Themes

Transesterification: mechanism, in fatty acids, in microalgae, uses

Because it is an equilibrium reaction, it must move in the desired direction using a considerable excess of the alcohol that is desired to be present in the final structure or eliminating the other.

Transesterification. CC0

The fact that the equilibrium of the reaction is reversible, with equivalent mechanisms for direct and inverse reactions, implies that transesterification is shown as the most appropriate example to start the study of nucleophilic substitution mechanisms in the acyl group when it is catalyzed. by an acid or by a base.

Transesterification mechanism

Transesterification reactions are generally acid or base catalyzed.

In bases

When a base is used as a catalyst, the reaction mechanism involves a simple nucleophilic substitution on the acyl group that is divided into two steps: the addition of the nucleophile and the elimination of the leaving group.

In acids

In the case of acid-catalyzed transesterification, the reaction is also divided into the two steps named above, but a proton transfer is required before and after each of these.

Acid catalyzed transesterification. MiPe / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Since the above mechanism can generate the formation of carboxylic acids by reacting the carbocation (product of the protonation of the carbonyl group) with water present in the reaction medium, acid-catalyzed transesterification reactions must be carried out in the absence of water.

In some cases, the transesterification can be carried out in a non-catalytic medium. However, this is only possible under supercritical conditions (higher temperature and higher pressure), which is not economically feasible.

Fatty acid transesterification

Vegetable oils, from seeds and fruits of plants , are mainly made up of triglycerides (glycerol triesters) that include three long-chain fatty acids responsible for the high molecular mass and low volatility of such oils.

Alcohols and catalysts

In addition to methanol, other short-chain alcohols such as ethanol, propanol, butanol, and pentanol are often used in the transesterification of fatty acids. However, methanol and ethanol are preferred:

  • Methanol due to its particular physicochemical properties.
  • Ethanol because it is less toxic and can be produced from renewable sources.

The catalysts used in the reaction can be acidic or basic. However, it is common for the process to be carried out in an alkaline environment, because the reaction efficiency is higher and, in addition, less damage is generated in the equipment used as a result of corrosion.

Biodiesel

Because the molecular masses of the methyl esters obtained are approximately one third of the molecular mass of the original triglyceride, their volatility is higher and they can be used in diesel engines.

Therefore, the mixture of methyl esters obtained from fatty acids is often called biodiesel. However, its composition will depend on the structure of the carboxylic acid chains that make up each triglyceride.

Transesterification in microalgae

Triglycerides can also be obtained from microalgae. These species not only synthesize them naturally, but also store them as a carbon energy reserve within their cells when they can no longer divide.

Advantage

With respect to vegetable oils, microalgae offer several advantages when it comes to their use as a raw material to obtain biodiesel. In addition to producing a greater amount of biomass due to the fact that they exhibit high growth rates, they do not represent a competition for the cultivation areas destined for food production.

Microalgae can grow in conditions as harsh as those found in non-arable land or fresh and salt water bodies such as ponds, lakes, seas and oceans.

Microalgae culture on a laboratory scale. CSIRO / Creative Commons Attribution 3.0 Unported.

Acid-catalyzed transesterification

Because microalgae oil contains more free fatty acids than vegetable oils, which produce adverse side reactions in a basic medium, an esterification process using acid catalysts should be carried out before the usual transesterification.

Given this, solid catalysts have great potential in the transesterification of microalgae. In addition to the fact that they can be used several times and easily separated from the reaction medium, pure glycerol is obtained without by-products, since the process is not affected by the moisture present in the raw materials and free fatty acids.

Applications of  transesterification

Transesterification is used mainly in the production of biodiesel; not only economic benefits are obtained, but reducing emissions of gases greenhouse . However, the great versatility of transesterification has allowed its use both in the laboratory and in industry.

Ester synthesis

When it comes to the synthesis of esters, transesterification usually has more advantages than the use of carboxylic acids and alcohols in certain particular cases.

Esters are soluble in most organic solvents, while some carboxylic acids have very poor solubility, making it difficult to carry out a homogeneous esterification. Then the ester-ester transformation from commercially available methyl and ethyl esters is particularly useful.

Polymerization

Transesterification is also useful in opening the lactone ring which results in the formation of a polyester.

Although today the production of polyethylene terephthalate (PET) is based almost entirely on the direct esterification of terephthalic acid, the transesterification reaction between dimethyl terephthalate and ethylene glycol used to be used in the production of this polyester.

Repeating unit of polyethylene terephthalate. ljfa-ag Diskussion / Public domain

On the other hand, transesterification is essential in the paint industry, since the curing (polyesterification) of alkyd resins makes it possible to improve the properties of paints in which vegetable oils are used as raw material.

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