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 - Online Tutor, Practice Problems & Exam Prep
Physical Properties of Fatty Acids Concept 1
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Physical Properties of Fatty Acids Example 1
Video transcript
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
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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
Video transcript
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.