Experiment 13: Grignard Reaction

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Grignard Reaction

Due Dates:

Friday, July 29 by 5 PM A/B

Chemical Safety Information:

Reagents and Solvents
1-Bromo-4-fluorobenzene carbon dioxide hydrochloric acid
magnesium sulfate deuterated chloroform diethyl ether
1,2-dibromoethane    

 

Organometallic Chemistry Lecture:

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

Chem 344 Organometallics Lecture - Notetaking Slides

Organometallics Problem Set (Answers)

CHEM 344 Organometallics Lecture 1 - old version with subtitles (slightly different to above version)

CHEM 344 Organometallics Lecture 2 - old version with subtitles (slightly different to above version)

Organometallics Lecture 1 - 2014 Notetaking Slides

Organometallics Lecture 2 - 2014 Notetaking Slides

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.

Fluorobenzene

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.

Fluorine shift table

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.