The chemical transformation of benzene under pressure is investigated, at room temperature and at 100 K, by means of infrared spectroscopy. Pressurization-decompression cycles in the 0-50 GPa pressure range have been performed to achieve the complete transformation of the monomer. The yellow-brownish recovered sample has been identified as an amorphous hydrogenated carbon (a-C:H). A correlation has been established between the pressure behavior of the frequencies of both Raman and infrared internal modes, and the corresponding vibrational energies in the S-1 excited state (B-1(2u)). From this comparison we conclude that pressure induces a mixing between the ground and the S-1 electronic states. The increased ring flexibility enhances the interactions among nearest-neighbor molecules inducing the formation of a network of interconnected benzene units where the aromatic character is lost. The bond breaking mainly occurs during the decompression cycle favored by the density decrease. Radical species form in this stage and rapidly propagate to give the denser a-C:H final product.