  The organic content of protoplanetary disks sets the initial compositions of
planets and comets, thereby influencing subsequent chemistry that is possible
in nascent planetary systems. We present observations of the complex
nitrile-bearing species CH3CN and HC3N towards the disks around the T Tauri
stars AS 209, IM Lup, LkCa 15, and V4046 Sgr as well as the Herbig Ae stars MWC
480 and HD 163296. HC3N is detected towards all disks except IM Lup, and CH3CN
is detected towards V4046 Sgr, MWC 480, and HD 163296. Rotational temperatures
derived for disks with multiple detected lines range from 29-73K, indicating
emission from the temperate molecular layer of the disk. V4046 Sgr and MWC 480
radial abundance profiles are constrained using a parametric model; the
gas-phase CH3CN and HC3N abundances with respect to HCN are a few to tens of
percent in the inner 100 AU of the disk, signifying a rich nitrile chemistry at
planet- and comet-forming disk radii. We find consistent relative abundances of
CH3CN, HC3N, and HCN between our disk sample, protostellar envelopes, and solar
system comets; this is suggestive of a robust nitrile chemistry with similar
outcomes under a wide range of physical conditions.
