Procedure:  250 mL 1-neck flask, stirbar, ozonator

                  Dissolved 2.777 g of ethyl sorbate (I) in 85 mL of absolute EtOH.  Stirred at rt.  Added 2.0 mL of a 0.05% solution of sudan red in EtOH.  Cooled to -78 C.  Passed O₃ through the solution.  After 65 min the solution had lost its deep red color and became pale. O₃ was stopped and N₂ was passed through the solution for 15 min as it warmed to rt.  Added 3.0 mL of Me₂S.  Stirred for 45 min.  The solvent was removed from the reaction mixture by rotary evaporation.  The product was isolated by Kugelrohr distillation.          

 

¹H NMR (CDCl₃, 400 MHz) d 9.75 (d, J = 7.6 Hz, CHO), 6.95 (dd, J = 16.0, 7.6 Hz, H3), 6.70 (d, J = 16.0 Hz, H2), 4.28 (q, J = 7.1 Hz, CO₂CH₂CH₃), 1.33 (t, J = 7.1 Hz, CO₂CH₂CH₃). 

 

notes

 

This is a good procedure for selective ozonolysis of the most reactive (ie. most electron rich) double bond of a diene.  In this case the dye is selected to have a reactivity in between that of the diene and the product enal.  Oxidation of the dye causes a change in color indicating the desired endpoint for isolation of the enal without overoxidation.  This reduces the difficulty in determining the quantity of O₃ delivered to the reaction to essentially a titration.