  Chiral molecules interact and react differently with other chiral objects,
depending on their handedness. Therefore, it is essential to understand and
ultimately control the evolution of molecular chirality during chemical
reactions. Although highly sophisticated techniques for the controlled
synthesis of chiral molecules have been developed, the observation of chirality
on the natural femtosecond time scale of a chemical reaction has so far
remained out of reach for isolated molecules. Here, we demonstrate a general
experimental technique, based on high-harmonic generation in tailored laser
fields, and apply it to probe the time evolution of molecular chirality during
the photodissociation of 2-iodobutane. These measurements show a change in sign
and a pronounced increase in the magnitude of the chiral response over the
first 100 fs, followed by its decay within less than 500 fs, revealing the
photodissociation to achiral products. The observed time evolution is explained
in terms of the variation of the electric and magnetic transition-dipole
moments between the lowest electronic states of the cation as a function of the
reaction coordinate. These results open the path to investigations of the
chirality of molecular reaction pathways, light-induced chirality in chemical
processes and the control of molecular chirality through tailored laser pulses.
