  The Voyager 1 flyby of Titan in 1980 gave a first glimpse of the chemical
complexity of Titan's atmosphere, detecting many new molecules with the
infrared spectrometer (IRIS). These included propane (C3H8) and propyne
(CH3C2H), while the intermediate-sized C3Hx hydrocarbon (C3H6) was curiously
absent. Using spectra from the Composite Infrared Spectrometer (CIRS) on
Cassini, we show the first positive detection of propene (C3H6) in Titan's
stratosphere (5-sigma significance), finally filling the three-decade gap in
the chemical sequence. We retrieve a vertical abundance profile from 100-250
km, that varies slowly with altitude from ~2 ppbv at 100 km, to ~5 ppbv at 200
km. The abundance of C3H6 is less than both C3H8 and CH3C2H, and we remark on
an emerging paradigm in Titan's hydrocarbon abundances whereby: alkanes >
alkynes > alkenes within the C2Hx and C3Hx chemical families in the lower
stratosphere. More generally, there appears to be much greater ubiquity and
relative abundance of triple-bonded species than double-bonded, likely due to
the greater resistance of triple bonds to photolysis and chemical attack.
