Question

In each pair, identify the fatty acid with the lower melting point. Explain. a. myristic acid and stearic acid b. stearic acid and linoleic acid c. oleic acid and linolenic acid

Short answer

a. Myristic acid, b. Linoleic acid, c. Linolenic acid.

Step-by-step solution

Compare Myristic Acid and Stearic Acid

Myristic acid (C14:0) has 14 carbon atoms while Stearic acid (C18:0) has 18 carbon atoms. Both are saturated fatty acids. Generally, shorter chain saturated fatty acids have lower melting points. Thus, myristic acid with fewer carbon atoms will have a lower melting point.

Compare Stearic Acid and Linoleic Acid

Stearic acid (C18:0) is a saturated fatty acid whereas linoleic acid (C18:2) is a polyunsaturated fatty acid with two double bonds. Presence of double bonds in fatty acids leads to kinks in the structure and prevents tight packing, resulting in a lower melting point. Therefore, linoleic acid has a lower melting point than stearic acid.

Compare Oleic Acid and Linolenic Acid

Oleic acid (C18:1) is a monounsaturated fatty acid with one double bond, whereas linolenic acid (C18:3) is a polyunsaturated fatty acid with three double bonds. More double bonds in a fatty acid molecule result in lower melting points due to increased kinks in the structure. Thus, linolenic acid has a lower melting point than oleic acid.

Key concepts

saturated fatty acid

Saturated fatty acids are a type of fat in which all carbon atoms are bonded to hydrogen atoms with single bonds. This means there are no double bonds between carbon atoms. An example of a saturated fatty acid is stearic acid. Saturated fats are typically solid at room temperature because their straight chains pack closely together, resulting in higher melting points.

Characteristics of saturated fatty acids include:

• Long straight chains
• No double bonds between carbon atoms
• Higher melting points
• Solid at room temperature

Examples include palmitic acid, stearic acid, and myristic acid.

unsaturated fatty acid

Unsaturated fatty acids have one or more double bonds between carbon atoms in their chains. There are two types: monounsaturated and polyunsaturated. Oleic acid is an example of a monounsaturated fatty acid with one double bond, while linoleic acid is a polyunsaturated fatty acid with more than one double bond. These double bonds create kinks in the chain, preventing tight packing and leading to lower melting points compared to saturated fatty acids.

Characteristics of unsaturated fatty acids include:

• One or more double bonds in the carbon chain
• Lower melting points due to kinks in the chain
• Typically liquid at room temperature

Examples include oleic acid (monounsaturated) and linolenic acid (polyunsaturated).

carbon atoms

Carbon atoms are the building blocks of fatty acids. The number of carbon atoms in a fatty acid chain affects its properties. For instance, myristic acid has 14 carbon atoms, and stearic acid has 18 carbon atoms. Generally, the more carbon atoms in a chain, the higher the melting point. This is because longer chains can pack tightly together, creating stronger intermolecular forces.

Properties that vary with carbon atoms include:

• Chain length
• Melting point
• Physical state at room temperature

In fatty acids, typical carbon chain lengths range from 4 to 24 carbon atoms.

double bonds

Double bonds in fatty acids refer to the presence of one or more pairs of carbon atoms connected by double bonds (C=C). The presence of these bonds introduces kinks in the fatty acid chain that prevent tight packing, thereby lowering the melting point.

Effects of double bonds include:

• Lower melting points due to disrupted packing
• Increased fluidity at room temperature
• Classification into monounsaturated (one double bond) and polyunsaturated (multiple double bonds)

Examples of fatty acids with double bonds include oleic acid and linolenic acid.