Red P+HI reduction

Question

My teacher told us that Red P and HI can reduce any functional group. Is it true? Or are there any groups it can't reduce?(I know this exact question has been asked before but I couldn't find the exact thing I wanted)

I need a list of functional groups which cannot be reduced but red P HI

One group that P/HI won't reduce is -COOH. Kiliani reduced glucose to hexanoic acid with this reagent. — user55119, Jun 17 '20 at 13:22
The linked question answers your question to the point. What could you not find there? — Aniruddha Deb, Jun 17 '20 at 13:23
@AniruddhaDeb the ans referred is scattered. I couldn't find many results from that — DatBoi, Jun 17 '20 at 13:26
To OP:See this. To @user55119: carboxylic acid can be reduced (for ex-benzoic acid reduced to toluene and another source saying lauric acid can be reduced) — Nilay Ghosh, Jun 17 '20 at 15:02
@Nilay Ghosh: Aromatic acids may be an exception. As to lauric acid, need it carefully. The commentary is shear speculation. Not a reference in sight. — user55119, Jun 17 '20 at 15:14
@user55119 forget to edit the previous comment. Granted that 2nd source was full of skeptical inforrmation but the 1st source also mentioned stearic acid with benzoic acid. Maybe, I need to find some authentic sources. — Nilay Ghosh, Jun 17 '20 at 15:39

score 5 · Answer 1 · answered Jun 17 '20 at 20:36

Imines survive, that's for sure. Some 20 years ago junkies developed a way of turning a relatively innocent drug, which was then legal (and which has since ceased to be legal in many jurisdictions for this very reason) into a much more potent substance. I'd rather not identify it precisely; let's just say it is prominently featured in a TV series with name that is an anagram of "Baking Bread".

In short, the first substance contained the $\ce{-OH}$ and $\ce{-NHCH3}$ groups, and the reduction took away the former, but not the latter. The synthesis could be done with surprisingly crude equipment, so the junkies did it every time they needed the product. They bought red phosphorus and crystalline iodine by kilograms, and carried them around in a form of pre-prepared mixture ready to go into the reaction. They used to call this mixture "Stendhal", because of "The Red and the Black".

So it... went for a while.

Red P+HI reduction

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