Intermediates for Electrophilic Aromatic Substitution
Methodology
MNDO/AM1/PM3 (Semiempirical MO)
Techniques Used
Building, Optimization, Analysis of
Bonding Parameters, Output Analysis,
Charge Density Analysis
Abstract. You will first optimize the structure
of phenol, and obtain the charge distribution on the aromatic carbons. You
will optimize the structures of two possible sigma-complexes involved in
the bromination of phenol, and find the energy of each intermediate to decide
which is more stable.
Procedure. Build and optimize phenol using a semiempirical
method. Record the enthalpy of formation of the molecule and the charge
distribution on the aromatic ring carbons. Build and optimize the cationic
sigma-complex formed by para and for meta attack
of Br+ upon phenol.
- Which is the more stable intermediate?
- Based upon the charge distribution in phenol, which site
is most likely to be attacked by Br+ to give the final bromophenol?
- Based upon the relative stabilities of the sigma-complexes,
which would be the most stable product to form?
- Do your answers for the preceeding questions agree?
Benchmarks. Phenol is brominated in the ortho
and para positions, almost exclusively.
Results
Copyright by Paul M. Lahti 1995.