Question

Draw cholesterol acetate. Is this molecule saponifiable? Explain.

Short answer

Yes, cholesterol acetate is saponifiable due to its ester linkage.

Step-by-step solution

Understand the Structure of Cholesterol

Cholesterol is a sterol molecule that consists of a complex hydrocarbon ring structure with a single hydroxyl group (-OH) attached. The structure includes three six-membered rings and one five-membered ring, making up a tetracyclic system with a hydroxyl group at the C3 position.

Convert to Cholesterol Acetate

To draw cholesterol acetate, replace the hydroxyl group (-OH) at C3 in cholesterol with an acetate group (-OCOCH₃). The acetate group is derived from acetic acid and involves an ester linkage between the -OH group of cholesterol and the carboxylic acid residue.

Examine the Ester Linkage for Saponifiability

An ester linkage in an organic molecule indicates that part of the molecule can undergo saponification. Saponification is a chemical reaction that involves breaking an ester into an alcohol and a carboxylate salt (often under basic conditions). In cholesterol acetate, the presence of an ester link at the C3 position means it is saponifiable.

Key concepts

Saponification Insight

Saponification is a fundamental reaction in organic chemistry, especially for esters. This process involves breaking down an ester bond in a compound to form alcohol and a salt of a carboxylic acid. This reaction occurs typically in the presence of a strong base such as sodium hydroxide (NaOH).

Here's what you need to know:

• The reaction turns the ester linkage (like that in cholesterol acetate) into alcohol and a salt.
• It is commonly associated with soap-making, as it breaks down fats (which are esters) into glycerol and soaps (salts of fatty acids).

In the context of cholesterol acetate, saponification would cleave the acetate ester at the C3 position. This would result in the formation of cholesterol (the alcohol) and a carboxylate salt derived from acetic acid.

Exploring Organic Chemistry

Organic chemistry is the branch of chemistry that deals with structures, properties, and reactions of organic compounds containing carbon. It's a vast field with a few key concepts integral to understanding molecules like cholesterol acetate.

In organic chemistry:

• Carbon’s ability to form stable covalent bonds with other elements, including itself, results in an immense variety of organic compounds.
• Functional groups like alcohols, esters, and acids determine the reactivity and properties of the compounds.

Cholesterol acetate serves as an excellent example, showing how the addition of various functional groups (like the ester group in cholesterol acetate) can alter a molecule's properties and reactivity, such as making it susceptible to saponification.

Understanding Sterol Structure

Sterol structures are a subgroup of steroids and are characterized by multiple rings of carbon atoms.

Key features:

• Consist of four fused carbon rings: three six-membered and one five-membered.
• A hydroxyl group (-OH) at one of the positions, often contributing to the molecule’s rigid structure and unique chemical properties.

Cholesterol is a typical sterol molecule. It possesses a tetracyclic ring system which provides rigidity to cellular membranes and serves as a precursor for other biologically significant compounds. Replacing the hydroxyl group at C3 with an acetate group forms cholesterol acetate, merging sterol and ester chemistry.

Role of Ester Linkage

Ester linkages are a common motif in organic molecules and play a crucial role in the structure of many biological compounds.

What is an ester linkage?

• Formed between an alcohol and an acid. In cholesterol acetate, this occurs between the hydroxyl group and the acetic acid.
• Characterized by the functional group -COO- connecting the two molecules.

In cholesterol acetate, the ester linkage at the C3 position is the key site for saponification. Such linkages are essential in storing and releasing energy, as found in fats and oils, by making molecules more reactive under specific conditions.