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Experiment 11: Saponification and

Transesterification: Making Soap and Biodiesel

Animal and vegetable fats are composed of triglycerides, esters of glycerol (1,2,3

trihydroxypropane) and “fatty acids” carboxylic acids with long, hydrophobic alkyl chains. In

this experiment we will be reacting fatty acids in two ways. The first, called “saponification” is

hydrolysis of the esters with hydroxide to form salts of the fatty acids or “soaps”. The second is

a transesterification reaction, conversion from the glycerol ester to another ester, in this case

you’ll be making the methyl ester. Esters of fatty acids are used as “biodiesel” an alternative

fuel.

O

O

O

O

O

O

n

n

n

NaOR

R=H or CH

3

OH

OH

OH

RO

O

n

Note, when R = H there is

a deprotonation step that results

in the sodium salt of the fatty acid.

Background: Soap

Soap works by helping greasy dirt particles

dissolve in water. The soap molecules have a hydro-

philic “head group” the carboxylate anion, which

interacts well with water and a long, greasy hydropho-

bic “tail” which interacts well with non-polar grease.

When soap molecules are dissolved in water they

organize themselves into clusteres called micelles with

all of the hydrophobic tails pointed inward and the

hydrophillic head groups pointed out into the sur-

rounding water.

When soap molecules help dissolve grease, the

hydrophobic tails become associated with the grease

and the soap molecules surrround it, forming a micelle

around the grease and bringing it into the water.

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“Soap scum” forms when soap becomes less soluble in water. This happens when divalent

counterions like Ca

2+

and Mg

2+

from the water become associated with two soap molecules.

The soap loses its ability to form micelles because the polar head group is now at the center of a

molecule with two hydrophobic tails. This pair of soap molecules is “greasy” by itself and

coats the tile in your tub (and you!) instead of washing away. “Soft” water has relatively low

concentrations of these divalent cations. Water softeners work by exchanging sodium cations

for calcium and magnesium ions.

Soap Scum

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Phase Transfer Catalysis

Since the fat or oil starting materials are not soluble in water, the saponification reaction

proceeds very slowly unless some pathway exists to bring the fatty ester and base into the

same phase. One method is to use alcohol as a solvent to solubilize the fat and base together

and then the alcohol is removed at the end to isolate the soap, but, as you can see from the

biodiesel experiment, this often results in ester products, rather than carboxylic acids. Another

approach is to use a tetraalkylammonium salt as a phase transfer catalyst to bring the base into

the oil phase thereby speeding up the hydrolysis.

In this experiment, partially hydrogenated vegetable shortening will be saponified using

aqueous sodium hydroxide with tetrabutylammonium bromide as a phase transfer catalyst.

Background: Biodiesel

Biodiesel is an alternative fuel produced from the transesterification (conversion from

one ester to another) of triglycerides such as vegetable oils and animal fats. Used cooking oils

may be easily recycled and used to make biodiesel. Since regular diesel engines like those in

tractor trailer trucks and heavy construction equipment can use biodiesel in their existing

engines, it is one of the simplest alternative fuels to use. The most commonly used fuel that

contains biodiesel is a mixture of 20% biodiesel and 80% regular petroleum diesel called B20.

Fuel that is 100% biodiesel is called B100. The US Navy is currently the world’s largest

consumer of biodiesel, using B20 for all non-tactical vehicles. Many other government

organizations are converting their vehicles to either biodiesel or ethanol based fuels.

Carbon dioxide (CO

2

) is a major greenhouse gas produced by combustion. Because

biodiesel is made from plants the amount of CO

2

given off when it is burned is the same as the

amount that the plant took in to make it. As you can see from the table below, many other

hazardous byproducts of the combustion of diesel fuel are reduced.

Average Biodiesel Emissions Compared to

Conventional Diesel according to EPA

Emission Type B100 B20

Total Unburned Hydrocarbons -67% -20%

Carbon Monoxide -48% -12%

Particulate Matter -47% -12%

NO

X

+10% +2% - -2%

Sulfates -100% -20% (estimated from B100)

The increased oxygen content of biodiesel (the presence of the esters) helps it to burn

more completely, reducing both unburned hydrocarbons and carbon monoxide emissions.

Sulfates aremajor components of acid rain and are nearly entirely reduced by using biodiesel.

One disadvantage to using biodiesel is the slight increase in emissions of nitrogen

oxides (NO

X

). Nitrogen oxides are a major component of smog and contribute to the

formation of ground-level ozone and acid rain. Another disadvantage to biodiesel is that it

increases in viscosity when it gets cold, making it unsuitable for use in cold weather.

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While biodiesel can be made from any triglyceride, even used cooking oils, the major

source of oil will be soybean oil. Soy beans are approximately 18% oil and their production is

currently in surplus. The Department of Energy estimates that by combining these sources

there is enough oil to produce 1.9 billion gallons of biodiesel, enough to replace 5% of the on-

road diesel used in the United States. According to the National Biodiesel Board, biodiesel

production has a positive energy balance, you get 3.24 times as much energy out of the

biodiesel produced as it takes to manufacture it. You will be asked to calculate whether it is

possible for the United States to produce enough soybeans to replace petroleum diesel with

biodiesel as part of your lab report.

Sources: TheNational Biodiesel Board www.biodiesel.org

The Department of Energy’s Alternative Fuels Data Center www.eere.energy.gov/afdc/

Soap Procedure:

Partially hydrogenated vegetable shortening (2.0 g) is placed in a 50 mL beaker along

with 2 mL of 25% aqueous sodium hydroxide and 0.020 g of tetrabutyammonium bromide. The

mixture is heated on steam bath with occasional stirring with a glass rod for 20 minutes, or

until the oil phase has disappeared and only soap solids and a clear aqueous phase remain. The

mixture is allowed to cool to room temperature then cooled in an ice bath while 5 mL of ice

cold water is slowly added. The suspension of soap solids is stirred while keeping it in the ice

bath, then suction filtered using a Buchner funnel. The soap solids are rinsed with a few mL of

ice cold water and air dried in the Buchner funnel.

Testing the Solubility and Foaming Properties of Soap in Soft and Hard Water.

A small amount of the soap is dispersed in about 20 mL of distilled water in a clean 50

mL beaker. The soap dispersion is transferred to a 60 mL separatory funnel and shaken

vigorously. The solubility and foaming properties of the resulting solution are observed and

recorded. The procedure is repeated using 20 mL of regular tap water. How do the solubility

and foaming properties of the soap in the hard water compare to the distilled water?

Biodiesel Procedure:

Dissolve 0.2g (two pellets) of NaOH in 10 mL of methanol in a beaker. Ensure the

NaOH is completely dissolved.

While the NaOH dissolves, begin slowly heating 50 mL of vegetable oil in a 125 mL

erlenmeyer. Place a thermometer in the oil and monitor its temperature. Transfer the stir bar

from the beaker of NaOCH

3

to the flask containing the oil with a stir bar retriever. With the oil

stirring quickly, slowly add the entire sodium methoxide solution to the oil. Place a

thermometer in the flask and gently heat the mixture to 45-50°C for 20 minutes. Do not let the

temperature go above 60°C. Transfer the reaction mixture to a 125 ml separatory funnel.

After about 15 minutes, separation can be observed.

Wait until there is good separation between the two products. Carefully drain off the

viscous bottom glycerol layer. Drain off the top biodiesel layer into a small erlenmeyer and

dry it with magnesium sulfate. Reheat the biodiesel to 60-70°C to decrease its viscosity and

suction filter it to remove the magnesium sulfate. Take a sample of the biodiesel (top layer)

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and record its pH range (alkaline, neutral or acidic) in your lab notebook. Obtain the mass and

volume of your dried biodiesel product, and compare it to the volume of vegetable oil you

started with. Also record the characteristics of your biodiesel (color, viscosity, odor) and

compare them to those of the original oil.

In your lab report:

Write a general mechanism for saponification and transesterification reactions

Discuss how soap works.

Discuss the results of the solubility and foaming experiment.

Explain the role of the phase transfer catalyst in the saponification experiment.

Is biodiesel really geen? Explain at least one argument in support of the idea that biodiesel is a

“greener” fuel. Also present one argument that biodiesel is not a greener fuel.

Using the data below, could the United states expect to grow enough soybeans to replace

petroleum diesel with biodiesel? Show your calculations.

Quantities in the United States

41 pounds of soy beans are needed to make 1 gallon of soydiesel

1 ton of soybeans makes 47.33 gallons of oil

1 bushel of soybeans is 60 pounds

By weight, soy beans are about 20% oil

60 pounds of soybeans yield 1.42 gallons of oil

38.1 average bushels of soybeans are produced per acre (year 2000)

54.4 gallons of soy bean oil are produced per acre

Agriculture in the United States

2.2 million farms and nearly 990 million acres (47.3% crops, 52.6% livestock/other)

74 million acres are in surplus

Energy in the United States

Soy diesel yield 117,093 BTUs per gallon; Gasoline yields 114,264 BTU per gallon

Soydiesel requires 23,620 BTUs per gallon to make

Gasoline requres about an equal amount of energy (BTUs) to make

Volume used in the United States:

30 billion gallons of diesel used per year

18 million barrels of petroleum used per day (about 6.6 billion barrels per year)

42 gallos of petroleum per barrel

277 billion gallons of petroleum per year