[Ak]

i was looking at my lecture notes and saw tht my prof used 1. CrO3 then 2. H3O+ at O°C and made an aldehyde out of a primary alcohol...in my text book it says CrO3 makes a carboxylic acid, so im just clarifying, does the acid work up make it into an aldehyde? Thanks

[macman104]

PCC is pyridinium chlorochromate, and utilizes the same CrO3 ion that chromic acid oxidation uses.  It is possible that is what the process was.

[movies]

PCC is quite different than CrO₃.  In fact, it was pretty much invented to solve this over-oxidation problem.  Oxidation of a primary alcohol with CrO₃ will give an acid, with PCC, an aldehyde.

It looks to me like an error.  Perhaps he did mean to put PCC instead of CrO₃.

[agrobert]

CrO₃ is in general too strong of an oxidant to stop at an aldehyde from a primary alcohol.  Although he may be referring to a more old school reaction in which the oxidation stops at the aldehyde because of the 0 ºC temp and of course careful timing.  Although PCC is an much better choice.

[macman104]

PCC is quite different than CrO₃.  In fact, it was pretty much invented to solve this over-oxidation problem.  Oxidation of a primary alcohol with CrO₃ will give an acid, with PCC, an aldehyde.

It looks to me like an error.  Perhaps he did mean to put PCC instead of CrO₃.

What I was saying though was that PCC is just a complex with CrO3 in it and without the water (since the water creates a hydrate of the aldehyde which can then be oxidized again).  I wasn't saying they were the same, but that they were related, and since I couldn't see the notes, possibly what was being discussed in the notes.

It's a moot point anyway, but just wanted to clarify, I wasn't saying they were the same thing.

[Ak]

i agree tht pcc would be better but my prof was taking up a question where you could only do a given reaction only once and PCC had already been done so im guessing he did use the "old school" way mentioned by agrobert.  Thanks

[limpet chicken]

Why not use the etard reaction?

chromyl chloride in DMSO solution gives aldehydes from primary alcohols, although I'm not sure offhand if its selective for benzylic alcohols or not, might be, but I don't THINK so.

[azmanam]

Is this for the synthesis question you posted earlier?  What did you come up with as a final answer?

[Ak]

@ azmanam:

its kinda for tht question because i was wondering if i could use the Cro3 and acid work up to make an alcohol to an aldehyde but the question my prof used this in was a different question...i did try the question and i came up with an answer, im just not sure if u can grignard this: MgBr - C - CH3
                            ||
                            CH2

[azmanam]

Interesting route.  Good work on a tough question.  Why not Jones the 2^o alcohol and save PCC for the last step?  Also, I'd be afraid you'd have trouble stopping the haloform reaction at the mono bromide.  What about aldol, oxid. then double wittig?  I'm also a little concerned about the triple elimination.  With the free alcohol I'd be wary of forming epoxides as side products. Have you already turned it in?

[Ak]

ya ive already handed it in....but its not worth tht much so i can lose a couple of marks...but i cant jones it because i have no idea what that is as it wasnt taught to us, and the aldol way never occured to me because i just learned it and forgot about it...and my prof said that the triple elimination is fine for this course as its not really that advanced.

[macman104]

Jones is the name of the oxidation with CrO3.  When you have a secondary alcohol, it is not reduced to a carboxylic acid, but stops at the ketone.  So you could have saved your more mild oxidation for when you needed it and used the Jones oxidation for the secondary alcohol.

[azmanam]

Jones reagent is CrO₃ with sulfuric acid.  Sometimes called chromic acid, H₂CrO₄. 

As for the triple elimination, since you've already turned it in, you could protect the alcohols with TMS or TES groups, eliminate with no problems, then oxidize straight to the aldehyde under Swern conditions (DMSO, oxalyl chloride, NEt₃) without deprotecting first.  Swern conditions are known to simultaneously deprotect smaller silyl ethers and oxidize to the aldehyde in one operation.

I think what was giving me so much trouble with your problem is that it seems like there should be some concise way to put the molecule together.  This is what we would call a 'step-y' synthesis.  not technically incorrect, but a lot of steps, and not very efficient.  But you're not in it for efficiency, just proof of concept, so your synthesis is probably fine.