ESTERIFICATION
This laboratory experiment is an example of transesteriflcation.(see class text chapter 17). Vegetable Oil, from natural sources like corn or olives, contains a mixture of “triglycerides”, which are tri-esters formed from the tri-alcohol glycerol, and long chain fatty acids. The chemical reaction performed in lab will convert these to esters of methanol (methyl esters), releasing the glycerol as the other organic product. Although esterification can occur under acid or base conditions, the laboratory reaction will involve a base.
[CENTER]
sodium hydroxide
Triglycerides + methanol <—> methyl esters + glycerol [/CENTER]The conversion of triglycerides or free fatty acids to methyl esters has developed interest as a potential alternate fuel source for diesel type engines. These esters are sometimes referred to as biodiesel. Free fatty acids are often found in ‘used’ vegetable oil. That is, oil that has been used for frying foods at elevated temperatures. Their presence is presumed to be the result of the reaction between water (from foods) and the triglycerides at elevated temperatures. The presence of free acids would make it necessary to use enough methanol to esterify them as well. Since this experiment utilizes pure vegetable oil (not previously used) we do not need to perform a titration to identify the levels of free acids present if ‘used’ oil was the reactant.
The reaction rate for transesterification depends on the concentrations of reactants (we use excess methanol) as well as any steric hindrance encountered because of the structures of the acid components of the tri-ester.
PROCEDURE:
Set up a 500mL round bottom flask with a reflux condenser, stirbar and mantle for heating. A thermometer should be immersed in the reaction mixture using one of the extra necks of the flask, and the other neck should be stoppered. Be sure that the thermometer is safely above the stirbar.
Using a large graduated cylinder, measure 200mL of com oil and add it to the flask. In another container, mix 20mL of methanol with 0.35g NaOH. The sodium hydroxide is extremely corrosive and care must be taken to protect yourself and the laboratory surfaces from contact with it. Be sure that all the solid has dissolved. Start the stirbar spinning, and using a funnel (to protect the neck of the flask) carefully add the basic methanol solution to the flask containing the com oil. This mixture will react at room temperature but would require a period of time that may exceed our lab time. Therefore we will heat the mixture to increase the reaction rate. Actual biodiesel processors heat the mixture to 120-130 degrees Fahrenheit.
Convert 125 F to Celsius temperature (our glass thermometers are Celsius and our digital thermometers which can be set to read F or C may be damaged by the basic reaction mixture at elevated temperatures)
The conversion equation is C = (F - 32)/1.8 and be sure to employ the parentheses
If the reaction mixture is heated to this temperature for at least 75 minutes, a sufficient yield of product should form. After the heating period, remove the heating mantle by raising the assembled glassware, and discontinue the stirring. The side product, glycerol, should begin to separate from the mixture, since it is more dense than the esters produced. Remove the stirbar. Set up a ring stand for support of the separatory funnel in the hood. Using a funnel, carefully pour the reaction mixture into your 250mL separatory funnel. The reaction mixture needs to remain undisturbed while the glycerol collects at the bottom of the flask. Put the labeled funnel in the hood. The label should include your name, the date and “biodiesel”.
Potential reaction problems:
If there were free fatty acids present, they would form soap (soap is a mixture of sodium salts of fatty acids) upon reaction with the base.
If there was not enough alcohol present or if water were present soap could also form. Incomplete reaction of the triglyceride (because of insufficient time or temperature) will result in mono- or diglyceride formation which can be observed as white chunks in the room temperature reaction mixture.
The reaction mixture must be purified: (during the next lab period)Gently move the ring stand from the hood to your work area. The glycerol layer (which may contain some excess methanol) is removed from the bottom via the stopcock.
Depending on the volume of liquid remaining in the funnel, it may be necessary to transfer the contents to a larger separatory funnel for the washing/extractions.
The product is ‘washed’(extraction) to remove impurities, primarily any soap that has formed and any excess methanol.
Use the text information on extraction to predict how much water to use, for each extraction taking into account the volume of liquid. (Rule of thumb: Total volume of extraction solvent should be equal to the volume of the other liquid, but divided into equal portions for repeated extraction steps, which produce more efficient results). Be extremely careful to avoid emulsion formation during the extraction process. Emulsions may take several hours to break up once they have formed.
Washing should be done several times with fresh water each time, until the wash water removed is clear (not cloudy or soapy).
Once you are finished with the washings, pour the (upper) layer containing the biodiesel esters out the top of the separatory funnel, into a labeled flask.
The ester product should be dried using sodium sulfate, and any remaining methanol may be removed by distillation (or rotary evaporation). The product may be verified using IR. There should be no evidence of free acid or alcohol, but should show the typical carbonyl peak for the ester.
I figured I would pass this along (it’s from my o-chem professor, Dr. Angela Parker). It has some useful information (and some not so useful–who has a heating mantle at home?). It’s really an easy process and this is why you get the internet chemists making biodesiel pages. We actually couldn’t find any “good” instructions on the internet. It’s like they know how to do it but they don’t know why. We were reading the one Volkswagen forum and the guy somehow managed to get carbons to just fly away somewhere. It was interesting.
