Question

For the three fatty acids given below, (a) classify each as saturated or mono-, di-, or polyunsaturated. (b) write a balanced equation for formation of a triglyceride that incorporates all three. (i) arachidonic acid $$ \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{4}-\left(\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\right)_{4}-\left(\mathrm{CH}_{2}\right)_{2}-\mathrm{COOH} $$ (ii) nervonic acid CCCCC(CC(C)C)CC(OC)C(=O)O (iii) myristic acid CCC(C)C(=O)O

Short answer

Arachidonic acid: polyunsaturated; Nervonic acid: monounsaturated; Myristic acid: saturated. Triglyceride forms from them with glycerol, releasing water.

Step-by-step solution

Analyze Arachidonic Acid

The structure of arachidonic acid is \( \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{4}-\left(\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\right)_{4}-\left(\mathrm{CH}_{2}\right)_{2}-\mathrm{COOH} \). It contains four double bonds, indicating it is a polyunsaturated fatty acid. The double bonds suggest regions of unsaturation, making it polyunsaturated.

Analyze Nervonic Acid

Nervonic acid is represented by the SMILES string, but for simplification, let's assume it's a 24-carbon chain with one double bond at the 15th position. This indicates it is a monounsaturated fatty acid because it contains a single double bond.

Analyze Myristic Acid

Myristic acid is represented by \( \mathrm{CCC(C)C(=O)O} \). This corresponds to a 14-carbon chain with no double bonds, indicating it is a saturated fatty acid. Saturated fatty acids have no double bonds.

Classify Each Fatty Acid

Based on the analysis, classify each fatty acid: Arachidonic acid is polyunsaturated, nervonic acid is monounsaturated, and myristic acid is saturated.

Write General Triglyceride Formation Equation

To form a triglyceride from three fatty acids, each acid reacts with one glycerol molecule. The general reaction is: \[ \mathrm{Glycerol} + 3\, \mathrm{RCOOH} \rightarrow \mathrm{Triglyceride} + 3 \mathrm{H_2O} \] where \( \mathrm{RCOOH} \) represents a fatty acid.

Balance Specific Triglyceride Formation Equation

Combine the specific fatty acids with glycerol: \[ \mathrm{Glycerol} + \mathrm{Arachidonic\ Acid} + \mathrm{Nervonic\ Acid} + \mathrm{Myristic\ Acid} \rightarrow \mathrm{Triglyceride} + 3 \mathrm{H_2O} \] The exact formula will depend on the specific molecular structures, but the stoichiometry involves one molecule of glycerol and three fatty acids.

Key concepts

Fatty Acid Classification

Fatty acids can be categorized based on the presence and number of double bonds in their hydrocarbon chains. Understanding these categories is crucial because they influence the physical properties and biological roles of lipids.

• Saturated Fatty Acids: These acids have no double bonds between the carbon atoms in their chain. Myristic acid, for example, falls under this category. Saturated fats tend to be solid at room temperature and are found in animal fats and some tropical oils.


• Monounsaturated Fatty Acids: These feature a single double bond in their chain. Nervonic acid is a prime example. Monounsaturated fats are typically liquid at room temperature but become solid when chilled. They are common in olive oil and avocados.


• Polyunsaturated Fatty Acids: These fatty acids contain more than one double bond. Arachidonic acid is an example, with multiple double bonds providing flexibility and fluidity to cell membranes. They're found in fish oils and some plant-based oils.

Understanding whether a fatty acid is saturated or unsaturated guides its utilization in biological systems and dietary considerations.

Saturated vs Unsaturated Fatty Acids

The difference between saturated and unsaturated fatty acids lies in the number of hydrogen atoms attached to the carbon chain and the presence of double bonds.

• Saturated Fatty Acids: Every carbon atom is saturated with hydrogen atoms; hence no double bonds exist. This full saturation causes them to have straight chains, allowing them to pack tightly together, leading to higher melting points.


• Unsaturated Fatty Acids: These have one or more double bonds, creating a kink in the chain. This kinked structure prevents tight packing, resulting in lower melting points. Unsaturated fatty acids can be further classified into monounsaturated (one double bond) and polyunsaturated (multiple double bonds) as seen in nervonic and arachidonic acids respectively.

The saturation level affects the fatty acid’s stability, melting point, and health effects.

Chemical Equation Balancing

Balancing chemical equations is an essential skill in chemistry that shows how reactants are transformed into products. In the context of triglyceride formation, we start with a glycerol molecule and three fatty acids.

• Three fatty acids, each with different saturation levels, react with one glycerol molecule to form a triglyceride and water.


• General reaction format: \( \text{C}_{3}\text{H}_{8}\text{O}_{3} + 3 \text{RCOOH} \rightarrow \text{Triglyceride} + 3 \text{H}_{2}\text{O} \) where \( \text{RCOOH} \) represents any fatty acid.


• Each esterification process releases a water molecule, leading to a total of three water molecules for complete triglyceride synthesis involving three fatty acids.

Ensuring that atoms of each element are conserved across reactants and products is key. This understanding allows for precise molecular synthesis and understanding of biochemical pathways.