Question

Chemical Reactivity of Lipids Soaps are salts of fatty acids and can be made by mixing triacylglycerols with a strong base such as \(\mathrm{NaOH}\). This saponification reaction produces glycerol and fatty acid salts. In a lab experiment, students saponify the triacylglycerol tristearin in the presence of \({ }^{18}\) O-labeled water. What saponification reaction products will contain the \({ }^{18} \mathrm{O}\) label?

Short answer

The fatty acid salts (soap) will contain the \(^{18} \mathrm{O}\) label.

Step-by-step solution

Understand the Saponification Reaction

Saponification is a chemical reaction between a triglyceride and a strong base such as NaOH, resulting in the formation of glycerol and soap (fatty acid salts). In our case, the triglyceride is tristearin, a triacylglycerol, and the base is NaOH. The reaction can be generalized as: \[ \text{Triacylglycerol} + 3 \text{NaOH} \rightarrow \text{Glycerol} + 3 \text{Soap} (\text{Fatty acid salts}) \] with water molecules involved in hydrolyzing the ester bonds of the triglyceride.

Analyze the Role of Labeled Water

In the presence of \(^{18}\mathrm{O}\)-labeled water, H\(^{18}\mathrm{O}\), the water participates in the hydrolysis of the ester bonds of tristearin. Each ester bond in the triglyceride is broken with the aid of a water molecule, replacing the acyl-oxygen bond with a hydroxyl (-OH) group.

Determine the Products Involved

When saponification occurs, the ester linkages in tristearin are broken to form glycerol and three molecules of stearate ion (soap). The oxygen from the labeled water (\(^{18}\mathrm{O}\)) will be incorporated into the carboxylate group of each resulting fatty acid salt (stearate). This is because the hydrolysis involves adding the water's oxygen to the oxygen of the stearate ion.

Identify the Product Containing the \(^{18}\mathrm{O}\) Label

Since labeled water provides the oxygen used to hydrolyze the ester bonds, the \(^{18}\mathrm{O}\) label will be incorporated into the carboxylate groups of the soap molecules (stearate ions). The glycerol does not incorporate the \(^{18}\mathrm{O}\) label as its structure is already complete with the natural oxygen from the ester linkage.

Key concepts

Triglycerides

Triglycerides are a type of lipid that plays a crucial role in energy storage in the body. Chemically, they are esters derived from glycerol and three fatty acids. Structurally, the glycerol backbone is connected to each fatty acid through ester bonds. This arrangement allows triglycerides to serve as an efficient energy source since they are densely packed in fat cells.
In the context of saponification, triglycerides like tristearin undergo chemical reactions with strong bases to produce simple molecules such as glycerol and soap. This process is significant because it transforms a large energy-rich molecule into smaller, useful components. Understanding the structure and function of triglycerides is essential for comprehending how they interact in chemical reactions.

Ester Bond Hydrolysis

Ester bond hydrolysis is a pivotal reaction in the breakdown of triglycerides during saponification. When a triglyceride reacts with a strong base like NaOH, the ester bonds linking the glycerol to the fatty acids break down. This reaction is catalyzed by water, which results in the formation of glycerol and fatty acid salts.
Both water and the base are crucial here. Water molecules, often labeled for tracking the reaction like the use of \(^{18}O\) in the exercise, participate in the hydrolysis by cleaving the bond and creating a hydroxyl group (-OH) on the glycerol fragment. As each ester bond is hydrolyzed, the fatty acids are released in the form of salts (or "soap" molecules).

• Water time and again proves its role as a critical agent in breaking ester bonds.
• Tracking labeled elements provides insight into reaction pathways and site-specific changes.

This knowledge is central to understanding how complex lipid molecules can transform in chemical processes such as saponification.

Fatty Acid Salts

Fatty acid salts are the primary products of triglyceride hydrolysis during saponification. These salts are essentially soap formed from the fatty acids released from the triglyceride. When the ester bonds in a triglyceride like tristearin are broken, the fatty acids are ionized into their corresponding salts.
In the process highlighted, each ester bond hydrolysis reaction incorporates oxygen from water into the carboxylate groups of these fatty acids. Thus, when using \(^{18}O\)-labeled water, the oxygen atom becomes part of the fatty acid salt, indicating the origin of the hydrolyzing water.

• In saponification, fatty acid salts function as detergents by efficiently emulsifying oil and water.
• The presence of labeled isotopes can help clarify the mechanisms of chemical reactions.

Understanding fatty acid salts' formation and function further explains their role in everyday products and various industrial applications.