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I am learning about carbonyl chemistry at the moment. So far, we have been able to make aldehydes and ketones from alcohols (via oxidizing agents). I can't help but wonder if there is possibly any way to make aldehydes from ketones or vice-versa through a series of synthetic steps. I would appreciate it if the synthetic steps are in the level of an undergraduate organic chemistry course.

Question: Synthetic steps for the interconversion between aldehydes and ketones?

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    $\begingroup$ You could reduce an aldehyde to a primary alcohol, dehydrate the alcohol to an alkene, hydrate it again with the hydroxyl on the other side (secondary alcohol), and then oxidize to get the ketone. $\endgroup$ – Karsten Theis Apr 21 at 3:42
  • $\begingroup$ @KarstenTheis yes, I was able to figure this out with the given answers, but I am still having trouble going in the reverse direction. Ketones to aldehydes. $\endgroup$ – James Bond Apr 21 at 3:55
  • $\begingroup$ Check out hydroboration vs. oxymercuration, e.g. by looking at this cheat sheet. Once you have the hydroxyl group on the carbon where you want it, you just have to make sure your oxidation does not go all the way to a carboxylic acid. $\endgroup$ – Karsten Theis Apr 21 at 4:02
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    $\begingroup$ @JamesBond For converting Ketones to aldehydes, what you can do is to perform a Bayer-Villiger Oxidation on the ketone to get an ester. The ester what you got can now be reduced with DIBAL-H (Diisobutylaluminium hydride) to get an aldehyde and the other part will leave as alcohol which you can again oxidise using PCC. $\endgroup$ – Soumik Das Apr 21 at 4:02
  • $\begingroup$ @SoumikDas Thanks! Makes perfect sense since I already know about the ester to aldehyde via DIBAL-H. The Bayer-Villiger Oxidation was the key to getting to the ester. Thanks a lot. If you want you can post your answer so I can accept it, I like this more. $\endgroup$ – James Bond Apr 21 at 4:19
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You can easily convert aldehydes to ketones and vice-versa by using simple organic synthetic steps which are very much well known.

Converting aldehydes to ketones

For converting aldehydes to ketones there are numerous pathways. Here I am mentioning the simplest two ways, one of which is by using Grignard reagent and other by using thioacetal intermediates.

You can react aldehydes with Grignard reagents ($\ce{R^2 -MgBr}$) and perform acidic workup to generate secondary alcohols. Then you can oxidise the alcohol to get a ketone by commonly used oxidising agents like PCC (pyridinium chlorochromate).

The other way is to react the aldehyde with a propane-1,3-dithiol to generate a cyclic thioacetal . Now, interestingly, the proton attached to the carbon now can be removed by strong base like n-Butyllithium, and generate Carbanion which can perform a $S_N2 $ reaction with an alkyl halide to generate cyclic thioketal. After that the thioketal can be hydrolysed using $\ce{HgCl2/CdCO3/H2O}$ to generate your desired ketone.

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Converting ketones to aldehydes

The easiest step for this conversion is to perform a Bayer-Villiger Oxidation reaction on the ketone to get an ester. Then that ester can be reduced by Diisobutylaluminium Hydride (DIBAL-H) and an aqueous workup will generate desired aldehyde (along with an alcohol). (If you wish you can further oxidise that alcohol also in case you want aldehyde of that part).

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There are two distinct ways to convert aldehydes into ketones and vice versa

An aldehyde is a carbonyl group on a primary carbon while a ketone is a carbonyl group on a secondary carbon. To interconvert the two, you can either keep the functional group on a given carbon and make/break carbon-carbon bonds. Or you can move the functional group along the existing carbon backbone without breaking carbon-carbon bonds.

Moving the carbonyl functional group

Enzymatic conversion from aldehyde to ketone and back is common in the biochemistry of carbohydrates. In glycolysis, for example, phosphorylated dihydroxyacetone is converted to glyceraldehyde. Sugars have a hydroxyl group alpha to the carbonyl group. In the enzymatic reaction, isomerization proceeds via an enediol intermediate (source):

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A similar acid/base mechanism is used in the interconversion of glucose to fructose. This is a commercially relevant reaction for making high fructose corn syrup.

Turn a primary carbon into a secondary carbon and vice versa

See Soumik Das' answer.

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