MediumChemistryHigh School (11–12)Organic Compounds with Functional Groups › Aldehydes, Ketones and Carboxylic AcidsNumerical
Ethanal (44.05 g/mol) reacts with HCN, in a trace of base, to give a cyanohydrin…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Ethanal () reacts with HCN, in a trace of base, to give a cyanohydrin. Hydrolysis of the cyanohydrin gives 2-hydroxypropanoic acid, also called lactic acid (). A chemist starts with of ethanal, and the overall yield is . Find the amount of ethanal, the theoretical mass of acid and the mass obtained. How many carbons does the acid have, compared with ethanal?
Need a reminder? The key ideaShow
A nucleophile attacks the carbonyl carbon, the carbon changes from to , and the oxygen then takes a proton. Alkyl groups lower the positive charge and crowd the carbon, so methanal > other aldehydes > ketones. HCN gives a cyanohydrin, NaHSO a bisulphite compound, alcohols with dry HCl an acetal, and compounds a C=N compound and water.
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. The ratio is one to one, so the theoretical mass is .
Step 3. Mass obtained .
Answer: , and . The acid has three carbons and ethanal has two, because the cyanide carbon becomes the carboxyl carbon.
Check. The product is heavier than the ethanal, as it should be, because HCN and water have been added.
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