Why is it called sigma complex?

An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. For historic reasons this complex is also called a Wheland intermediate, after American chemist George Willard Wheland (1907–1976). They are also called sigma complexes.

What is meant by Sigma complex and pi complex?

sigma complex = 1) the formation of a sigma complex follows the initial formation of a complex known as a pi complex. 2) sigma complex involve the actual bonding. 3) lower energy and higher stability. pi complex= 1) In contrast t the sigma complex, the pi complex does not involve actual bonding.

How sigma complex is formed?

When the electrophile adds to the aromatic ring, it produces a carbocation intermediate. The formation of the sigma complex in electrophilic aromatic substitution has a higher activation energy than the formation of a carbocation in electrophilic addition to an alkene (Figure 13.1).

What is the sigma complex in EAS and why is it important?

Since sigma complex formation is the rate determining step of EAS reactions, benzene derivatives are divided into two groups based on how the substituent stabilizes or destabilizes the positively charged sigma complex. The EAS reaction of a substituted ring with an activating group is faster than benzene.

What are wheland intermediates?

Wheland intermediate (arenium ion) A cation that appears in the *electrophilic substitution of *aromatic compounds.

What is sigma complex in benzene?

The sigma complex usually refers to the structure of protonated benzene C6H7+ having the proton attached to a particular carbon by a sigma bond, rather than having the proton loosely attached to the ring via the pi orbitals. See arenium ion (Wikipedia) for more details.

What is π complex?

Pi ligands are a class of organometallic ligand with extended π systems that include linear molecules including ethylene, and allyl, and cyclic molecules such as cyclopentadienyl. As a dative L-type ligand, these molecules have a direct affect on the reactivity of the organometallic complex.

Is Arenium ion?

The resonance-stabilized carbocation intermediate of electrophilic aromatic substitution of arenes is called the arenium ion.

What is the best leaving group?

Good leaving groups are weak bases. They’re happy and stable on their own. Some examples of weak bases: halide ions (I-, Br-, Cl-) water (OH2), and sulfonates such as p-toluenesulfonate (OTs) and methanesulfonate (OMs). The weaker the base, the better the leaving group.

How do I know if my wheland is intermediate?

Electrophilic addition of this active electrophile to the aromatic ring gives the Wheland intermediate, which is identified by UV–Vis spectra recorded under operando conditions at 573 K on the industrially used H-ZSM-5 zeolite catalyst.

What is Benzenonium ion?

Answer. 1. Solution. It is a stable allylic carbocation: the C of benzene attacked by E+ becomes sp3 hydridized the other five C′s are sp2 hybridised and their remaining p atomic orbitals overlap sideways to form a delocalized π e- cloud.