Cоставить цепочку из С2H6+CL2(свет)----->X-->+NA---X---+Br2-->X-->+Na---->X (c3)
1) С2H6 + CL2 => C2H5Cl + HCl
2) 2C2H5Cl + Na => C4H10 + 2NaCl
3) C4H10 + Br2 => C4H9Br + HBr
4) 2C4H9Br + 2Na => С8H18 + 2NaBr