The efficient partial oxidation of hydrocarbons to valuable chemicals without formation of CO2 is one of the great challenges in heterogeneous catalysis. The ever-decreasing cost of renewable electricity and the superior control over reactivity qualify electrochemistry as a particularly attractive means of addressing this challenge. Yet, to date, little is known about the factors regulating hydrocarbon oxidation at the atomic level. A relevant showcase reaction is propene electro-oxidation to key industrial commodity chemicals, such as acrolein, acrylic acid and propylene oxide. In this study, we investigate the partial electrochemical oxidation of propene on high-surface area Pd electrodes using a combination of electrochemical measurements, advanced product characterization and theoretical modeling. We report a new reaction product, propylene glycol, and high selectivity towards acrolein. We further identify key reaction intermediates and propose a mechanism dictated by the surface coverage of organic species formed in situ, where stable reactant adsorption at low coverage determines the selectivity towards allylic oxidation at high coverage. Our fundamental findings enable advances in partial hydrocarbon oxidation reactions by highlighting atomic surface structuring as the key to selective and versatile electrochemical catalyst design.