[Chemistry Class Notes] on Transesterification Pdf for Exam

Transesterification is an organic reaction in which an alcohol’s R group is substituted for an ester’s R’ group. In most cases, this is achieved by applying an acid or base catalyst to the reaction mixture. It can also be achieved with the aid of enzyme catalysts (such as lipases). The exchange of an R’ group from alcohol with an R’’ group from an ester in a transesterification reaction is illustrated in the diagram below.

This article will study the transesterification meaning and transesterification process in detail.

When this reaction is catalyzed by an acid catalyst, the carbonyl group is converted by the donation of a proton to it. Base catalysts, on the other hand, take a proton away from the alcohol group, creating a strongly nucleophilic alkoxide ion.

It should be noted that transesterification can be used to produce esters with relatively large alkoxy groups from methyl and ethyl esters. This is normally achieved by heating the ester (methyl or ethyl) with the acid/base catalyst and large alkoxy alcohol, then evaporating the smaller alcohol to push the equilibrium reaction in the desired direction.

Transesterification Mechanism

Here is given transesterification process step by step:

In Basic Medium

Step 1

The basic medium deprotonates the alcohol, which results in the formation of an alkoxide ion. This alkoxide strikes the carbonyl carbon of the ester with a nucleophilic attack, resulting in the formation of an intermediate. As seen in the diagram below, the double bond between the carbonyl carbon and the oxygen is broken, and the negative charge is transferred to the carbonyl oxygen.

Step 2

The initial ester reactant’s R’ group serves as a leaving group, and it is removed from the intermediate. The bond pair of electrons is maintained by the oxygen, resulting in the creation of a new alkoxide. Finally, as shown below, the double bond between the carbonyl carbon and the negatively charged oxygen is reformed.

In Acidic Medium

Step 1

The acidic medium first protonated the carbonyl oxygen. The oxygen becomes more electron-withdrawing as a result of the positive charge, activating the carbonyl carbon against a nucleophilic attack.

Step 2

The nucleophilic nature of the alcohol is due to the presence of two lone pairs on the oxygen. This oxygen binds to the carbonyl carbon via a nucleophilic attack. An intermediate is formed as a result of this.

Step 3

This intermediate undergoes an intramolecular proton transfer, in which the positive charge is transferred from the oxygen of the alcohol to the oxygen of the ester, as shown below.

Step 4

The carbon-oxygen bond is broken because the protonated oxygen acts as a leaving group. The oxygen atom maintains the bond pair, and the positive charge is relayed to the carbonyl oxygen via the carbonyl carbon (the carbon-oxygen double bond is reformed, as illustrated below).

Step 5

The carbon-oxygen bond is broken because the protonated oxygen acts as a leaving group. The oxygen atom maintains the bond pair, and the positive charge is relayed to the carbonyl oxygen via the carbonyl carbon (the carbon-oxygen double bond is reformed, as illustrated below).

Applications of Transesterification

1. Polyester Production

Polyester synthesis is the largest-scale application of transesterification. Diesters are transesterified with diols to form macromolecules in this application. Dimethyl terephthalate and ethylene glycol, for example, react to produce polyethylene terephthalate and methanol, which is then evaporated to speed up the reaction.

2. Methanolysis and Biodiesel Production

Transesterification also includes the reverse reaction, methanolysis. Polyesters have been recycled into individual monomers using this method (see plastic recycling). It’s also used to make biodiesel from fats (triglycerides). One of the first applications was for this conversion. Before World War II, heavy-duty vehicles in South Africa were powered by transesterified vegetable oil (biodiesel).

3. Fat Processing

In the food industry, fat interesterification is used to rearrange the fatty acids of triglycerides in edible fats and vegetable oils. For example, a solid fat with mostly saturated fatty acids could be transesterified with vegetable oil with a lot of unsaturated acids to make a spreadable semi-solid fat with a mix of both kinds of acids.

4. Synthesis

Enol derivatives are difficult to make using other methods, so transesterification is used to make them. Transesterification of vinyl acetate, which is readily available, produces vinyl ethers.

Did You Know?

1. Triglycerides are a type of lipid (fat) found in the bloodstream.

2. Your body transforms any calories it doesn’t need right away into triglycerides when you feed. Triglycerides are contained in the fat cells of your body. Hormones then release triglycerides to provide nutrition in between meals.

3. You may have high triglycerides if you eat more calories than you burn on a regular basis, especially from high-carbohydrate foods (hypertriglyceridemia).

4. High triglyceride levels may contribute to artery hardening or thickening (arteriosclerosis), which increases the risk of stroke, heart attack, and heart disease. Highly high triglycerides can also cause acute pancreas inflammation (pancreatitis).

5. Obesity and metabolic syndrome — a cluster of disorders that involves too much weight around the waist, high blood pressure, high triglycerides, high blood sugar, and abnormal cholesterol levels — are also associated with high triglycerides.