Report of the Grignard Reaction
VII. Calculations
Crude product mass: 202.824 g - 197.984 g = 5.032 g
Theoretical yield: 4.5 mL bromobenzene x 1.5 g/mL = 6.75 g
Percent (experimental) yield: (5.032 g / 6.75 g) x 100% = 74.63%
TLC observation: All three spots appeared at the same position on the TLC plate.
VIII. Results and Discussion
The Grignard reaction can take a while to get going, and that is why the procedure includes
steps
meant to keep the reaction stable and moving forward. Grignard reagents are very sensitive to
water and oxygen, so the setup has to stay as dry as possible. Another issue is that magnesium
does not always react quickly on its own. When the magnesium is crushed into a finer form, it
exposes more surface area, which helps it react faster. Heating the mixture and adding
bromobenzene in THF slowly also helps the reaction start, but the overall process can still be
slow as the reagent forms.
The first 30-minute reflux matters because it gives magnesium time to react with bromobenzene
under controlled conditions to form phenylmagnesium bromide, which is the Grignard reagent.
Reflux keeps a steady temperature, which helps the reaction proceed without overheating. Too
much heat can damage the Grignard reagent and can also increase side reactions.
The second 30-minute reflux is needed after adding methyl benzoate because the Grignard
reagent then does nucleophilic addition to build the carbon framework that leads to
triphenylmethanol. This step is also done carefully because the reaction is exothermic. If
reagents are added too fast, the temperature can rise quickly, which can lower product quality or
create unwanted byproducts.
Overall, the Grignard synthesis involves highly reactive intermediates, so slow addition and
longer reflux times help keep the chemistry under control. These choices usually improve the
chance of a solid yield and a cleaner final product.
IX. Conclusion
The melting point of the triphenylmethanol sample was 159 C, which is close to the reported
literature range of 160-164 C. Since the measured value is only slightly below the expected
range, this suggests the product was fairly pure. The percent yield was about 75%, and the
small
drop from a perfect yield was most likely caused by typical experimental losses, especially
during
transfer and extraction steps.
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The TLC results suggested the crude sample initially contained impurities, but after the
extraction
workup and using the rotovap, those impurities were removed. Petroleum ether is nonpolar, so it
traveled higher on the TLC plate with the solvent system. The larger spot corresponds to the
product (triphenylmethanol), while the smaller spot near the top corresponds to petroleum ether.
X. Answers to Post-Laboratory Questions
There were no post-laboratory questions for this experiment.
