MediumChemistryHigh School (11–12)Organic Compounds with Functional Groups › Aldehydes, Ketones and Carboxylic AcidsNumerical
A student makes the Grignard reagent from 7.85 g of bromobenzene (157.01 g/mol) and pours it over…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student makes the Grignard reagent from of bromobenzene () and pours it over crushed solid carbon dioxide. Dilute acid is then added, and benzoic acid () is obtained in a yield of . Find the amount of bromobenzene, the theoretical mass of benzoic acid and the mass obtained. How does the number of carbon atoms of the acid compare with that of the Grignard reagent?
Need a reminder? The key ideaShow
The carboxyl carbon is carbon 1. Acids come from oxidation of primary alcohols, aldehydes and alkylbenzenes (a benzylic hydrogen is needed), from hydrolysis of nitriles, amides and esters, and from RMgX with CO, which adds one carbon. Two hydrogen bonds join acid molecules in a dimer, so the molar mass found in benzene is near double.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. .
Step 2. Theory: .
Step 3. Obtained: .
Answer: , and . The acid has one carbon more: seven, against six in the Grignard reagent.
Watch out. The Grignard reagent must be kept dry. Water would destroy it and give benzene.
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