Experiment 11: Electrophilic Aromatic Substitution – Nitration

Chemical Safety Information:

Experimental Spectra:

Reaction A (fluorobenzene)

Stock fluorobenzene ¹H-, ¹H{¹⁹F}, ¹⁹F-, ¹³C-NMR FID (FOR REFERENCE ONLY)

Sample ¹H-, ¹H{¹⁹F}, ¹⁹F-, ¹³C-NMR EAS Nitration of fluorobenzene crude product (not available for submission for credit)

Stock FID ¹H-, ¹H{¹⁹F}, ¹⁹F-, ¹³C-NMR EAS Nitration of fluorobenzene crude product (available for submission for credit, see laboratory manual for details)

Stock GC-MS EAS Nitration of fluorobenzene crude product (available for submission for credit, see laboratory manual for details)

Reaction C (bromobenzene)

Stock bromobenzene ¹H-NMR FID (FOR REFERENCE ONLY)

Sample ¹H-NMR EAS Nitration of bromobenzene crude product (not available for submission for credit)

Stock FID ¹H-NMR EAS Nitration of bromobenzene crude product (available for submission for credit, see laboratory manual for details)

Sample ¹H-NMR EAS Nitration of bromobenzene recrystallized product (not available for submission for credit)

Stock FID ¹H-NMR EAS Nitration of bromobenzene recrystallized product (available for submission for credit, see laboratory manual for details)

Stock GC-MS EAS Nitration of bromobenzene crude product (available for submission for credit, see laboratory manual for details)

Frequently Asked Questions:

Q1) Why does my Gaussian09 job keep failing when I try to optimize the arenium cation intermediates?

A1)  Assuming that you are using WebMO to submit the job to the correct queue, choosing the correct job type, and selecting correct charge/multiplicity, the most likely issue is that you have a really bad starting geometry and Gaussian cannot figure out how to fix it.

Due to the WebMO clean-up tools not being able to fix the really short bond distances, odd dihedral angles, and strange planarity often drawn by users, it is easy to generate a bad starting geometry for an arenium cation.  Shown below is the error message at the bottom of the WebMO drawing window that can be seen when mechanics fails to clean up a bad starting structure.

The easiest fix is to inspect your molecule after cleanup to make sure it has a carbon atom with a geometry that is consistent with an sp³ hybridized electronic structure.  If adjustments need to be made; use the adjustment tool and try cleaning up after making useful changes.  The examples shown below are two common drawing problems that the clean-up cannot easily overcome.  On the left, the arenium cation contains a planar sp³hybridized carbon atom; this will result in an error or a bizarre output.  On the right, the arenium cation has a nitro group with incredibly small (<0.6 Angstrom) bond distances; this will result in an optimization failure because the atoms are too close for the software to accomodate their electrons.

badly planarized carbon atom with 4 bonds	badly short N-O bond distances

Demonstration of creating the p-arenium cation for the nitration of bromobenzene.

Q2) Why does my WebMO/Gaussian09 optimized arenium cation look so strange?

A2) It is always a good idea to inspect your output structure, even if it says completed in the WebMO jobs list.  Sometimes, the output is not the desired structure you’re looking for.  This can happen with very badly drawn arenium intermediates quite easily.  If your starting structure looked like the one from A1 above on the left, it is likely that Gaussian09 will simply move the nitro group away from the molecule until it can make a reasonable geometry for bromobenzene.  Recall that Gaussian09 cannot easily break an enforced planarity.  As a result, the only path to a lower energy structure is to remove the nitro group to a safe distance producing a bizarre output structure.  See the structures below for two examples of bad output geometries that can arise from a planar starting geometry.

Q3) I’ve tried to follow the directions and example video, but I still can’t get the p-arenium cation to be C_s symmetry.  How can I fix it?

A3)  A correctly symmetrized (C_s) structure will have a mirror plane running down the C-Br bond, through the para carbon atom and the nitrogen and hydrogen connected to the sp³ carbon.  If you build it C_s, WebMO will find a C_s symmetry, but this can be a bit of a challenge.  If you get a reasonable structure that is C₁, you can actually used Gaussian09 to find the symmetry for you.

1. Build a reasonable arenium.  Make sure the structure has about the correct approach of the NO₂⁺ group and a tetrahedral carbon where the NO₂⁺ is attached to the ring.
2. Try optimizing it.
3. If it was a good starting structure, it will now be very, very nearly C_s.  Gaussian cannot automatically change the symmetry without additional commands that are not the default.
4. If you open the optimized C₁ structure, you should now be able to open it in a new job window and have WebMO detect a near C_s symmetry.
5. Click on the red C_s button, re-optimize and it will be perfect.