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We aim to understand the rich chemical composition of AFGL 2591, aprototypical isolated high-mass star-forming region.  Based on HIFI and JCMT data, the molecular abundances of species found in theprotostellar envelope of AFGL 2591 were derived with the Monte Carlo radiativetransfer code RATRAN, assuming either constant values or 1D stepwise radialprofiles as abundance distributions. The reconstructed 1D abundances werecompared with the results of time-dependent gas-grain chemical modeling,considering ages of 10,000 to 50,000 years, cosmic-ray ionization rates of 0.5to 50 times 10^-16 s^-1, uniformly-sized 0.1-1 micron dust grains, a dust/gasratio of 1%, and several sets of initial molecular abundances with C/O <1 and>1.  Constant abundance models give good fits to the data for CO, CN, CS, HCO+,H2CO, N2H+, C2H, NO, OCS, OH, H2CS, O, C, C+, and CH. Models with an abundancejump at 100 K give good fits to the data for NH3, SO, SO2, H2S, H2O, HCl, andCH3OH. For HCN and HNC, the best models have an abundance jump at 230 K. Thetime-dependent chemical model can accurately explain abundance profiles of 15out of these 24 species. The jump-like radial profiles for key species likeHCO+, NH3, and H2O are consistent with the outcome of the time-dependentchemical modeling. The best-fit model has a chemical age of 10-50 kyr, a solarC/O ratio of 0.44, and a cosmic-ray ionization rate of 5 x 10^-17 s^-1; grainproperties and external UV intensity do not affect the calculated chemicalstructure much.  We thus demonstrate that simple constant or jump-like abundance profilesagree with time-dependent chemical modeling for most key C-, O-, N-, andS-bearing molecules. The main exceptions are species with very few observedtransitions (C, O, C+, and CH), with a poorly established chemical network(HCl, H2S) or whose chemistry is strongly affected by surface processes(CH3OH).

The HIFI spectral survey of AFGL 2591 (CHESS). III. Chemical structure of the protostellar envelope