3. Oleic acid, shown below, is

(A) A saturated fatty acid

(B) An unsaturated fatty acid

(C) Insoluble in water

(D) Soluble in acetone

Oleic Acid Is an Unsaturated Fatty Acid 

Correct Answer

(B) An unsaturated fatty acid

Introduction

Oleic acid is one of the most important naturally occurring fatty acids found in plants and animals. It is the major fatty acid present in olive oil, canola oil, sunflower oil, peanut oil, avocado, nuts, and many biological membranes. Because of its health benefits and widespread occurrence, oleic acid is frequently asked about in competitive examinations such as CSIR NET, GATE Biotechnology, IIT JAM, CUET PG, NEET PG, UPSC, and university entrance examinations.

The key feature that distinguishes oleic acid from saturated fatty acids is the presence of one carbon-carbon double bond (C=C) in its hydrocarbon chain. This single double bond classifies oleic acid as a monounsaturated fatty acid (MUFA).

Understanding the structure of oleic acid makes it easy to solve similar MCQs in biochemistry and lipid chemistry.

Understanding the Structure of Oleic Acid

Oleic acid is chemically known as:

cis-9-Octadecenoic acid

Its molecular formula is

C₁₈H₃₄O₂

Important structural characteristics include:

• 18 carbon atoms
• One carboxyl (-COOH) group
• One carbon-carbon double bond
• Long hydrocarbon chain
• Cis configuration of the double bond

The double bond introduces a bend (kink) in the molecule, preventing close packing of adjacent molecules.

This is why oils rich in oleic acid remain liquid at room temperature.

Why Option (B) Is Correct

Oleic Acid Is an Unsaturated Fatty Acid

The defining feature of an unsaturated fatty acid is the presence of at least one carbon-carbon double bond (C=C).

Oleic acid contains one double bond, making it a monounsaturated fatty acid (MUFA).

Characteristics include:

• Contains one C=C bond
• Lower melting point
• Liquid at room temperature
• More flexible hydrocarbon chain
• Healthier dietary fat compared to many saturated fats

Therefore,

✅ Option (B) is the correct answer.

Detailed Explanation of Every Option

H3: Option (A) A Saturated Fatty Acid

Why It Is Incorrect

A saturated fatty acid contains only single carbon-carbon bonds throughout its hydrocarbon chain.

General features include:

• No double bonds
• Straight hydrocarbon chain
• Molecules pack tightly together
• Usually solid at room temperature

Examples include:

• Palmitic acid
• Stearic acid

Since oleic acid clearly contains one C=C double bond, it cannot be classified as saturated.

Therefore, Option (A) is incorrect.

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Option (B) An Unsaturated Fatty Acid

Why It Is Correct

Unsaturated fatty acids possess one or more double bonds.

Oleic acid has:

• One double bond
• Cis configuration
• Bent molecular structure
• Lower melting point
• Greater membrane fluidity

Since there is exactly one double bond, oleic acid belongs to the category of monounsaturated fatty acids (MUFA).

Examples of monounsaturated fatty acids include:

• Oleic acid
• Palmitoleic acid

Hence,

Option (B) is correct.

Option (C) Insoluble in Water

Scientific Explanation

Oleic acid contains:

• One small polar carboxyl group
• One very long non-polar hydrocarbon chain

The hydrocarbon chain dominates the molecule, making it hydrophobic.

Water is highly polar.

According to the principle:

“Like dissolves like.”

Polar substances dissolve in polar solvents.

Non-polar substances dissolve in non-polar solvents.

Therefore, oleic acid is practically insoluble in water.

Why This Option Is Not the Best Answer

Although this statement is true, the question asks:

Oleic acid is

The primary classification based on structure is that it is an unsaturated fatty acid.

Hence examiners expect the structural identity rather than an additional physical property.

Option (D) Soluble in Acetone

Scientific Explanation

Acetone is an organic solvent.

Oleic acid dissolves well in:

• Acetone
• Ether
• Chloroform
• Benzene
• Alcohol

Therefore,

the statement is scientifically true.

Why It Is Not the Correct Choice

The question focuses on identifying the chemical nature of oleic acid rather than one of its physical properties.

Hence Option (D) is not considered the best answer.

What Makes Oleic Acid Unsaturated?

The carbon-carbon double bond reduces the number of hydrogen atoms attached to the carbon chain.

Compare:

Saturated fatty acid

No double bond

Maximum hydrogen atoms

Example:

Stearic acid

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Unsaturated fatty acid

Contains one or more double bonds

Fewer hydrogen atoms

Example:

Oleic acid

Types of Fatty Acids

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Properties of Oleic Acid

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Biological Importance of Oleic Acid

Oleic acid performs several important biological functions.

Energy Storage

Stored in triglycerides and used as an energy source.

Cell Membrane Fluidity

Its double bond increases membrane flexibility.

Heart Health

Oleic acid is associated with:

• Reduced LDL cholesterol
• Improved HDL cholesterol
• Better cardiovascular health

Signal Molecule

Acts as a precursor for several biologically active molecules.

Sources of Oleic Acid

Natural sources include:

• Olive oil
• Avocado
• Canola oil
• Peanut oil
• Almonds
• Hazelnuts
• Sunflower oil
• Sesame oil

Saturated vs Unsaturated Fatty Acids

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Why Competitive Exams Ask This Question

This MCQ tests whether students can identify fatty acids from their chemical structures.

Students should immediately recognize:

• Double bond present → Unsaturated
• No double bond → Saturated

This concept is repeatedly tested in:

• CSIR NET Life Sciences
• GATE Biotechnology
• IIT JAM Biotechnology
• CUET PG
• NEET PG
• M.Sc. Entrance Exams
• University Biochemistry Papers

Key Takeaways

• Oleic acid contains 18 carbon atoms.
• It possesses one carbon-carbon double bond.
• It is classified as a monounsaturated fatty acid (MUFA).
• It is insoluble in water because of its long hydrophobic hydrocarbon chain.
• It is soluble in organic solvents such as acetone.
• The presence of one double bond is the key feature used to identify it in structure-based MCQs.