Chemists have figured out how to produce an industrially important reaction in a very economical way. Usually, adding a hydrochloric acid across a double bond involves two steps—an oxidation and a reduction—and requires a 1:1 ratio of both the oxidant and the reductant. Now Tobias Ritter and coworkers at the Max Planck Institute for Kohlenforschung and RWTH Aachen University devised a method that requires only adding a photocatalyst and visible light to the starting material and hydrochloric acid (Nat. Catal. 2023, DOI:10.1038/s41929-023-00914-7). This new finding represents reactivity that’s the opposite of a well-established mechanism in organic chemistry, and it’s a way to make commodity chemicals with less waste. In college organic chemistry class, students usually learn about the Markovnikov rule: when adding a mineral acid across a carbon-carbon double bond, the compound that comes from the stablest carbocation is the one that will form. Anti-Markovnikov reactions, which have the opposite selectivity, are well known to come out of radical reactions of mineral acids, but these require some sort of stoichiometric initiator. This reaction does not, Ritter says. The researchers used cheap available mineral acids, such as HCl, and added them across simple olefins to form an alkyl chloride, he says. This is not the first time that chemists have made an alkyl chloride from an olefin, Ritter says. “There are a lot of other robust, cheaper methods than what we have here,” he says. But this is the first time anyone has done this reaction with high atom economy, he says. The real target for industrial chemists is taking a cheap feedstock chemical and adding water across a double bond to make primary alcohols, a starting material for a massive amount of industrial chemicals, Ritter says. This new method represents a step toward making those reactions with less waste.