Site selectivity in electrophilic aromatic substitution

It is well known in organic chemistry that when an substituted benzene undergoes electrophilic attack, the substituent group on the ring can affect both the reaction rate and the site of the attack.  These effects classify the substituent groups into two classes.  One is called ortho-para directors where the substituent group directs the incoming electrophile to mainly the ortho and para positions.  The other is called meta directors where the substituent group directs direct the incoming electrophile to only the meta position.  For example, let consider two substituent groups: -NH₂ and -NO₂. 

Procedure:  Using tools in Avisto.  You can download these tools from Astonis.

1. Generate 3D structures for both molecules.  There are two ways to do this.  One is to use MolLib to send out to workbench the molecular structure of benzene then use MolDesign to edit the benzene structure to add the substituent groups.  The other option is to use the MarvinSketch tool to draw the molecule then convert it into 3D.

2. Use Basic Qchem Edu or Basic QChem tool to find the stable structure for these molecules. 

3. View the results by PsiViewer by double click on the data icons. 

• Atomic Partial Charges:  Right-click on the graphic window of PsiViewer to select 'Labels' option to display the atomic partial charges.  By default, these are Mulliken charges as shown in the figure below.

From these partial charges, it is clear that the NH₂ group is the ortho

-para director whereas the NO₂ group is the meta director.

• Electrostatic Potential (ESP):  Another possibility is to plot the electrostatic potential around the molecules.  Below are the ESP iso-value surfaces for these two molecules.

These ESP surfaces also illustrate the site selectivity for electrophilic substitution on these two substituted benzene molecules.

Exercise:  Repeat the calculations for -CH₃, -CF₃, -OH, and -CN groups.