顺反异构 C2H5 C2H5 C=C C2H5 C>H5 H H-C-G H H CH3 CH3 CH3 CH3 H H HC C H C2 H5 C2 H5 H C=C C2H5 C2H5 H H CH3 CH3 H H CH3 H H CH3 顺反异构