Experiment 13: Grignard Reaction

Chemical Safety Information:

Organometallic Chemistry Lecture:

CHEM 344 Organometallic Chemistry Lecture (current version, without subtitles)

Chem 344 Organometallics Lecture – Notetaking Slides

Organometallics Problem Set (Answers)

Experimental Spectra:

¹H-NMR 26-Jul-2016

GC-MS 26-Jul-2016 Instrument not resolving product — USE STOCK DATA

Sample ¹H-NMR, ¹³C-NMR, and ¹⁹F-NMR spectra of benzoic acid derivative (not available for submission for credit)

Stock GC-MS spectrum of benzoic acid derivative (available for submission for credit, see laboratory manual for details)

Stock IR spectrum of benzoic acid derivative (available for submission for credit, see laboratory manual for details)

4-fluoro-bromobenzene ¹H-NMR FID (FOR REFERENCE ONLY)

Stock FID ¹H-NMR Grignard Product (available for submission for credit, see laboratory manual for details)

Stock FID ¹⁹F-NMR Grignard Product (available for submission for credit, see laboratory manual for details)

Frequently Asked Questions:

Q1) How can we determine the reactivity of the C-X bonds toward Mg?

A1)  The substrate in this reaction has two C-X bonds which have different reactivities toward magnesium.  There is only a single Grignard reagent formed in the first step of this reaction.  In order to determine which C-X bond is more reactive, the presence of the other halogen atom in the product of the reaction can be probed.  If the C-F bond is more reactive than the C-Br bond then the Br should be attached to the aromatic ring in the product and vice versa.

Computational/Theoretical Prediction: The reaction of an alkyl or aryl halide with Mg to form a Grignard reagent is dependent upon the C-X bond dissociation energy (X = halogen).  In a series of R-X molecules in which the halogen changes, the C-X bond dissociation energy is correlated to the C-X bond length.  The relationship between C-X bond reactivity, bond dissociation energy, and bond length was explored in Chapter 4.

NMR: The presence of a fluroine atom in the product can be detected by ¹H-NMR, ¹³C-NMR, and ¹⁹F-NMR spectroscopy.  The I = 1/2 ¹⁹F nucleus is 100% abundant.  Both ¹H-NMR and ¹³C-NMR spectroscopy will show the presence of ¹⁹F nuclei through their coupling to ¹H- and ¹³C-nuclei.  Representative ¹H-¹⁹F and ¹³C-¹⁹F coupling constants for fluorobenzene are shown below.

Fluroine is also directly detectable via ¹⁹F-NMR spectroscopy, which will unabiguously identify its presence in the reaction product.  A chemical shift table for fluorine NMR is provided below from Prof Hans J. Reich.

Bromine has two NMR active nuclei (I = 3/2), ⁷⁹Br and ⁸¹Br, but both have large quadrupole moments which make their nuclei almost undetectable in a normal NMR experiment.

EI-MS: Detection of fluorine or bromine atoms in the product via EI-MS is trivial based upon the mass of the molecular ion.  Recall that the ⁷⁹Br and ⁸¹Br isotopes are almost equally abundant in any molecule that contains Br atom, and so a molecule containing a Br atom displays a distinctive isotope pattern.