HardChemistryHigh School (11–12)Organic Compounds with Functional Groups › Aldehydes, Ketones and Carboxylic AcidsConceptualNumerical
A chemist wants to reduce only the ester of ethyl 4-oxopentanoate, and keep the ketone, to make…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A chemist wants to reduce only the ester of ethyl 4-oxopentanoate, and keep the ketone, to make 5-hydroxypentan-2-one. Lithium aluminium hydride alone would reduce both groups. Write a route, and name the reagent in each step. How many steps does it have? Find the molar mass of the product, , using , and for C, H and O.
Need a reminder? The key ideaShow
Reactivity towards nucleophiles: acyl chloride > aldehyde > ketone > ester > acid > amide. NaBH reduces aldehydes and ketones, LiAlH every carbonyl group, and cold DIBAL-H takes an ester or a nitrile to the aldehyde. A Grignard reagent uses 1 equivalent on an aldehyde, ketone or nitrile and 2 on an ester or acyl chloride. One LiAlH holds 4 hydrides, and a ketone takes 1 and an ester 2. An acetal protects a carbonyl group.
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. Protect. Ethylene glycol and a little acid turn the ketone into a cyclic acetal.
Step 2. Reduce. Lithium aluminium hydride in dry ether, then water, reduces the ester to a primary alcohol. The acetal is untouched.
Step 3. Deprotect. Dilute aqueous acid turns the acetal back into the ketone. The product is 5-hydroxypentan-2-one.
Number of steps. 3.
Step 4. Molar mass. .
Answer: 3 steps (protect, reduce, deprotect), and .
What if? Sodium borohydride would reduce the ketone and spare the ester, which is the wrong group for this target.
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