User`s guide

7. Exercises
24. Carbocation Intermediate Analysis of Electrophilic Aromatic
Substitution Reactions
Aromatic substitution reactions proceed via a carbocation intermediate. The product distribution
of an aromatic substitution reaction may be predicted from the relative stabilities of the
carbocation intermediates. The aldehyde group is meta-directing for aromatic substitution
reactions.
C
O
H
+
Cl
C
O
H
Cl
H
C
O
H
Cl
+
H
Product ortho meta para
C
6
H
4
CHOCl 19% 72% 9%
Build planar benzaldehyde and optimize the geometry with a PM3 calculation. View the partial
charges of the ortho, meta, and para carbons (on the side of the ring closest to the H atom of the
aldehyde group).
Click
New Job Using This Geometry, and open the molecule in the WebMO Editor. Add a
chlorine atom to the ortho carbon (on the H atom side of the aldehyde group). Adjust the double
bond to the ortho carbon to a single bond. Select the CHCl group, choose
Clean-Up: Selection
Only
, and choose Clean-Up: Comprehensive so that only the new change is cleaned up.
Optimize the geometry of this intermediate structure with a PM3 calculation, setting the charge
appropriately for a carbocation.
Repeat this to compute the PM3 energies of the meta and para intermediates.
Construct a table with columns for isomer (ortho, meta, para), partial charge, intermediate energy,
and observed chlorination percentage. Comment on your results.
H. Potential Energy Surface Scans
25. Rigid Potential Energy Scan of CH Bond Dissociation (Pro)
Build CH
4
and clean up its geometry. Use Tools: Z-Matrix and Fix All coordinates. Scan the C1-
H5 bond length from 0.7 to 2.7 in 20 steps. Perform a Hartree-Fock STO-3G Coordinate Scan
calculation.
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