Question

Explain why oleic acid ( 18 carbons, one cis bond) has a lower melting point than stearic acid, which has the same number of carbon atoms but is saturated. How would you expect the melting point of trans-oleic acid to compare with that of cis-oleic acid?

Short answer

Oleic acid has a lower melting point than stearic due to less efficient packing from the cis bond. Trans-oleic acid likely has a higher melting point than cis-oleic but still lower than stearic acid.

Step-by-step solution

Understanding the Composition

Oleic acid and stearic acid both contain 18 carbon atoms. Oleic acid has one cis double bond, while stearic acid is fully saturated without any double bonds.

Impact of Cis Double Bond

The cis double bond in oleic acid introduces a kink in the hydrocarbon chain, preventing tight packing of the molecules. This decreases the intermolecular Van der Waals forces, leading to a lower melting point.

Structure of Stearic Acid

Stearic acid lacks any double bond, resulting in a straight chain that allows for tight packing. This ability to pack closely results in stronger intermolecular forces, and thus, a higher melting point compared to oleic acid.

Comparing Trans and Cis Double Bonds

Trans-oleic acid has a trans double bond that does not introduce a significant kink in the chain, allowing for tighter packing than the cis configuration. This results in intermolecular forces more similar to saturated fats, potentially raising the melting point closer to stearic acid.

Key concepts

Oleic Acid

Oleic acid is a monounsaturated fatty acid with a chain composed of 18 carbon atoms. What makes oleic acid unique is the presence of a single cis double bond at the ninth carbon position in its structure. This cis double bond creates a kink in the molecule's otherwise straight hydrocarbon chain. The kink prevents the molecules from packing closely together, which impacts its physical properties, most notably its melting point. Oleic acid is commonly found in various vegetable oils, such as olive oil, and is an essential component in a healthy diet.
Understanding oleic acid helps explain molecular behavior and functionality in biochemistry, particularly in its roles within cell membranes and energy storage.

Stearic Acid

Stearic acid is another fatty acid akin to oleic acid in that it also contains 18 carbon atoms. However, unlike oleic acid, stearic acid does not have any double bonds and is fully saturated. This lack of double bonds allows the hydrocarbon chains of stearic acid to remain straight.
Because there are no kinks or bends in the chain, stearic acid molecules can pack together more tightly in a solid state. This tight packing results in stronger intermolecular Van der Waals forces, contributing to a higher melting point. Stearic acid is found in animal fats and is frequently used in the production of soaps, candles, and cosmetics.

Cis and Trans Double Bonds

Double bonds in fatty acids can exist in either cis or trans configurations, each affecting the molecules differently. In oleic acid, the cis configuration results in a kink, which disrupts tight packing, lowering the melting point.
This property is vital for maintaining fluidity in cell membranes. In contrast, trans double bonds, as found in trans-oleic acid, do not create such kinks. Hence, they allow fatty acids to pack more like saturated fats. This packing increases intermolecular forces, potentially causing higher melting points.

• Cis configuration: kinked, less tight packing, lower melting point.
• Trans configuration: straighter chain, tighter packing, higher melting point.

Melting Point

The melting point of a substance is the temperature at which it changes from a solid to a liquid. In fatty acids, melting point is influenced by molecular structure. Longer chains and closer packing due to the absence of kinks lead to stronger intermolecular forces and higher melting points.
As oleic acid contains a cis double bond, the resulting kink lowers the melting point compared to stearic acid. In contrast, the straight and tightly packed stearic acid exhibits higher melting points. Furthermore, trans double bonds, like those in trans-oleic acid, lead to higher melting points than their cis counterparts due to tighter molecular packing.

Lipid Structure

Lipids are a diverse group of compounds integral to biological systems. Their structure significantly affects their physical and chemical properties. Fatty acids, like oleic and stearic acids, are key components of lipids, contributing to functions ranging from energy storage to membrane structure.
The presence of double bonds, as in oleic acid, introduces bends or kinks in the lipid structure, affecting how they interact with one another and with water. Understanding the contrast between saturated and unsaturated fatty acids provides insights into lipid behavior. The spatial arrangement and length of hydrocarbon chains are critical for the functionality of biological membranes and lipoproteins, impacting fluidity and permeability.

• Saturated fatty acids: straight chains, tight packing, solid at room temperature.
• Unsaturated fatty acids: kinked chains, less dense packing, liquid at room temperature.