Question

Br Mg ether

1. Why is already-heated water added to the water bath under the reaction mixture as opposed to simply heating the water directly with a hot plate during the Grignard reaction?
2. Why is the initial portion of bromobenzene in ether added before additional ether is added?
3. Why is additional ether added before the second portion of bromobenzene is added?
4. During what part of the purification procedure is any unreacted acetophenone removed from the product? Be specific. Answer the question assuming that there is some unreacted acetophenone.
5. Based on your observations during the isolation of the product, is the product soluble in diethyl ether? In pure hexane? Support both answers with observations.

Answer 1

a) Most Grignard reactions occur in solvents such as anhydrous diethyl ether or tetrahydrofuran (THF) because the oxygen in these solvents stabilizes the magnesium reagent. ... In small scale reactions, the solvent vapours do not have enough space to protect the magnesium from oxygen.

The resulting “Grignard reagent” acts as both a good nucleophile and a strong base. ... Its basic property means that it will react with acidic compounds, such as carboxylic acids, phenols, thiols and even alcohols and even water; therefore, reaction conditions must be free from acids and strictly anhydrous.

So the water bath is heated so that it gets steamed and vapourised so that Grignard reagent do not get any change to interact with water.

b) & c)The reason why bromobenzene substrate is added in two portions, rather than one full portion, is because adding it all at once would cause an increase of the product 1-phenylethanone and very little phenylmagnesium bromide would be produced.

d) Excess of acetophenone from chalcone can be removed by filtration, then washing with methanol under vacuum OR Recrystallize the chalcone from an organic solvent such as ethyl acetate as it is solid. The excess of acetophenone will remain in the solvent.

It may be necessary to add more hexane to dissolve the crystals, as the vacuum filtration process tends to evaporate the hexane in the filter flask leaving biphenyl crystals in the bottom of the flask.