How do you make aldehyde from carboxylic acid?

Notice how a carboxylic acid, with a #("C"="O")-"OH""# functional group, has one more oxygen than an aldehyde, with a #("C"="O")-"H"# functional group.

That means you'll have to somehow reduce a carboxylic acid one step to get to an aldehyde.

Two common ways to reduce a carbonyl compound is with #"NaBH"_4# or #"LiAlH"_4#. The order of reactivity goes as follows:

#stackrel("NaBH"_4 " is suitable")overbrace("Acyl Halide" > "Anhydride" > "Aldehyde" > "Ketone") > stackrel("LiAlH"_4 " is suitable")overbrace("Ester" ~~ "Carboxylic Acid" > "Amide" > "Carboxylate Ion")#

Note that It is difficult to reduce a carboxylic acid to an aldehyde without accidentally reducing it further into an alcohol because #NaBH_4# is not reactive enough, but #LiAlH_4# is very reactive.

There are other ways to make a carboxylic acid more reactive, which will open up more pathways we can use, and one of them is to turn it into an acyl chloride.

So, we will have to do this in two or more steps. Two ways I can think of:

Like I said earlier, #"LiAlH"_4# makes the reduction go all the way to the primary alcohol, but you can partially oxidize back to the aldehyde. PCC, pyridinium chlorochromate, is one method to do so.

Alternatively, you can turn the carboxylic acid into an acyl chloride using #"SOCl"_2# (thionyl chloride) in pyridine, and from there a reduction via a "poisoned" #"LiAlH"_4# variant only partially reduces the acyl chloride. That may be due to steric hindrance, as #"LiAl"["OC"("CH"_3)_3]_3"H"# is very bulky.