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Hydrolysis, Hydrogenation, Saponification and Rancidity of oils, Drying oils

In chemistry and biology, hydrolysis is the process of decomposing a substance by using water as one of the reactants.


In chemistry and biology, hydrolysis is the process of decomposing a substance by using water as one of the reactants. The reversible chemical equation may be represented by the equation AB + HOH ⇌ AH + BOH if A and B in the compound represent atoms or groups, and water represents water by the formula HOH. Besides water, hydrolysis reactants and products may consist of neutral or ionic molecules, depending on the organic compounds involved.

Water reacting with an ester of a carboxylic acid can provide an example of how hydrolysis between organic compounds takes place; all such esters have the general formula RCO - OR′, where R and R′ represent combining groups (representing for instance, the methyl group, CH3, in which case the ester is methyl acetate). The slowest step of the hydrolysis is when an oxygen atom from the water molecule forms a covalent bond with a carbon atom from the esters. A hydrogen ion attaches to the nascent alcohol molecule quickly after the carbon-oxygen bond breaks in the ester. Here is the equation representing the reaction from beginning to end:

RCO – OR' + H2O ----> RCO + OH + R’ + OH

Carboxylic acid molecules are denoted by RCO - OH, and alcohol molecules by R′ - OH, while dashes represent breaks or formations of covalent bonds.


  • Hydrogenation reactions are made up of three basic ingredients: hydrogen, a substrate, and catalysts. The catalysts aid the reaction when temperatures and pressures are low.
  • Hydrogenation can be achieved using heterogeneous or homogeneous catalysts.
  • In addition to converting alkenes to alkanes, hydrogenation reactions can reduce substrates in a range of ways.
  • The health consequences of incomplete hydrogenation reactions have been linked to circulatory diseases.
The process of hydrogenation refers to the addition of hydrogen atoms (usually unsaturated compounds) to compounds. For these reactions to take place under normal temperature and pressure conditions, a catalyst is usually required. Although gaseous hydrogen is commonly used as the hydrogen source for hydrogenation reactions, there are other options. As an alternative to hydrogenation, dehydrogenation involves removing hydrogen from a compound. The products of hydrogenation have the same charge as their reactants, unlike those of protonation or hydride addition.

Three components are necessary for hydrogenation reactions: the hydrogen source, the substrate, and the catalyst. Various catalysts and substrates will result in different reaction temperatures and pressures. A hydrogenation reaction occurs to make an alkane. Compounds add hydrogen in a sympathetic manner, so it enters from the least hindered end and adds from the same end. The hydrogenation reactions of alkenes, alkynes, and aldehydes result in a change of alkenes, alkanes, aldehydes, and ketones to alcohols, amides to amines, and esters to secondary alcohols.

Catalyst of hydrogenation

The hydrogenation of organic compounds does not occur without metal catalysts when the temperature is below 480 degrees Celsius. Catalysts bind the hydrogen molecule and allow the hydrogen to react with the substrate. The catalyst facilitates the reaction between hydrogen and substrate by binding the hydrogen molecule. There are a number of known active catalysts which can function at lower temperatures and pressures, including palladium, rhodium, and ruthenium. There is ongoing research to procure catalysts that do not contain precious metals, and can function at temperatures and pressures lower than precious metal catalysts. Raney nickel, for example, has been developed successfully, but it requires high temperatures and pressures.

There are two types of catalysts: homogeneous catalysts and heterogeneous catalysts. Solvents containing unsaturated substrates can dissolve homogeneous catalystsIndustry-wide heterogeneous catalysts are soluble in solvents that contain substrates, but not in solvents that contain catalysts. The support for heterogeneous catalysts is usually carbon or oxide, and metal catalysts are often attached to metal surfaces. These materials can be affected by the choice of their supports, which can affect their activity. Most hydrogen used in commercial products comes from hydrogen gas, which is commercially available.

It is estimated that 25 kcal/mol of energy are released during the hydrogenation of vegetable oils and fatty acids. A heterogeneous catalyst is capable of achieving hydrogenation because of this mechanism. The first step in bonding to the catalyst is the unsaturated bond, followed by the hydrogen dissociation into atomic hydrogen on the catalyst. As a result, the hydrogenation process becomes irreversible when the additional atom is added, causing the hydrogen to attach irreversibly to the substrate. In homogeneous catalysis, hydrogen is bound to the metal through oxidative addition to produce dihydride complexes. Through migratory insertion, the substrate is bound to the metal, and one hydrogen atom from the metal can then be transferred to the substrate. With the transfer of the second hydrogen from the metal, the newly formed alkane is dissociated simultaneously through reductive elimination from the substrate.


The use of soap plays a key role in maintaining good health and hygiene. Objects and skin surfaces must be cleaned with soap to remove dirt and oil. The soaps we use for bathing, cleaning, washing, and other duties around the house are widely used. As a necessity in everyday life, soap has become an essential household item. Where does soap come from? Saponification is how soap is made. The process of soap making or saponification is explained in a clear and concise manner here. Making soap is called soapification. The salts in soaps are simply long-chain fatty acids in a potassium or sodium form. Saponification occurs when an ester reacts with an inorganic base to form soap and alcohol. Triglycerides are typically converted to soap when reacting with potassium hydroxide or sodium hydroxide (lye).

Rancidity of oils

Chemically, rancidity, or rancidification, is caused by the aerial oxidation of unstructured fats, which results in the unpleasant taste or odor of foods and other products. Ultimately, unsaturated components of fats can transform into hydroperoxides, which break down into alcohols, volatile aldehydes, esters, ketone, and hydrocarbons, some of which are odourless. Sometimes oil becomes rancid (rancid oil) from the breakdown of fats in it, or milk may become rancid from not heating it in a humid environment, etc. Rancidity appears when oil decomposes (rancid oil), or milk decomposes due to unheated humid conditions.

Types of rancidity

  • Microbial rancidity
  • Oxidative rancidity
  • Hydrolytic rancidity

Drying oils

When exposed to air, this relatively unsaturated oil oxidizes and polymerizes, resulting in a dry, hard film. Based on the ease of oxidation and polymerization, vegetable and fish oils are classified as non-drying, semi-drying, or drying. Paints, inks, and varnishes use drying oils because of their film-forming properties. As a result of their diallylic composition (two double bonds separated by methylene groups, for example, CHCHCH2CHCH) or conjugated composition (two carbon-carbon double bonds separated by a single bond), oils can be reactive with oxygen. A correlation exists between the number of diallylic groups per molecule and the drying nature of the oil. Typically, dry oils have a fn of at least 2.2; semidrying to nondrying oils have a fn of less than 2.2.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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