Question

Write equations for the following reactions, representing the reactants and products using structural formulas. (a) the formation of ethyl acetate from acetic acid and ethanol (b) the hydrolysis of glyceryl tristearate (the triester of glycerol with stearic acid, a fatty acid)

Short answer

a) Acetic acid and ethanol form ethyl acetate, b) Hydrolysis of glyceryl tristearate forms glycerol and stearic acid.

Step-by-step solution

Understanding the Reaction Given

For part (a), we need to find the structural formulas of acetic acid, ethanol, and ethyl acetate. Acetic acid is represented as \( \text{CH}_3\text{COOH} \) and ethanol as \( \text{CH}_3\text{CH}_2\text{OH} \). Ethyl acetate, the ester formed, is represented structurally as \( \text{CH}_3\text{COOCH}_2\text{CH}_3 \). For part (b), glyceryl tristearate is hydrolyzed to produce glycerol and three molecules of stearic acid.

Writing the Equation for Part (a)

In the formation of ethyl acetate, acetic acid reacts with ethanol in a process called esterification. The structural reaction equation is: \[ \text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_3\text{COOCH}_2\text{CH}_3} + \text{H}_2\text{O} \]. This shows the carboxyl group of acetic acid reacting with the hydroxyl group of ethanol to release water and form ethyl acetate.

Writing the Equation for Part (b)

The hydrolysis of glyceryl tristearate involves breaking it down into glycerol and stearic acid. Glyceryl tristearate has a complex structure consisting of glycerol bound to three stearic acid molecules. The structural reaction is: \[ \text{C}_3\text{H}_5(\text{O}_2\text{C}_{17}\text{H}_{35})_3 + 3 \text{H}_2\text{O} \rightarrow \text{C}_3\text{H}_5(\text{OH})_3 + 3 \text{C}_{17}\text{H}_{35}\text{COOH} \]. Glycerol (\( \text{C}_3\text{H}_5(\text{OH})_3 \)) is produced along with three molecules of stearic acid (\( \text{C}_{17}\text{H}_{35}\text{COOH} \)).

Key concepts

Esterification Reaction

Esterification is a fundamental chemical reaction in organic chemistry. It typically involves the combination of a carboxylic acid with an alcohol, leading to the formation of an ester and water as a byproduct.
During this process, the hydroxyl group of the carboxylic acid and the hydrogen atom from the alcohol's hydroxyl group are removed, producing water.
The remaining parts of the acid and alcohol bond to form an ester. For example, in the formation of ethyl acetate:

• Acetic acid (\( \text{CH}_3\text{COOH} \)) provides the carboxyl group.
• Ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)) contributes its alcoholic group.
• The ester ethyl acetate (\( \text{CH}_3\text{COOCH}_2\text{CH}_3 \)) is formed along with water.

This reaction is usually catalyzed by an acid, like sulfuric acid, which acts as a catalyst to speed up the process without being consumed itself.

Hydrolysis of Esters

Hydrolysis of esters is the chemical process where esters are broken down into their corresponding acids and alcohols with the addition of water.
It is essentially the reverse of an esterification reaction. Hydrolysis can occur under two conditions: acidic or basic, known as saponification when in basic condition.
In acidic hydrolysis, water and the ester react in the presence of an acid to form an alcohol and carboxylic acid. In basic hydrolysis (saponification), however, a strong base like sodium hydroxide is used, yielding the alcohol and a carboxylate salt instead of the free acid.
As an example, consider glyceryl tristearate, an ester composed of glycerol and three stearic acid molecules:

• During hydrolysis, water molecules break the ester bonds.
• The reaction releases glycerol (\( \text{C}_3\text{H}_5(\text{OH})_3 \)).
• It also yields three molecules of stearic acid (\( \text{C}_{17}\text{H}_{35}\text{COOH}\)).

Structural Formulas

In organic chemistry, structural formulas are crucial for understanding molecules' specific arrangements.
They provide insight into the type and number of atoms in a molecule, and how they are bonded together.
These representations extend beyond simple molecular formulas by illustrating the positioning and connectivity of atoms.

• For example, the structural formula for acetic acid is \( \text{CH}_3\text{COOH} \), specifying the arrangement of carbon, oxygen, and hydrogen.
• Ethanol's structural formula, \( \text{CH}_3\text{CH}_2\text{OH} \), shows the two-carbon chain and hydroxyl group.
• Understanding these helps predict the reactivity and interaction of molecules during reactions.

Structures can be represented in various ways, including line-angle drawings which simplify the display by omitting hydrogen atoms bonded to carbons.

Fatty Acids

Fatty acids are long-chain hydrocarbons with a terminal carboxyl group (\( \text{-COOH} \)).
They play a vital role in biological systems, serving as key components of triglycerides and cellular membranes.
Naturally occurring fatty acids usually have an even number of carbon atoms, owing to their biosynthetic origin from acetate units.

• Saturated fatty acids, like stearic acid, have no double bonds. This saturated nature makes them solid at room temperature.
• Unsaturated fatty acids contain one or more double bonds, which introduce kinks in the hydrocarbon chain, affecting melting points and physical states.

Stearic acid (\( \text{C}_{17}\text{H}_{35}\text{COOH}\)) is a common saturated fatty acid.
Due to its long carbon chain, it is hydrophobic, which is crucial for forming membranes and energy reservoirs in cells.
Fatty acids are not only important in nutrition but also in industry, where they are used to synthesize soaps, detergents, and cosmetics.