  We present the detection of ethanol (C2H5OH), acetone (CH3COCH3), and
propanal (C2H5CHO) toward the cyanopolyyne peak of TMC-1. These three O-bearing
complex organic molecules are known to be present in warm interstellar clouds,
but had never been observed in a starless core. The addition of these three new
pieces to the puzzle of complex organic molecules in cold interstellar clouds
stresses the rich chemical diversity of cold dense cores in stages prior to the
onset of star formation. The detections of ethanol, acetone, and propanal were
made in the framework of QUIJOTE, a deep line survey of TMC-1 in the Q band
that is being carried out with the Yebes 40m telescope. We derive column
densities of (1.1 +/- 0.3)e12 cm-2 for C2H5OH, (1.4 +/- 0.6)e11 cm-2 for
CH3COCH3, and (1.9 +/- 0.7)e11 cm-2 for C2H5CHO. The formation of these three
O-bearing complex organic molecules is investigated with the aid of a detailed
chemical model which includes gas and ice chemistry. The calculated abundances
at a time around 2e5 yr are in reasonable agreement with the values derived
from the observations. The formation mechanisms of these molecules in our
chemical model are as follows. Ethanol is formed on grains by addition of
atomic carbon on methanol followed by hydrogenation and non-thermal desorption.
Acetone and propanal are produced by the gas-phase reaction between atomic
oxygen and two different isomers of the C3H7 radical, where the latter follows
from the hydrogenation of C3 on grains followed by non-thermal desorption. A
gas-phase route involving the formation of (CH3)2COH+ through several
ion-neutral reactions followed by its dissociative recombination with electrons
do also contribute to the formation of acetone.
