Experiment 11: Electrophilic Aromatic Substitution - Acylation

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Dates:

Due Dates:

Supplemental Handout for Electrophilic Aromatic Substitution - Acylation

Experimental Spectra:

Stock ¹H-NMR of toluene

Stock ¹H-NMR EAS Friedel-Crafts Acylation product

Frequently Asked Questions:

Q1) Why are the structures drawn below not resonance structures of the same molecule?

A1)  This depiction of acylium resonance represents a fundamental misunderstanding about the definition and use of resonance structures.  Individual resonance structures, by definition, are not real represenations of the molecule.  But, a collection of two or more resonance structures taken together attempt to capture the bonding and electronic structure of the molecule.  In order for the resonance structures to meaningfully sum to a reasonable representation of the actual molecule, the geometry cannot change from one structure to another.  Specific to the case above, the oxygen atom moves location from one structure to another; this IS NOT allowed.  Acyllium ions are not bent as depicted on the left structure; the structure on the right is more meaningful.

Q2)  Does it matter whether I draw two bonds or three bonds between the carbon and oxygen atoms when making my input structure for the acylium electrophile?

A2)  All that matters is that your submitted input structure have atoms in about the correct locations and be the right symmetry.  It does not matter at all how many bonds WebMO thinks exists between any atoms.  All that Gaussian09 needs is the x,y,z coordinates of each atom; any bonds that are drawn are not used in the calculation.  In fact, all of the input structures below get the same final geometry when optimized (without any regard to the odd bonds drawn in the right two).  Notice that the atoms have not moved location and that the charge is +1 in all structures.