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  1. Problems
  2. /A student makes the Grignard reagent from 7.85 g of bromobenzene (157.01 g/mol) and pours it over…
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…

StemPy LabContributed by StemPy Lab

Try 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 7.85 g7.85\ \text{g}7.85 g of bromobenzene (157.01 g/mol157.01\ \text{g/mol}157.01 g/mol) and pours it over crushed solid carbon dioxide. Dilute acid is then added, and benzoic acid (122.12 g/mol122.12\ \text{g/mol}122.12 g/mol) is obtained in a yield of 70 %70\ \%70 %. 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?

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Work it out on paper, then type your number. The answer itself stays hidden.

mmol
g
g
Need a reminder? The key ideaShowHide

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 CO2_22​, 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

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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. n=7.85157.01=0.0500 mol=50.0 mmoln = \dfrac{7.85}{157.01} = 0.0500\ \text{mol} = 50.0\ \text{mmol}n=157.017.85​=0.0500 mol=50.0 mmol.

Step 2. Theory: 0.0500 mol×122.12 g/mol=6.11 g0.0500\ \text{mol} \times 122.12\ \text{g/mol} = 6.11\ \text{g}0.0500 mol×122.12 g/mol=6.11 g.

Step 3. Obtained: 6.11 g×0.70=4.27 g6.11\ \text{g} \times 0.70 = 4.27\ \text{g}6.11 g×0.70=4.27 g.

Answer: 50.0 mmol50.0\ \text{mmol}50.0 mmol, 6.11 g6.11\ \text{g}6.11 g and 4.27 g4.27\ \text{g}4.27 g. 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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  1. Names, preparation and physical properties of carboxylic acidsCarboxylic Acids: Structure, Acidity and Reactions

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