction of a tert -butyl group.
The Friedel -Crafts alkylation is a reversible reaction. Therefore, alkyl radicals of alkyl aromatics by heating and under influence of a catalyst can be transferred to other aromatics.
Kinetic and thermodynamic control
Under relatively mild reaction conditions, alkyl groups are ortho -and para- directing. The result is the final kinetically controlled. At higher temperature or with strong Lewis acids are preferentially formed the thermodynamically more stable meta- products.