The Grignard Reaction
Organic Chemistry 2580 Laboratory, Section 01
Experiment 18
10/31/18
I.
Abstract
The Grignard reaction is considered to be one of the most important reactions in
organic chemistry with great importance and elegance. This reaction results in high
yields and can be performed using 1, 2, 3, and aromatic halides. In this laboratory
experiment the goal was to synthesize benzoic acid by preparing a Grignard reagent
from bromobenzene. Under anhydrous conditions magnesium metal was added to
bromobenzene to form phenylmagnesium bromide which was then reacted with CO 2
and to give an intermediate benzoate ion which was then in turn reacted with HCl to
give the product, benzoic acid. The % yield and melting range were determined to be
62% and 114-115C respectively.
II.
Introduction
The alkyl group of the prepared Grignard reagent acts as a very strong base and
will thus react with acids to generate an alkane. However, this also means that any
compound that contains H+ will donate that proton and in turn will destroy the Grignard
reagent. This is why the glassware were all baked in the oven and dried subsequently
over a flame.
Iodine crystals are used to activate the magnesium metal. When
phenylmagnesium bromide is reacted with CO2 it’s CO2 that acts as the electrophile to
yield the intermediate benzoate ion. HCl is then added in order to neutralize the
benzoate ion protonating it and converting it to benzoic acid. H2O serves as a solvent for
the magnesium salts.
Extraction steps were performed based on polarity and solubility differences in
order to remove impurities or by-products such as biphenyl which is formed by
phenylmagnesium bromide reacting with bromobenzene. Below is the mechanism for
the reduction of diphenylmethanol from benzophenone:
III.
Results
Name
Benzoic acid
Molar Mass
122.12 g/mol
Yield
4.61 g
Structural
formula
Melting range
114-115C
Theoretical yield
7.44
% Yield
62%
Appearance
IV.
White solid powder
Conclusion
In this laboratory experiment the goal was to synthesize benzoic acid by
preparing a Grignard reagent from bromobenzene. Under anhydrous conditions
magnesium metal was added to bromobenzene to form phenylmagnesium bromide
which was then reacted with CO2 and to give an intermediate benzoate ion which was
then in turn reacted with HCl to give the product, benzoic acid. The % yield and melting
range were determined to be 62% and 114-115C respectively.
The % yield was higher than that reported in the lab manual, which was 60%.
This ensures that all the products were recovered, transferred over, and filtered. In
addition to having the expected theoretical yield, the melting range was observed to be
114-115C which is a bit low compared to that reported in the lab manual, being 122C.
This might be due to contaminants or perhaps the products not being completely dried.
In addition, the melting range could be due to the heating apparatus heating up too fast
thus the sample recovered wasn’t entirely pure and could have contained biphenyl.
Overall, the experiment was conducted without any issues and the results were
as expected. There were concerns about the experiment due to the specific
requirements the experiment required to be executed without complications. For
instance, the presence of water would have reacted with phenylmagnesium bromide
which in turn would ruin the reaction. Also, excess heat could have affected the
outcome leading to the production of biphenyl. However, the % yield and melting range
verified that the experiment was conducted properly in accordance to the lab manual.