  We present time dependent chemical models for a dense and warm O-rich gas
exposed to a strong far ultraviolet field aiming at exploring the formation of
simple organic molecules in the inner regions of protoplanetary disks around T
Tauri stars. An up-to-date chemical network is used to compute the evolution of
molecular abundances. Reactions of H2 with small organic radicals such as C2
and C2H, which are not included in current astrochemical databases, overcome
their moderate activation energies at warm temperatures and become very
important for the gas phase synthesis of C-bearing molecules. The
photodissociation of CO and release of C triggers the formation of simple
organic species such as C2H2, HCN, and CH4. In timescales between 1 and 10,000
years, depending on the density and FUV field, a steady state is reached in the
model in which molecules are continuously photodissociated but also formed,
mainly through gas phase chemical reactions involving H2. The application of
the model to the upper layers of inner protoplanetary disks predicts large gas
phase abundances of C2H2 and HCN. The implied vertical column densities are as
large as several 10^(16) cm^(-2) in the very inner disk (< 1 AU), in good
agreement with the recent infrared observations of warm C2H2 and HCN in the
inner regions of IRS 46 and GV Tau disks. We also compare our results with
previous chemical models studying the photoprocessing in the outer disk
regions, and find that the gas phase chemical composition in the upper layers
of the inner terrestrial zone (a few AU) is predicted to be substantially
different from that in the upper layers of the outer disk (> 50 AU).
