Electron Withdrawing Groups and Steric Effects on the Methanogenic Toxicity

Aims: This work highlights the influence of the additivity of withdrawing electronic groups attached to the aromatic ring on the inhibition of the production of methane gas by the methanogenic bacteria acetoclastic.

Study Design: Anaerobic digestion of pig manure, anaerobic toxicity essay, Effect of additivity and the number of electron withdrawing groups on the methanogenic toxicity, relationship between the methanogenic toxicity, electron and steric effects.

Place and Duration of Study: Department of Chemistry, University of Kinshasa, DR Congo, from March 2012 to January 2013.

Methodology: The toxicity to acetoclastic Methanogenic archaea was performed with the standard method of serum bottles, digested pig manure was utilized as inoculums and acetate as substrate. The methane gas volume produced was measured by serum bottles liquid displacement systems (Mariotte flask system).

Results: The obtained results show that the increase of the number of electron withdrawing groups on the aromatic ring does not imply by force the increase of the Methanogenic toxicity (MT) of these aromatic compounds, in comparison to those which has only one electron withdrawing group. ¶We notice that Nitrobenzene and Chlorobenzene have shown higher toxicity than their ortho-, meta- and para-Nitro and Chloro groups isomers, respectively. ¶Moreover for these groups, the meta-isomers show more toxicity than the para-isomers, which in their turns are more toxic than the ortho-isomers.

Conclusion: The overall obtained results indicate that as for electronic effects of various withdrawing substituents on the aromatic ring, the steric constraints of those substituents also have a considerable effect on MT.