  We attempt to determine the molecular composition of disks around young
low-mass stars in the $\rho$ Oph region and to compare our results with a
similar study performed in the Taurus-Auriga region. We used the IRAM 30 m
telescope to perform a sensitive search for CN N=2-1 in 29 T Tauri stars
located in the $\rho$ Oph and upper Scorpius regions. $^{13}$CO J=2-1 is
observed simultaneously to provide an indication of the level of confusion with
the surrounding molecular cloud. The bandpass also contains two transitions of
ortho-H$_2$CO, one of SO, and the C$^{17}$O J=2-1 line, which provides
complementary information on the nature of the emission. Contamination by
molecular cloud in $^{13}$CO and even C$^{17}$O is ubiquitous. The CN detection
rate appears to be lower than for the Taurus region, with only four sources
being detected (three are attributable to disks). H$_2$CO emission is found
more frequently, but appears in general to be due to the surrounding cloud. The
weaker emission than in Taurus may suggest that the average disk size in the
$\rho$ Oph region is smaller than in the Taurus cloud. Chemical modeling shows
that the somewhat higher expected disk temperatures in $\rho$ Oph play a direct
role in decreasing the CN abundance. Warmer dust temperatures contribute to
convert CN into less volatile forms. In such a young region, CN is no longer a
simple, sensitive tracer of disks, and observations with other tracers and at
high enough resolution with ALMA are required to probe the gas disk population.
