Chemical Properties of Fats

Oxidation and thermal effects

Unlike carbohydrates, fats are readily flammable substances, which is why they historically served as materials for lighting and heating. Subjecting them to high temperatures below the flash point causes thermal decomposition (destruction). The molecule breaks down, forming volatile, acrid fragments in the form of bluish vapors—this is precisely the phenomenon we observe when oil burns on a skillet.

The most critical process is fat oxidation (rancidity). The mechanism involves unsaturated acid radicals being oxidized by atmospheric oxygen.

• Reaction products: short-chain aldehydes and ketones are formed.
• Biological significance: dietary lipids acquire a characteristic unpleasant odor and rancid taste.
• Industrial application: the ability of vegetable oils to oxidize and polymerize (when chemical bonds form between radicals) is utilized in the production of boiled linseed oil (drying oil) and oil paints. The oil within them forms a durable film structure, i.e., it "dries". To accelerate this reaction, siccative catalysts are added (e.g., cobalt, lead, or manganese salts).

Hydrolysis reactions

Since fats are chemically esters, they are capable of hydrolyzing with the cleavage of ester bonds. When interacting with pure water, the process proceeds extremely slowly due to the complete insolubility (hydrophobicity) of the molecules.

Of utmost importance is alkaline hydrolysis, also known as saponification:

1. The reaction proceeds rapidly upon heating fat with alkali solutions (e.g., $NaOH$).
2. Reaction products: free glycerol ($C_3H_8O_3$) and sodium or potassium salts of higher fatty acids are formed, which constitute regular soap.

Hydrolysis conditions vary greatly depending on the medium:

• In industry, molecules are cleaved using superheated water (temperatures above $200^\circ C$ and pressures up to 25 atmospheres).
• In living organisms, dietary fats are hydrolyzed under mild conditions owing to enzymes (lipases) and the essential emulsifying action of bile.

Addition reactions

These transformations proceed with the cleavage of $C=C$ double bonds and are characteristic only of fats containing unsaturated fatty acid residues.

A) Hydrogenation The essence of the process lies in the addition of hydrogen, which allows liquid vegetable oils (sunflower, cottonseed) to be converted into solid products.

• Reaction conditions: the process requires a temperature of about $200^\circ C$, elevated pressure (2 atm), and the presence of a metal catalyst, most commonly nickel ($Ni$).
• Features: hydrogenation is typically reversible and non-exhaustive. For instance, a linoleic acid radical may not be fully reduced, but rather reduced only to an oleic acid radical.
• Significance: the technology forms the basis of margarine production. However, the reaction inevitably produces trans-configurations of acids that are absent in natural products, which significantly reduces their nutritional value and safety.

B) Halogenation (iodination) Iodine molecules are capable of quantitatively adding across double bonds in radicals. This property is utilized in analytical chemistry: by determining the mass of bound iodine, the degree of unsaturation of the molecule is calculated.

Chemical structure analysis

Understanding these properties allows the structure of unknown fats to be determined through calculation problems:

• Combustion product analysis: knowing the amount of released carbon dioxide and water makes it possible to find the gross formula (e.g., $C_{57}H_{108}O_6$). Comparing it with the formula of a fully saturated fat $C_n H_{2n-4}O_6$ (where the deduction of four hydrogen atoms is due to three $C=O$ groups), we observe a deficit of two more $H$ atoms. This proves the presence of exactly one double bond in the radical.
• Calculation using the iodine value: the ratio of the number of moles of reacted iodine to the number of moles of fat directly indicates the total number of $C=C$ bonds. For example, a 5:1 ratio means that five double bonds are hidden in the triglyceride structure (which may correspond to one oleic and two linoleic acid residues).

Structural isomers are determined based on the identified acid residues. Depending on the position of the unique radical (at position 1, 2, or 3 of the glycerol backbone), the molecule may be symmetrical or asymmetrical.

Class and exam questions

What else to study on this topic

• Structural isomerism of triglycerides: symmetrical and asymmetrical isomers.
• The mechanism of trans fat formation during non-exhaustive hydrogenation.
• The biochemical role of bile in lipid emulsification and hydrolysis.
• Algorithms for determining the gross formula of an ester based on its complete combustion products.

Other topics in "Chapter 18. Major Classes of Bioorganic Compounds"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.