осуществить превращение ch4->ch3br->c2h6->c2h5br->c2h4->c2h5br
CH4 + Br2 = CH3Br + HBr
2CH3Br + 2Na = C2H6 + 2NaBr (РЕАКЦИЯ ВЮРЦА)
C2H6 + Br2 = C2H5Br + HBr
C2H5Br + KOH = C2H4 + H2O + KBr
C2H4 + HBr = C2H5Br