Biodiesel

Biodiesel is a mixture of methyl esters of long-chain carboxylic acids. It is used as a fuel in diesel engines, either on its own or blended with diesel from crude oil.It is made by reacting vegetable oils (triglycerides) with methanol in the presence of a catalyst, typically a strong alkali such as sodium or potassium hydroxide. The methanol swaps places with the glycerol in each ester group, so the three fatty acid chains leave as three separate methyl esters, and glycerol is released.triglyceride + 3CH₃OH → 3RCOOCH₃ + CH₂(OH)CH(OH)CH₂OHFor example, a chain from octadecanoic acid ends up as methyl octadecanoate, C₁₇H₃₅COOCH₃. The methyl esters are much smaller molecules than the original oil, so biodiesel is far less viscous and burns well in an engine.Worked example: 0.40 mol of a triglyceride reacts completely with methanol. The ratio of triglyceride to methyl esters is 1 : 3, so it gives 3 × 0.40 = 1.2 mol of methyl esters, uses 1.2 mol of methanol and releases 0.40 mol of glycerol.Biodiesel is renewable, because the oils come from crops that can be regrown. It can be close to carbon neutral, since the crops absorb carbon dioxide as they grow. It is not completely carbon neutral, because energy from fossil fuels is used to farm, process and transport the crops.