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Ultracompact (UC) HII regions constitute one of the earliest phases in theformation of a massive star and are characterized by extreme physicalconditions (Go>10^5 Habing field and n>10^6 cm^-3). The UC HII Mon R2 is theclosest one and therefore an excellent target to study the chemistry in thesecomplex regions.  We carried out a 3mm and 1mm spectral survey using the IRAM 30-m telescopetowards three positions that represent different physical environments in MonR2: (i) the ionization front (IF) at (0",0"); two peaks in the molecular cloud(ii) MP1 at the offset (+15",-15") and (iii) MP2 at the farther offset(0",40"). In addition, we carried out extensive modeling to explain thechemical differences between the three observed regions.  We detected more than thirty different species. We detected SO+ and C4Hsuggesting that UV radiation plays an important role in the molecular chemistryof this region. We detected the typical PDR molecules CN, HCN, HCO, C2H, andc-C3H2. While the IF and the MP1 have a chemistry similar to that found in highUV field and dense PDRs like the Orion Bar, the MP2 is more similar to lowerUV/density PDRs like the Horsehead nebula.  We also detected complex molecules that are not usually found in PDRs (CH3CN,H2CO, HC3N, CH3OH and CH3C2H). Sulfur compounds CS, HCS+, C2S, H2CS, SO and SO2and the deuterated species DCN and C2D were also identified. [DCN]/[HCN]=0.03and [C2D]/[C2H]=0.05, are among the highest in warm regions.  Our results show that the high UV/dense PDRs present a different chemistryfrom that of the low UV case. Abundance ratios like [CO+]/[HCO+] or[HCO]/[HCO+] are good diagnostics to differentiate between them. In Mon R2 wehave the two classes of PDRs, a high UV PDR towards the IF and the adjacentmolecular bar and a low-UV PDR which extends towards the north-west followingthe border of the cloud.

Spectral line survey of the ultracompact HII region Mon R2