During ethanol fermentation, glucose and other sugars in the corn (or sugarcane or other crops) are converted into ethanol and carbon dioxide.
C6H12O6 → 2 C2H5OH+ 2 CO2 + heat
Like any fermentation reaction, the fermentation is not 100% selective, and other side products such acetic acid, glycols and many other products are formed to a considerable extent and need to be removed during the purification of the ethanol. The fermentation takes place in aqueous solution and the resulting solution after fermentation has an ethanol content of around 15%. The ethanol is subsequently isolated and purified by a combination of adsorption and distillation techniques. The purification is very energy intensive.
During combustion ethanol reacts with oxygen to produce carbon dioxide, water, and heat:
C2H5OH + 3 O2 → 2 CO2 + 3 H2O + heat
Starch and cellulose molecules are strings of glucose molecules. It is also possible to generate ethanol out of cellulosic materials. That, however, requires a pretreatment that splits the cellulose into glycose molecules and other sugars that subsequently can be fermented. The resulting product is called cellulosic ethanol, indicating its source.
Ethanol may also be produced industrially from ethene (ethylene), by hydrolysis of the double bond in the presence of catalysts and high temperature.
C2H4 + H2O → C2H5OH
By far the largest fraction of the global ethanol production, however, is produced by fermentation