oxidation/reduction ochem

[yellowjellybean]

To oxidize a primary alcohol into an aldehyde without further oxidation into a carboxylic acid what reagent should you add in conjunction with chromic acid?

A. Pyridine and HCl
B. Toluene and HCl
C. Pyridine and sulfuric acid
D. Toluene and sulfuric acid

The answer is A but I have no idea why because that is an oxidizing agent, wouldn't that just further oxidize it?

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[NuttyEngDude]

I memorized PCC. So when I saw this that was the first thing that popped into my head. Unfortunately I cant tell you more than just "PCC" (ochem is my weakspot).

But... PCC stands for Pyridinium chlorochromate and you can get that from pyridine, HCl, and chromic acid. I hope someone provides you (and me) with a better explanation but that was my line of reasoning.

 

[circulus vitios]

I really suck at o-chem so someone else should verify this or more likely prove me wrong, but I think the goal is to find which combination gives a weak acid. Pyridine can accept a proton from a strong acid; toluene cannot accept any protons. HCl is monoprotic which is ideal because a diprotic acid would give side reactions i.e. the resulting carbonyl of the aldehyde could be protonated by the HSO4- ion.

 

[yellowjellybean]

This is what Wikipedia says "Although not widely used, PCC offered the advantage of the selective oxidation of alcohols to aldehydes, whereas many other reagents were less selective." I was just confused because TBR didn't say anything about this so I thought it would oxidize all the way to a carboxylic acid.

 

[Hemorrage]

The whole point of using PCC is to be able to stop the oxidation at the aldehyde.. otherwise it continues onto the c-acid.

 

[Vurday]

When oxidizing primary alcohols, what happens with reagents in acidic conditions is that the original aldehyde that forms can be protonated to form a geminal diol, which further reacts by oxidizing to the carboxylic acid stage.

In order to prevent this, you should run your reaction in basic conditions. This is where the pyridine comes in. The nitrogen in pyridine can accept protons and therefore act as a base. This prevents protonation of the aldehyde into the geminal diol, causing your reaction to stop at the aldehyde stage.

I would pick HCl over H2SO4 since the sulfuric acid can also readily protonate the aldehyde which may cause oxidation to the carboxylic acid.

Source: http://www.utdallas.edu/~biewerm/11alc.pdf

 

[NuttyEngDude]

these response certainly helped me out (hopefully yellowjellybean too), thanks guys
👍