Polyynes were synthesized by arc discharge between two copper electrodes in alkanes for the first time. The as-grown polyyne samples were separated through high-performance liquid chromatography and the size-selected polyynes C2nH2 (n = 4-6) were investigated by surface-enhanced Raman scattering spectroscopy. The formation rate of polyynes has been found to increase with rising carbon-hydrogen ratio of the alkanes, and the alkanes affect the length of polyyne chains. The reaction enthalpy from alkane to C-2 radicals decreases with increasing carbon-hydrogen ratio, which causes the energy consumption in the whole reaction to differ and leads to different formation rates.