Physical Properties of Fatty Acids : Study with Video Lessons, Practice Problems & Examples

Fatty acids' physical properties, such as solubility and melting point, are influenced by carbon chain length and the number of pi bonds. Solubility in water decreases as carbon chain length increases due to increased non-polarity. Conversely, melting point increases with longer carbon chains but decreases with more pi bonds. For example, stearic acid has a high melting point, while oleic acid, with one pi bond, drops significantly. Understanding these relationships is crucial for grasping lipid behavior in biological systems.

Physical Properties of Fatty Acids Concept 1

When it comes to the physical properties of fatty acids, we look at two things: it's determined by the carbon chain length and their number of pi bonds. Now, here if we're taking a look at solubility in water, we're going to say that the length of the carbon chain is inversely proportional to solubility in water. Because remember, as your carbon chain lengthens, you gain more carbons becoming more nonpolar, and therefore, you're less soluble in a polar solvent like water. So here we're going to say that as we increase our carbon chain length, we're going to decrease the solubility in water. Remember, inversely proportional means they're opposites of each other. If one goes up, the other one has to come down. In this case, the longer your carbon chain becomes, the less soluble you become in water.

Physical Properties of Fatty Acids Example 1

In this example, it says, which of the following fatty acids would have the lowest solubility in water? Now remember, water itself represents a polar solvent. And we want the lowest solubility. Here, remember there's an inverse relationship between the carbon chain length and its solubility in water. The longer your carbon chain length is, the less soluble you'll be in water. So if we take a look here, we have different types of fatty acids, and we're going to say this is 16 carbons, 18 carbons, 12 carbons, and then 20 carbons. So here we're going to say the answer has to be option d. This particular fatty acid, because it possesses the most number of carbons, it's going to have the longest carbon chain length, which will result in the lowest solubility in a polar solvent like water. So again, our final answer here will be option d. Here we have Arachidic Acid which has 20 carbons and no pi bonds.

Physical Properties of Fatty Acids Concept 2

In this video, we'll take a look at our second physical property of fatty acids, Melting Point. Now, here we're going to say Carbon Chain Length and the number of Pi Bonds have opposing effects on the melting point of fatty acids. Here, when we take a look at Carbon Chain Length, we're going to say that Length is directly proportional to Melting Point. Meaning that if we increase our Carbon Chain Length, then we're going to increase our melting point. If we take a look here at this graph, we have our Temperature on the y-axis, and number of carbons on our x. And we can see that as our number of carbons increases, our temperature or melting point increases. Here we have 12 carbons, 16 carbons, 20, and 24 carbons. Here we have our lauric acid, we have palmitic acid, here we have arachidonic acid, and we have lignoceric acid. And we can see as the number of carbon chains increases, the temperature is going up.

On the opposite side, we see the opposite occur. Here we're going to say the number of Pi bonds is inversely proportional to melting point, meaning that if I increase the number of Pi bonds, I'm going to decrease my melting point. If we take a look here, we have stearic acid which is well over 60 degrees Celsius in terms of temperature. Just adding 1 pi bond to go to Oleic acid causes a sharp decrease within our melting point. Oleic Acid here is somewhere around 11 to 13 degrees or so in terms of melting point. And if we add another pi bond, now we have 2 where we have Linoleic acid, it's going to go below 0 degrees Celsius in terms of melting point. So here we can see a great precipitous drop in our melting point as the number of Pi Bonds increases.

So remember, there is an opposite effect. Increasing the carbon chain length increases my melting point. Increasing the number of pi bonds decreases my melting point.

Physical Properties of Fatty Acids Example 2

Here in this example question, it says within each pair, determine the fatty acid with a greater melting point. Now remember, if we increase our number of carbons, then that's going to cause an increase in my melting point. And if I increase my number of pi bonds, that's going to cause a decrease in my melting point. If we take a look here at our 2 fatty acids, we have Stearic Acid with 18, Oleic Acid with 18, so, we can't rely on the number of carbons. What we rely on instead is the number of Pi Bonds. Stearic Acid has 0 Pi Bonds, Oleic Acid has 1. Remember, an increase in the number of pi bonds decreases my melting point. So, Stearic Acid would be higher in terms of melting point.

Next, we have Linolenic Acid, which has 18 carbons and 3 pi bonds. And we have Met Acid, which has 23 carbons. Both of them have the same number of pi bonds, but they have different numbers of carbons. Since Met Acid has more carbons, it'll have a higher melting point. Next, we compare fatty acids with 18 carbons and 1 pi bond to those with 16 carbons and 1 pi bond. Again, the number of pi bonds is the same, but the one with 18 carbons has more carbon, so it's going to have a higher melting point.

Finally, we have 18 carbons in 4 pi bonds, 20 carbons in 3 pi bonds. Two forces are at work here. First, our second fatty acid has more carbons which should result in a higher melting point and in addition to this, it has one less Pi bond which would also cause an increase in my melting point. So, for those two reasons, this fatty acid would have a higher melting point.

These are the different fatty acids based on each pair that would have the greater melting point.

Here’s what students ask on this topic:

How does the carbon chain length affect the solubility of fatty acids in water?

The solubility of fatty acids in water is inversely proportional to the carbon chain length. As the carbon chain length increases, the fatty acid becomes more non-polar, reducing its solubility in a polar solvent like water. This means that longer carbon chains result in lower solubility. For example, a fatty acid with a 12-carbon chain will be more soluble in water than one with a 24-carbon chain. This relationship is crucial for understanding how fatty acids behave in biological systems, particularly in terms of their interaction with water and other polar substances.

What is the relationship between the number of pi bonds and the melting point of fatty acids?

The number of pi bonds in a fatty acid is inversely proportional to its melting point. As the number of pi bonds increases, the melting point decreases. For instance, stearic acid, which has no pi bonds, has a high melting point of over 60°C. Adding one pi bond to form oleic acid significantly lowers the melting point to around 11-13°C. Adding another pi bond to form linoleic acid drops the melting point even further, below 0°C. This relationship is important for understanding the physical state of fatty acids at different temperatures and their functionality in biological membranes.

Why does increasing the carbon chain length of a fatty acid increase its melting point?

Increasing the carbon chain length of a fatty acid increases its melting point because longer chains have more van der Waals interactions, which require more energy to break. This means that as the number of carbons in the chain increases, the fatty acid becomes more solid at room temperature. For example, lauric acid with 12 carbons has a lower melting point compared to lignoceric acid with 24 carbons. This property is essential for understanding the structural integrity and functionality of lipids in biological systems, especially in cell membranes.

How do the physical properties of fatty acids influence their behavior in biological systems?

The physical properties of fatty acids, such as solubility and melting point, significantly influence their behavior in biological systems. Fatty acids with longer carbon chains are less soluble in water, affecting their transport and storage. The melting point determines whether a fatty acid is solid or liquid at body temperature, influencing membrane fluidity and function. For instance, saturated fatty acids with no pi bonds are solid at room temperature, making membranes more rigid, while unsaturated fatty acids with pi bonds are liquid, increasing membrane fluidity. Understanding these properties helps in grasping lipid metabolism and cell membrane dynamics.

What is the effect of adding pi bonds to a fatty acid on its melting point?

Adding pi bonds to a fatty acid decreases its melting point. Pi bonds introduce kinks in the fatty acid chain, preventing tight packing and reducing van der Waals interactions. For example, stearic acid (no pi bonds) has a high melting point, while oleic acid (one pi bond) has a significantly lower melting point. Adding more pi bonds, as in linoleic acid (two pi bonds), further decreases the melting point. This property is crucial for understanding the fluidity of cell membranes and the role of unsaturated fats in maintaining membrane flexibility.