Aromatic Substitution

Background and actual explanation

The global reaction, named after Friedel and Crafts (1877), is:

The mechanism is represented by the following steps:

· Formation of electrophile (E+),

· Reaction of the arene double bound with the electrophile leading to s complex formation.

· Removal of the proton by breaking the C-H simple bond and restore of the aromatic system

The reaction of the electrophile E⁺ with the arene is the slow step since it results in the loss of aromaticity even though the resulting cation is still resonance stabilized.

The general case will be exemplified for Friedel-Crafts Alkylation of Benzene;

Rearrangement of electrophile and other electrophilic aromatic substitution like nitration, halogenation, acylation are described in the book.

This reaction allows alkyl groups to be attached to benzene rings via an electrophilic substitution reaction.

For the Friedel-Crafts alkylation, an alkyl chloride reacts with a benzene ring in the presence of an aluminum chloride catalyst:

The mechanism for this substitution occurs in two parts.

First the AlCl₃ catalyst reacts with the alkyl chloride (a Lewis acid) to give AlCl₄^- and a electrophile (carbocation):

Other Lewis acids such as BF₃, FeCl₃ or ZnCl₂ can also be used.

In a second step, the p electrons of the aromatic nucleu act as a nucleophile, attacking the electrophilic carbocation. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate:

In the third step a removal of the proton from the sp³ C bearing the alkyl group reforms the C=C and the aromatic system, generating HCl and regenerating the active catalyst

PROPOSED MECHANISM

At beginning it is necessary to be underlined the contradictions between actual mechanism proposed by quantum mechanic and known concepts of physical chemistry and catalysis concepts.

A catalyst is a substance used only in small quantity in order to modify the activation energy of intermediate state, and after reaction the catalyst remain unmodified. Of course after a long period of usage, the catalyst looses its catalytically properties.

In comparison with this definition, any organic reaction where these so called ,,Lewis compounds” catalysts are used, need a stoechiometric quantity of them.

The second condition –reuse of catalyst is not fulfilled - in any reaction ,,catalyzed” by Lewis compound after a single reaction process this ,,Lewis” compound can’t be reused.

Having these experimental facts, in proposed mechanism ,,Lewis compounds” are not catalyst, but a simple reagents.

Actual quantum mechanic suppose that in case of alkyl chloride reaction with FeCl₃, an ionic compound is formed.

It was demonstrate from 1957 (Olah G.), the existence of molecular complex in case of CH₃Br*AlBr₃, compound soluble in CH₃Br. It was demonstrating the fact that these molecular compounds are not ionized, and are not conductors of electricity. In this case, how is possible in the presence of an aromatic compound (non polar) to appear cations and anions? Actual quantum mechanic does not offer an explanation for this …

In proposed mechanism there is a reaction between alkyl chloride and FeCl₃, with formation of a molecular complex after next mechanism.

In a first step the C-Cl bound from alkyl chloride suffer a homolitic cleavage and chlorine radical and an alkyl radical are generated.

These radicals attack a pair of coupled unbound electrons of Fe atom and two new covalent bounds are formed. The structure of molecular complex is bipyramidal.

In this way the use of stoechiometric quantity of FeCl₃ and solubility of intermediary complex in non polar solvents is entirely justified.

Roginski experiment, made in 1937, can be very easy interpreted in proposed mechanism. The original experiment consists in reaction, at low temperature, between an alkyl chloride with a labeled chlorine isotope and AlCl₃. After separation of components (destroying the bypiramidal complex) the radioactive chlorine was found also in AlCl₃.

F. Fairbrother, 1937 performed the Friedel Crafts reaction with (CH₃)₃CCl having a radioactive chlorine and AlCl₃ using non radioactive chlorine. The obtained HCl contain 25% of radioactive chlorine.

Both reactions suppose an intermediate state of equal equivalence for all 4 chlorine atoms, and looking at the radical intermediate structure, this is achieved in actual proposed mechanism.

The formed complex has a covalent nature so the absence of conductibility is entirely justified.

There is no charge transfer in this process of complex formation because the reaction is completely radicalic.

Further, the Fe-C covalent bound from complex suffer a homolitic cleavage and another two radicals are obtained.

The alkyl radical attacks the aromatic nucleus and a complex radical is formed.

In order to simplify the representation for benzene only the C-H bounds and the magnetic moments perpendicular on the plane of molecule are represented (the magnetic moments which form actual pi bound in quantum mechanic). It should be understood that any line of hexagon is formed from two opposite magnetic moments (for details see the organic compound bound on site). The six magnetic alternate magnetic moments in the benzene molecule are responsible in the proposed theory for the aromatic character of benzene. When alkyl radical attacks a magnetic moment of a C from benzene, the aromatic character is perturbed, and a new bond C-C is formed. The C at which the attack is performed, in this intermediate, forms four simple covalent bounds: three with closed C atoms and one with hydrogen.

This intermediate is not a stable one, consequently the system evolves to a regeneration of aromatic state. The four substituted carbon loose an atom of hydrogen (as radical) and after a reorientation of magnetic moments and methyl group, the aromatic state is regenerated.

Hydrogen radical reacts with radical FeCl₄ radical and HCl and FeCl₃ are obtained.

It is possible to be observed how a wrong concept of Lewis acid and basis was followed by a completely wrong mechanism for a lot of organic reactions. The proposed theory denies the existence of Lewis acid and basis and a new concept of acidity will be defined.