Alcohol fuels are oxygenated fuels containing one or more oxygen atoms per molecule. Monohydric acyclic saturated alcohols make a homologous series with general formula CnH2n+1OH. The lower members like methanol, ethanol are completely miscible with water, and as you go down the series, miscibility decreases as the larger alkyl group can make the molecule more hydrophobic. The boiling point and volatility of the alcohols increases as molecular weights increase in the series. Practically any of the small organic molecules in the alcohol family can be used as a fuel. However, alcohols that have been considered as motor fuels are methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH), and butanol (C4H9OH). Propanol has two isomers, и-propanol, and iso-propanol, whereas butanol has four isomers, и-butanol, sec-butanol, isobutanol and ferf-butanol. Nevertheless, ethanol is the most popular alcohol fuel due to lower toxicity and easy accessibility via fermentation methods.
Biobutanol is another renewable alcohol fuel under development, and can be prepared by fermentation as well as catalytic routes. Unfortunately, the rise in alcohol concentration causes butanol to be toxic to the fermentation microorganisms, killing them off after a period of time. This makes the butanol fermentation process expensive. Therefore, new modifications are constantly being researched, including the modification of enzymes and genes involved in biobutanol formation via fermentation. However, with the current state of technology, methanol and bioethanol fuels are technically and economically more suitable for internal combustion engines .