Does RedP+HI reduce all carbon functional groups to alkane? I've seen the related questions 1 and 2, they're not duplicates 

So. when introducing Red P+HI reduction, our teacher said that it is the strongest reducing agent, and it can reduce any functional group i.e. all of these: carboxylic acid,  acid anhydride, ether, amide, acid halide, carbonyl, and nitrile

However, today I stumbled upon a question in my book involving a step in the synthesis of ibuprofen, where they did not reduce the carboxylic acid group even after using RedP+HI. I searched a lot on the internet, and finally came across this paper which confirms this:



paper - notice they've only reduced the alcohol in step (ii)

This now makes me wonder if what I was taught is actually correct. Which brings me to the question:


  Does RedP+HI really reduce all carbon functional groups to alkane?
Nope, in general it doesn't. You are correct. See also for example: 


A. Aramini, M. R. Sablone, G. Bianchini, A. Amore, M. Fanì, P. Perrone, A. Dolce, M. Allegretti, Tetrahedron 2009, 65 2015–2021. DOI: 10.1016/j.tet.2009.01.005:






M. Dobmeier, J. M. Herrmann, D. Lenoir, B. König, Beilstein J. Org. Chem. 2012, 8, 330–336. DOI: 10.3762/bjoc.8.36 Open access PDF. (This also includes your one as a reference, too.):





If it did, it would be a pretty poor reagent since it would have no functional group tolerance, so not very useful. However, it is not unlikely that under special conditions it might be able to reduce carboxylic groups as well.