a, Ultraviolet–visible spectral changes showing the decay of 2 (1 mM) and formation of 3 on addition of 2-PPA (50 mM). Inset, time courses of the absorbance change of 2 at 526 nm for the reaction with 2-PPA (filled circles) and for the natural decay (open circles). b, Second-order rate constants determined in the reactions of 2 with pentanal (1°-CHO; triangles), 2-methylbutanal (2°-CHO; circles), and pivalaldehyde (3°-CHO; squares). Standard deviation is <10% of the data used in the plot.