Electrophilic Attack on Benzene and Aromatic Species

Electronic Nature of Aromatic Rings and Electrophilic Attack

• The structure of benzene is characterized by a high concentration of electron density due to its delocalized $\pi$ orbitals located above and below the plane of the ring-carbon atoms.

• This concentration of negative charge serves to shield the ring-carbon atoms from potential attack by nucleophilic reagents.

• Conversely, the electron-rich nature of the $\pi$ cloud promotes attack by cations ($X^+$) or electron-deficient species, which are categorized as electrophilic reagents.

Characteristics of $\pi$ Complexes

• The initial phase of an electrophilic reaction typically involves an interaction between the approaching electrophile and the delocalized $\pi$ orbitals of the aromatic system.

• These interactions result in the formation of so-called $\pi$ complexes, such as those observed in the reaction between methylbenzene (toluene) and hydrogen chloride ($HCl$).

• Example of $\pi$ complex formation:

  • Methylbenzene reacts with $HCl$ at $-78^{\circ}C$ to form a $1:1$ complex.

  • This reaction is readily reversible.

• Experimental evidence regarding bonding in $\pi$ complexes:

  • When the reaction is repeated using deuterochloride ($DCl$), a $\pi$ complex is formed, but no exchange of deuterium with the hydrogen atoms of the benzene nucleus occurs.

  • The lack of deuterium exchange during the formation and decomposition of the complex confirms that no actual covalent bond (specifically no $C-D$ bond) is formed between a ring-carbon atom and the proton/deuteron.

Varieties and Applications of Charge Transfer Complexes

• Aromatic hydrocarbons form $\pi$ complexes with several species, including:

  • Halogens.

  • Silver ions ($Ag^+$).

  • Picric acid ($2,4,6-(O_2N)_3C_6H_2OH$).

• Charge Transfer Complexes (Adducts):

  • The interaction with picric acid produces stable, colored crystalline adducts.

  • The melting points of these adducts are used as a standard method to characterize specific aromatic hydrocarbons.

• Structural Geometry of Complexes:

  • In the specific complex formed between benzene and bromine, the halogen molecule ($Br_2$) is positioned centrally and oriented at right angles (perpendicular) to the plane of the benzene ring.

Formation of $\sigma$ Complexes and Wheland Intermediates

• The presence of a compound with an electron-deficient orbital, such as a Lewis acid (e.g., $AlCl_3$), changes the nature of the interaction between the electrophile and the aromatic ring.

• In these conditions, a type of complex known as a $\sigma$ complex is formed, also referred to as a Wheland intermediate.

• Evidence for $\sigma$ complex formation:

  • If $DCl$ is used in the presence of a Lewis acid, rapid exchange of deuterium with the hydrogen atoms of the nucleus takes place.

  • This exchange indicates that the $H^+$ or $D^+$ has become covalently bonded to a specific ring-carbon atom.

• Nomenclature for $\sigma$ complexes includes:

  • Wheland intermediate.

  • Arenium ions.

  • Arenonium ions.

  • Carbocation intermediate.

• Structure and Charge Distribution in $\sigma$ Complexes:

  • The positive charge in a $\sigma$ complex is shared across the remaining five carbon atoms of the nucleus through the $\pi$ orbitals.

  • In the transition to this state, the involved carbon atom changes its hybridization, and the deuterium and hydrogen atoms attached to that carbon are situated in a plane.