Comparison between the observed and modeled abundances in the Horsehead nebula for HCO and the six organic molecules listed in the first column of Table 1, at two typical early and late times. The model results are represented by solid curves and the observations by vertical error bars which refer to a 50% error in the abundances (Guzman et al. 2014), and horizontal lines, which are solid where our model can reproduce the observed abundances within their error bars and dashed where the agreement is only within a standard factor of ten. The grayish boxes indicate the approximate A_V ranges at which the observations were performed in each region, according to the corresponding published articles.