Esters and Esterification – Their Significance in Industrial Processes

Esters such as ethyl ethanoate are compounds that are formed when an alkanol reacts with carboxylic acids in a process known as esterification. Water is also produced as a bi-product. This process is very slow are reversible at room temperature. It is catalyzed by a high concentration of hydrogen ions. Methyl ethanoate and ethyl ethanoate are two examples of esters.

Esters are the main constituents of most naturally occurring fats and oil. Many of these esters have pleasant smells and are largely responsible for the nice fragrances of flavors of fruits and flowers. For instance, pentyl ethanoate has the odor of bananas and is usually called banana oil, while octyl ethanoate has the odour of an orange.

Ethyl ethanoate, one of the simplest esters and most common, is prepared by the esterification between glacial ethanoic acid and ethanol at 150*C in the presence of concentrated tetraoxosulpahte(VI) acid. Generally, ethyl ethanoate and other esters are used as solvents in industries and laboratories. These solvents are used for quick-drying substances such as paints, varnishes, adhesives and lacquers. Solvents are also used for cellulose nitrate. Thinner water, a solvent, is a mixture of esters which includes pentyl ethanoate.

Physical Properties of Ethyl Ethanoate(Ester)

 (i) Ethyl ethanoate is a colorless volatile liquid with a very pleasant smell

(ii) Ethyl ethanoate has a boiling point of 75*C

(iii) Ethyl ethanoate is only slightly soluble in water. However, it dissolves readily in organic solvents like ethanol, benzene and ethoxyethane

 

Esters
 

Chemical Properties of Ethyl Ethanoate(Ester)

 (i) Ethyl ethanoate is hydrolyzed by water to give its component alkanol and acid. It is catalyzed by an alkali or a dilute acid.

(ii) Ethyl ethanoate reacts with ammonia to form ethanol and ethanamide

(iii) Ethyl ethanoate is usually reduced to ethanol by hydrogen from very strong reducing agents such as lithium tetrahydridoaluminate(III).

Formation of Esters and Esterification

Esters are formed by acid-catalysed reaction of carboxylic acids and alcohols. They usually have fruit-like odours and are used in perfumes and artificial flavourings. They can also react in a polymerisation reaction to make polyester plastics.

Esters are made by warming a mixture of carboxylic acid and alcohol in the presence of an acid catalyst such as concentrated sulfuric acid. The reaction is known as Fischer esterification.

Role Of Carboxylic Acid in Ester Formation

Carboxylic acid is a group of organic molecules in which a carbon atom is bonded to an oxygen atom through a double bond and to a hydroxyl (–OH) group through a single bond. A fourth bond links the carboxyl group to a hydrogen (H) atom or to another univalent combining group such as an alkyl or aryl group. These two functional groups allow the compound to participate in a wide range of reactions.

One of the most common reactions is the synthesis of esters by reaction between a carboxylic acid and an alcohol under the action of a strong acid catalyst. The result is a mixture of two substances in which the ester is the major product and the alcohol is the minor product. This is an equilibrium reaction and it follows Le Chatelier’s Principle. It is possible to force the position of equilibrium in one direction or the other by using a larger excess of the cheaper reactant.

Other reactions of carboxylic acids include nucleophilic substitution reactions in which a partially positively charged carboxylate ion substitutes for an electron pair on an alcohol. This produces a series of acid derivatives known as acyl chlorides and acyl anhydrides. It is also possible to carry out addition reactions across the C=O bond in which the carboxylic acid acts as an electrophile and the alcohol acts as a nucleophile.

Esterification is a particularly important reaction of carboxylic acids because it provides a method for the preparation of organic compounds that have high boiling points. The higher boiling points of esters than of their parent acids are the result of Van der Waals dispersion forces and hydrogen bonds.

An example of this is the reaction between ethanoic acid and methanol in the presence of sodium hydroxide to produce the ester hexyl propanoate. This type of esterification reaction can also be carried out in the presence of a variety of other weak acids. In order to do this the reagent is usually treated with an acid catalyst such as trimethylphosphine. In this case the reaction is called a Fischer esterification.

Reactions Of Alcohol to Form Esters

Carboxylic acid and alcohol are both organic molecules and when mixed form an ester. The reaction is a condensation reaction and is exothermic, more energy is released when the bonds are formed than the energy it takes to break them. This is why an ester is a solid at room temperature and a liquid when dissolved in water. An ester can be formed from carboxylic acids that contain oxygen, such as acetic acid (CH3COOH), or non-oxygenic acids, such as formic acid, HCOOH. The latter are more rarely used in the laboratory as they have a toxic effect on the organisms that react with them.

An easy way to prepare esters is to heat the carboxylic acid with the alcohol in a test tube containing concentrated sulfuric acid. This will cause a gas to be produced which can be smelled and also causes the acid to decompose to water and carbon dioxide.

This is called Fischer esterification. It is a good example of how the concentration, pressure or volume of the reactants can influence the equilibrium of the reaction. By putting a large excess of the alcohol into the mixture, it is possible to drive the reaction towards completion with a high yield of the desired product.

The rate of the reaction depends on the chemical structure of the alcohol and the acid as well as the type of acid catalyst used. The simple alcohols methanol and ethanol, for example, react very quickly because they do not have carbon atom side chains that would hinder the reaction with the carboxylic acid. In contrast, formic acid (HCOOH) and methyl formate react very slowly because of the long distances between the carbon atoms in these molecules.

This reaction is important in both organic synthesis and the chemical industry because it is a route to valuable chemicals such as solvents, flavouring agents and surface active compounds. The reactions of fatty acids with carboxylic acids to produce esters are also important in the production of some pharmaceuticals. This is because these compounds are generally considered to be more ‘biologically stable’ than other synthetically produced drugs, which often have a number of harmful side effects.

Reaction Catalyst

A catalyst is a substance that speeds up a reaction by acting as both reactant and product. It is not consumed during the course of the reaction, but rather remains in a different phase than the reactants (for example, solid) while promoting the formation of the reaction products. The most common liquid acid catalysts used in esterification reactions are sulfuric and nitric acids. In this case, the catalyst serves as a dehydrating agent, pushing water away from the reaction, thus facilitating the formation of the ester. Liquid acid catalysts are typically accompanied by the use of alkyl halides, which can be problematic in terms of environmental hazard as they are potential greenhouse gases, ozone depletors, carcinogens and ecological poisons.

An alternative to these corrosive liquid acids are heterogeneous acid catalysts. These catalysts are typically solid in nature, and they are used to eliminate corrosion problems as well as to reduce the amount of work needed to separate the reaction products. Heterogeneous acid catalysts can also be recycled and reused, which significantly reduces production costs and the amount of waste produced.

The use of heterogeneous acid catalysts in esterification processes is becoming increasingly popular. This is because they can replace the use of liquid acid and also reduce environmental hazards. The main advantage of heterogeneous acid catalysts is that they are not toxic and do not require the use of alkyl halides. Additionally, they are much cheaper than liquid acid catalysts and can be recycled for reuse indefinitely.

One example of a heterogeneous acid catalyst that is widely used for esterification is calcined Zn-Mg-Al zirconia, which has been found to be highly active in the esterification of lauric acid with diethylene glycol (1:60) under optimized conditions. It is also reusable for six cycles, which is significantly higher than the recyclability of Amberlyst-15 that loses its activity after the third cycle.

Saturated fatty esters are commonly used as solvents and softening agents in textile, cosmetic, biodiesel and polymer industries. These esters are formed by heating fatty acids with alcohols. These esters are also known as simple esters because they have a relatively low number of carbon chains and are more volatile than other types of ester molecules. The scent of saturated fatty esters is influenced by the type of functional group attached to the carboxylic acid. For example, propyl acetate has the scent of pears, while butyl acetate smells like apples.

Solubility Of Esters

The solubility of a substance depends on its chemical structure as well as the type and concentration of solvent. Usually, substances that dissolve easily are called solutes and those that do not dissolve are called solvents. Solubility is important because it determines how easy it is to transport the solute, or how readily it can be absorbed into another substance.

The esterification reaction is one of the most important reactions in organic synthesis and in the chemical industry. It is widely used in the production of fragrances, plastics, biodiesel and solvents.

Esters are formed by a condensation reaction between an alcohol and a carboxylic acid in the presence of a catalyst. It is a reversible reaction and is an important building block in the manufacture of many other chemicals such as polymers, dyes, perfumes and pesticides.

There are a number of different ways that esters can be produced. The most common is the Fischer-esterification reaction between an alkanol and a carboxylic acid, which requires the use of a strong acid catalyst. Other methods for producing esters include the reaction of acid chlorides, anhydrides or nitriles with alcohols; and the trans-esterification of two esters.

The solubility of an ester is determined by the nature of its solvent and its chemical structure. In general, polar solvents dissolve polar solutes and non-polar solvents dissolve non-polar solutes. In addition, the size of the carbon-oxygen bond in the carboxylic acid also has an effect on its solubility.

A large, branched carboxylic acid will have a greater degree of solubility than a small, straight-chain carboxylic acid. The same principle applies to the solubility of ester products; long-chain esters are more soluble than short-chain esters.

Temperature also affects the solubility of an ester. This is because the process of dissolving an ester is endothermic, or heat-added. An increase in temperature will stress the equilibrium and cause it to shift to the right, thereby increasing the solubility of the product.

To test the solubility of an ester, a sample of the product is added to a known solvent such as water or ethanol and stirred vigorously. If the sample is clear, it is soluble; otherwise it is not. This is a simple test that can be performed on a laboratory scale.