TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The method comprises (d) purification of hydrocarbons obtained in step (c). The fermentable raw material is pretreated prior to step (a) with an acid, a base, a solvent, a physical process or enzymes. The step (b) is performed at a pH of 5.5-6.5. The step (b) is carried out with a redox potential of -500 to -600 mV. The step (b) is performed in the presence of ethanol. The steps (a) and (b) are performed together in a single step. The steps (a) and (b) are carried out by providing fermentable feedstocks in a co-culture with a microorganism capable of hydrolyzing cellulose and one or more bacteria(s) of the genus Megasphaera. The volatile fatty acids in step (b) are continuously extracted from the fermentation broth. The short-chain fatty acids remaining in the fermentation broth in step (b), after extraction of the elongated fatty acids, are reused in steps (a) or (b) to produce alkene by Hofer-Moest or Kolbe electrolysis. The volatile fatty acids obtained in step (b) are extracted from the fermentation broth with an extractant and/or solvent. The volatile fatty acids obtained in step (b) are extracted with a water-repellent membrane. The fermentation is carried under ceiling or bubbling methane, biogas or a mixture of methane, carbon dioxide and hydrogen in varying proportions, in the presence of an additive as inhibitor of coenzyme M reductase in the fermentation liquid. The extraction is carried out in a two-phase fermenter. Preferred Components: The bacterium Megasphaera is Megasphaera elsdenii. The fatty acids obtained in step (b) are at least 5C, preferably 5-8C fatty acids, in which at least 50% of the fatty acids are 6C fatty acids, and at least 90% of the fatty acids are 5C fatty acids. The hydrocarbons obtained in step (c) are alkanes with a chain of at least 8C, preferably 8-14C. The extractant comprises organophosphorus compounds and tertiary or quaternary fatty amines. The solvent is alcohols, esters, alkanes or mixtures of hydrocarbons. The fermentable raw materials are byproducts of sugar or starch produced from plants, sugars obtained by hydrolysis of biomass of terrestrial origin optionally preceded by a delignification and sugars obtained by hydrolysis of biomass of aquatic origin, and industrial, urban, agricultural or livestock effluents or wastes. EXAMPLE - No suitable example given.