Question

Hydrolysis of Lipids Name the products of mild hydrolysis with dilute \(\mathrm{NaOH}\) of a. 1-stearoyl-2,3-dipalmitoylglycerol b. 1-palmitoyl-2-oleoylphosphatidylcholine.

Short answer

a. Glycerol, sodium stearate, sodium palmitate; b. Glycerol, sodium palmitate, sodium oleate, choline, and phosphate.

Step-by-step solution

Understanding Mild Hydrolysis with NaOH

Mild hydrolysis of lipids using dilute NaOH, also known as saponification, breaks ester bonds in triglycerides or phospholipids. This results in the formation of glycerol and sodium salts of fatty acids or sodium salts of phosphorylated derivatives.

Hydrolysis of 1-stearoyl-2,3-dipalmitoylglycerol

This triglyceride consists of three esterified fatty acids: one stearic acid and two palmitic acids. Mild hydrolysis with NaOH will break the ester linkages, resulting in glycerol and the sodium salts of stearic acid and palmitic acid. Thus, the products are glycerol, sodium stearate, and sodium palmitate.

Hydrolysis of 1-palmitoyl-2-oleoylphosphatidylcholine

This phospholipid will undergo hydrolysis to break its ester bonds. The products include glycerol, sodium palmitate from the palmitoyl ester, sodium oleate from the oleoyl group, and choline alongside inorganic phosphate groups from the phosphatidylcholine.

Key concepts

Saponification

Saponification is a chemical reaction that results from the hydrolysis of fats and oils (triglycerides) in the presence of an alkali, typically sodium hydroxide (NaOH). As the name suggests, it is the method used to make soap. During saponification, the ester bonds between the glycerol and fatty acids in triglycerides are broken.
This leads to the formation of glycerol and fatty acid salts, commonly known as "soap." The process significantly relies on the ability of the sodium ion in NaOH to break and sit in the ester bonds, creating these new compounds. Here is a simple breakdown of the key processes involved:

• The alkali cleaves the ester bonds of the triglyceride.
• Glycerol is liberated as a byproduct.
• The fatty acids combine with sodium to form sodium salts.

This reaction highlights the incredible transformation of substances typically considered non-reactive under normal conditions into valuable products like soap.

Triglycerides

Triglycerides are the main form of stored fat in the body and an essential energy source. They consist of a single molecule of glycerol bound to three fatty acids via ester bonds. Because of their structure, when subjected to saponification, triglycerides break down into glycerol and fatty acids.
The fatty acids released from the triglyceride can bind with sodium, forming sodium salts of the corresponding fatty acids. In the case of the triglyceride from the exercise, 1-stearoyl-2,3-dipalmitoylglycerol, it yields glycerol along with sodium stearate and sodium palmitate upon hydrolysis. Key characteristics of triglycerides include:

• They are hydrophobic, which means they do not mix with water.
• Their fatty acid chains can vary in length and saturation level.
• They serve as a significant source of energy upon oxidation.

Their structure and ability to be saponified make them valuable both biologically and commercially.

Phospholipids

Phospholipids are vital components of cell membranes. Unlike triglycerides, they contain two fatty acids and a phosphate group attached to glycerol. This unique composition allows them to form bilayers, which are critical for cell membrane structure.
When a phospholipid like 1-palmitoyl-2-oleoylphosphatidylcholine undergoes hydrolysis, its ester bonds are broken, and it releases components like sodium palmitate, sodium oleate, glycerol, and choline. The phosphate group remains attached to glycerol, emphasizing the phospholipid's dual nature:

• They have both hydrophobic and hydrophilic properties.
• The phosphate group forms the "head," interacting with water.
• The fatty acids form the "tails," avoiding water and helping structure membranes.

This structural complexity accounts for phospholipids' diverse functionality in biological systems and their remarkable adaptability in synthetic applications.

Ester Bond Cleavage

Ester bond cleavage is a crucial step in the hydrolysis of lipids. This process involves breaking the ester linkage that attaches fatty acids to glycerol in triglycerides and phospholipids. Ester bonds are relatively stable but can be hydrolyzed effectively using alkaline substances like NaOH.
The process of cleaving ester bonds relies on the nucleophilic attack by hydroxide ions. This attack breaks the ester, releasing a carboxylate ion and an alcohol group. In the exercise context, this results in the formation of glycerol and the corresponding fatty acid salts.

• Cleaving ester bonds converts non-polar molecules into polar components.
• This reaction is the foundational step in converting oils into soap.
• The stability of ester bonds makes them crucial in commercial and industrial applications.

Understanding ester bond cleavage is essential for appreciating many biochemical and industrial processes, including the making of soap and biodiesel.