  We have carried out high-angular resolution (1.4") observations in the
continuum at 3.1mm and in the N2H+ 1-0, CH3CN 5_k-4_k and 13CS 2-1 lines using
the Plateau de Bure Interferometer (PdBI) towards the intermediate mass (IM)
protostar IRAS21391+5802 (IC1396N). In addition, we have merged the PdBI images
with previous BIMA (continuum data at 1.2mm and 3.1mm) and single-dish (N2H+
1-0) data to have a comprehensive description of the region. The combination of
our data with BIMA and 30m data show that the bipolar outflow associated has
completely eroded the initial molecular globule. The 1.2mm and 3.1mm continuum
emissions are extended along the outflow axis tracing the warm walls of the
biconical cavity. Most of the molecular gas, however, is located in an
elongated feature in the direction perpendicular to the outflow. A strong
chemical differentiation is detected across the molecular toroid, with the N2H+
1-0 emission absent in the inner region.This chemical differentiation can be
understood in terms of the different gas kinetic temperature. The
[CH3CN]/[N2H+] ratio increases by 5 orders of magnitude with gas temperature,
for temperatures between 20K and 100K. The CH3CN abundance towards IRAM 2A, the
most massive protostellar core, is similar to that found in hot corinos and
lower than that expected towards IM and high mass hot cores. This could
indicate that IRAM 2A is a low mass or at most Herbig Ae star (IRAM 2A) instead
of the precursor of a massive Be star. Alternatively, the low CH3CN abundance
could also be the consequence of IRAM 2A being a Class 0/I transition object
which has already formed a small photodissociation region (PDR).
