The stepwise mechanism begins with nitrogen extrusion, forming an α-ketocarbene. The α-ketocarbene can either undergo a 1,2-alkyl shift, to give the ketene product, or can undergo a 4π electrocyclic ring closure, to form an antiaromatic oxirene. This oxirene can reopen in two ways, to either α-ketocarbene, which can then form the ketene product. There are two primary arguments for stepwise mechanisms. The first is that rate constants of Wolff rearrangements depend on the stability of the formed carbene, rather than the migratory aptitude of the migrating group. The most definitive evidence is isotopic scrambling of the ketene, as predicted by an oxirene intermediate, which can only occur in the stepwise path. In the scheme below, the red carbon is 13C labelled. The symmetric oxirene intermediate can open either way, scrambling the 13C label. If the substituents R1 and R2 are the same, one can quantify the ratio of products stemming from the concerted and stepwise mechanisms; if the substituents are different, the oxirene will have a preference in the direction it opens, and a ratio cannot be quantified, but any scrambling indicates some reactant is going through a stepwise mechanism. In photolysis of diazo acetaldehyde, 8% of the label is scrambled, indicating that 16% of product is formed via the oxirene intermediate. Under photolysis, the biphenyl (R1=R2=phenyl) substrate shows 20–30% label migration, implying 40–60% of product goes through the oxirene intermediate. α-diazocyclohexanone shows no label scrambling under photolytic conditions, as it is entirely s-cis, and thus all substrate goes through the concerted mechanism, avoiding the oxirene intermediate. Isotopic labeling studies have been used extensively to measure the ratio of product stemming from a concerted mechanism versus a stepwise mechanism. These studies confirm that reactants that prefer s-trans conformations tend to undergo stepwise reaction. The degree of scrambling is also affected by carbene stability, migratory abilities, and nucleophilicity of solvent. The observation that the migratory ability of a substituent is inversely proportional to amount of carbene formed, indicates that under photolysis, there are competing pathways for many Wolff reactions. The only Wolff rearrangements that show no scrambling are s-cis constrained cyclic α-diazo ketones. Mechanistic conclusion Under both thermolytic and photolytic conditions, there exist competing concerted and stepwise mechanisms. Many mechanistic studies have been carried out, including conformational, sensitization, kinetic, and isotopic scrambling studies. These all point to competing mechanisms, with general trends. α-Diazo ketones that exist in the s-cis conformation generally