4 C3H8O + 2 K2Cr2O7 + 8 H2SO4 → 3 C4H8O2 + 2 K2SO4 + 2 Cr2(SO4)3 + 12 H2O
This is an oxidation-reduction (redox) reaction:
12 C-II - 12 e- → 12 C-I (oxidation)
4 CrVI + 12 e- → 4 CrIII (reduction)
C3H8O is a reducing agent, K2Cr2O7 is an oxidizing agent.