3.4.1: Environment- Synthesis of Biodiesel Fuel
- Page ID
- 50053
Biological oils are different from petroleum oils ("regular" diesel fuel) in molecular structure and properties.
C12H23, dodecane |
A triglyceride: glyceryl trilinoleate |
Because of their large size and consequent large intermolecular attractions, the viscosity of biological oils is generally too high for use in conventional diesel engines. Biological oils also burn a little less readily, and with a sootier flame than petroleum diesel. Biological oils can be used in conventional diesel engines if they are preheated to reduce their viscosity, but this requires an auxiliary electrical heater until the engine warms up. For these resons, bilogical oils require processing for use as biodiesel.
A biological oil is an ester, which is a type of organic compound having the atom linkage shown below.
![]() The ester linkage. R and R' represent chains of carbon and hydrogen atoms. R = -CH3 and R' = -C18H35O2 for the methyl stearate in Example 1 |
glycerol |
The ester linkage in biological oils is created when a glycerol molecule reacts with organic acids. The glycerol molecule has a chain of 3 carbon atoms, each with an -OH (alcohol) group on it. The figure below shows how an organic alcohol reacts with a organic acid. Organic chemists abbreviate molecular structures--the "zig-zag" lines in the figure represent carbon chains with a C atom at each "zig" or "zag". Each carbon has 4 bonds, and if fewer than 4 are shown, it's assumed that they go to H atoms. So the alcohol is C2H5OH (ethanol), and the acid is acetic acid (or ethanoic acid, CH3COOH) in the Figure:
Since glycerol has 3 -OH groups, 3 long chain organic "fatty acids" attach to make the bulky "triglyceride".
But just as easily as esters can be made from alcohols and acids, they can switch alcohols or acids. In the presence of a strong base catalyst, like NaOH, a triglyceride can react with 3 small alcohol molecules, like methanol (CH3OH), which replace the glycerol "backbone", making 3 separate esters of lower molecular weight

Quite often a mixture of two or more products is formed. For example, when a vegetable oil reacts with methanol, only one or two of the acids may be displaced from the glycerine, producing only 1 or 2 FAMEs.
- C3H5(C18H35O2)3 + NaOH + 2 CH3OH → C3H5(C18H35O2)2(OH) + 2 C17H35COOCH3
- C3H5(C18H35O2)3 + NaOH + 1 CH3OH → C3H5(C18H35O2)(OH)2 + 1 C17H35COOCH3
- C3H5(C18H35O2)3 + NaOH + 3 CH3OH → C3H5(OH)3 + 3 C17H35COOCH3
Usually, a large excess of methanol and sodium hydroxide are added, so that the reaction produces the maximum amount of FAME.
But in the case of a transesterification, even though none of the reactants is completely consumed, no further increase in the amounts of the products occurs. We say that such a reaction does not go to completion. When a mixture of products is produced or a reaction does not go to completion, the effectiveness of the reaction is usually evaluated in terms of percent yield of the desired product. A theoretical yield is calculated by assuming that all the limiting reagent is converted to product. The experimentally determined mass of product is then compared to the theoretical yield and expressed as a percentage:

EXAMPLE 1 When 100.0 g C3H5(C18H35O2)3 gas and 15.0 g CH3OH are mixed at 55°C with NaOH catalyst, they react to form 90.96 g C17H35COOCH3 methyl stearate biodiesel. Calculate the percent yield.
Solution We must calculate the theoretical yield of NH3, and to do this, we must first discover whether N2 or H2 is the limiting reagent. For the balanced equation
- C3H5(C18H35O2)3 + NaOH + 3 CH3OH → C3H5(OH)3 + 3 C17H35COOCH3
- stearin + sodium hydroxide + 3 CH3OH → glycerol + 3 methyl stearate
The stoichiometric ratio of the reactants is






C3H5(C18H35O2)3(s) | + 3 CH3OH (l) | → 1 C3H5(OH)3 (l) | + 3 C18H35O2)3CH3(s) | ||
---|---|---|---|---|---|
m, g | 100.0 g | 15.00 g | 90.96 g | ||
M, g/mol | 891.5 | 32.04 | 298.5 | 92.1 | |
n present, mol | 0.1122 mol | 0.4682 mol | |||
n actual, mol | 0.1122 | 0.3366 | 0.1122 | 0.3366 | |
m actual, mass | 100.0 | 10.78 | 10.33 | 100.47 |

Transesterification is a classic example of a reaction which does not go to completion.
References
- ↑ en.Wikipedia.org/wiki/Diesel_fuel
Contributors
Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.
Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.