hydroxymethyl

The story

1. Proto-Indo-European*wed-
2. Proto-Indo-European*wódr̥
3. Proto-Hellenic*údōr
4. Ancient Greekῡ̆̔́δωρ (hū̆́dōr)
5. Ancient Greekὑδρο- (hudro-)
6. Frenchhydro-
7. Proto-Indo-European*ǵenh₁-
8. Proto-Indo-European*ǵénh₁os
9. Proto-Hellenic*génos
10. Ancient Greekγένος (génos)
11. Frenchhydrogène
12. Englishhydrogen
13. Proto-Indo-European*h₂eḱ-der.?
14. Ancient Greekὀξύς (oxús)
15. Ancient Greekγένος (génos)
16. Frenchoxygène
17. Englishoxygen
18. Proto-Indo-European*swel-der.?
19. Ancient Greekῡ̔́λη (hū́lē)
20. Englishhydroxyl
21. Englishhydroxy-
22. Proto-Indo-European*médʰu
23. Proto-Hellenic*métʰu
24. Ancient Greekμέθυ (méthu)
25. Ancient Greekῡ̔́λη (hū́lē)
26. GermanMethy
27. Englishmethyl
28. English todayhydroxymethyl

From hydroxy- + methyl.