Question

Draw the structure of a triacylglycerol whose components are glycerol and three oleic acid acyl groups.

Short answer

Draw a glycerol backbone with three esterified oleic acid chains.

Step-by-step solution

Understanding the Basics

Triacylglycerols, also known as triglycerides, consist of a glycerol backbone and three fatty acid chains. In this exercise, we need to identify glycerol and oleic acid before we can draw the structure.

Identify the Glycerol Backbone

Glycerol is a three-carbon alcohol, each carbon bearing a hydroxyl \((OH)\) group. Its structure can be represented as: \[ \text{CH}_2\text{-OH} - \text{CH} - \text{-OH} - \text{CH}_2\text{-OH} \] This forms the backbone of our triacylglycerol.

Identify Oleic Acid

Oleic acid is an 18-carbon monounsaturated fatty acid with one double bond between the 9th and 10th carbon. Its structure can be written as: \[ \text{CH}_3 - (\text{CH}_2)_7 - \text{CH} = \text{CH} - (\text{CH}_2)_7 - \text{COOH} \]. In the triacylglycerol, we use the oleate form (deprotonated acid): \[ \text{CH}_3 - (\text{CH}_2)_7 - \text{CH} = \text{CH} - (\text{CH}_2)_7 - \text{COO}^- \].

Esterification Reaction

Attach the oleate groups to the glycerol backbone via an ester linkage by replacing the hydroxyl hydrogen in glycerol with the oleate carboxyl carbon. Each hydroxyl group on glycerol bonds with the carbonyl carbon of the oleate group, forming: \[ \text{CH}_2\text{-OCOR} - \text{CH}\text{-OCOR'} - \text{CH}_2\text{-OCOR''} \], where \( R \), \( R' \), and \( R'' \) are oleate chains.

Assemble the Triacylglycerol Structure

Combine the components to draft the full structure: A glycerol backbone with three oleic acid-derived acyl groups attached. Each acyl group is joined with the glycerol by an ester linkage. This results in a triacylglycerol molecule with three identical oleic acid chains attached to glycerol.

Key concepts

Glycerol Backbone

Glycerol is a simple, small molecule, yet plays a critical role in forming triacylglycerols. Structurally, glycerol can be visualized as a three-carbon chain where each carbon atom is linked to a hydroxyl group \( \text{OH} \). Consequently, its chemical formula is \((\text{C}_3\text{H}_8\text{O}_3)\).

Key features of glycerol include:

• Three carbon atoms: This forms the primary "backbone" to which fatty acids attach.
• Hydroxyl groups: Each carbon atom has one hydroxyl group, providing the attachment points for esterification.
• Structure: It can be represented with the structure \[ \text{CH}_2\text{-OH} - \text{CH} - \text{-OH} - \text{CH}_2\text{-OH} \], which is the foundation of the triacylglycerol molecule.

These hydroxyl groups are crucial as they enable the formation of ester bonds with fatty acids. In triacylglycerols, this configuration allows the molecule to attach three fatty acid chains, producing a typical stored form of energy in living organisms.

Oleic Acid

Oleic acid is one of the three fatty acids commonly found in triacylglycerols. It is a monounsaturated fatty acid, meaning it contains one double bond between its carbon atoms.

Important characteristics of oleic acid include:

• 18-carbon chain: This long chain contributes significantly to the hydrophobic nature of triacylglycerols.
• Monounsaturation: The presence of a single double bond between the 9th and 10th carbon atoms affects the fluidity and melting point of the fat.
• Chemical structure: Represented as \[ \text{CH}_3 - (\text{CH}_2)_7 - \text{CH} = \text{CH} - (\text{CH}_2)_7 - \text{COOH} \], this shows the placement of the double bond in the middle of the chain.

In the context of triacylglycerols, oleic acid is often converted into its carboxylate form, known as oleate, to facilitate binding with glycerol. This conversion involves deprotonating the acid, leading to the slightly different structure \[ \text{CH}_3 - (\text{CH}_2)_7 - \text{CH} = \text{CH} - (\text{CH}_2)_7 - \text{COO}^- \].

Esterification Reaction

The esterification reaction is a key process in forming triacylglycerols. It involves attaching the fatty acids to the glycerol backbone through the formation of ester bonds.

Here's how esterification works:

• Reaction Mechanism: The reaction occurs when the hydroxyl hydrogen \( \text{H} \) in glycerol is replaced by the carbonyl carbon \( \text{C} \) of an oleate, forming an ester linkage.
• Ester Link Formation: This bond is a result of removing a water molecule \( \text{(H}_2\text{O)} \) that forms from the reaction between alcohol (glycerol) and carboxylic acid (fatty acid) components.
• Structure Post-Esterification: The molecular structure post-reaction appears as \[ \text{CH}_2\text{-OCOR} - \text{CH}\text{-OCOR'} - \text{CH}_2\text{-OCOR''} \], where \( R, R', \) and \( R'' \) represent the attached fatty acid chains.

This reaction is essential as it links each oleic acid-derived acyl group to the glycerol backbone, forming the complete triacylglycerol structure that is highly relevant in energy storage within living cells.