The boiling point of propanol is 97 °C ,much higher than that ...

Question

The boiling point of propanol is 97 °C ,much higher than that of either ethanethiol (37 °C) or chloroethane (13 °C) , even though all three compounds have similar MWs. Explain.

Accepted Answer

Hi, everyone. Our next problem says state the main reason why the boiling point of Cyclin all 140.4 °C is much higher than that of one butane thiol, 98.2 °C and tetra fluoro methane, negative 127.8 °C. Despite having similar molecular weights, a cyclops has a much higher boiling point because it has a ring structure. B starts the same way because it has much more carbon C again starts the same way because it can exhibit hydrogen bonding or D starts the same way because it has the most electron element. So let's think about the structure of each of these. So Cyclin, all we have a five carbon ring with that alcohol group and then we have one butane thy. So we have a four carbon chain with an sh group on carbon one. So draw that there and then tetra fluoro methane is just a carbon with four fluorine groups around it. So we can draw are foreign groups in that tetrahedral geometry around the carbon. So let's think about the differences in boiling point which as you can see are quite large between the three with the P cyclops, all being the highest and tetra fluor methane being by far the lowest and what molecular interactions determine that. Well, in this case, we're talking about in intermolecular forces IMF because the stronger those molecules are held together, the higher the boiling point as it will be more difficult to separate those molecules to boil the substance. So when we think about these intermolecular forces, let's think about them in terms of strongest to weakest the stronger the IMF the higher the boiling point. So we have ion dipole forces as the strongest. But they don't apply here, we don't have any ionic compounds and then hydrogen bonds followed by dipole dipole interactions. And then finally, dispersion forces, which are the only thing you have in nonpolar molecules or just your electron cloud, you know, at times the electrons on one side or the other. And you can have these momentary very weak interactions. So first of all, we noticed that our tetra fluoro methane, well fluorine carbon, of course, there's a great difference in electron negativity fluorine is much more electron. So that will be quite a polar bond. But in this case, we have four fluorine in a tetrahedral arrangement. So the dipole forces cancel out in this case because of the geometry of this molecule. So this is a nonpolar molecule, it is only has these dispersion forces which is why you see that very low boiling point. Now, let's look at our cyclin all versus butane dial. Well, they both will have dile dippo oxygen and sulfur are more electron than hydrogen. So each of those atoms are going to have this partial negative charge. However, let's think about where they are on the periodic table. Sulfur is a row below oxygen. So oxygen is much more electron than sulfur. So, as a result, while our alcohol can participate in hydrogen bonding, we don't really classify compounds with sulfur or with thiol groups as having hydrogen bonding. They do participate in dile dile interactions because again, that is a polar region of the molecule, the sulfur is more electron. And just to note, I just switched around, I'd written it as sh but since it's on the left side of the molecule, it really should be written hs So just to be slightly more clear, I've switched that around. But again, we have these dile dile interactions, but they don't usually get categorized as hydrogen bonds because they're not as strong as the interactions. You see when your electron element is oxygen or fluorine or nitrogen, one of those on that top right part of the periodic table. So the reason Cyclin all has this very high boiling point is its participation in hydrogen bonding. So that's choice c that will have the most significant effect on the boiling point here. But let's take a brief look at our other answer choices to understand why they're not as good of an answer as choice C So choice A says that it's higher because of having a ring structure. Well, the physical structure of our molecule of course will affect boiling point in terms of how well the molecules can stack together when they're in a solid or liquid form and how they interact with each other in those forces. But in terms of the magnitude of the forces, the magnitude of the hydrogen bonding caused by that oh group is much stronger than any difference between the non polar region of a ring versus a chain. So it's just not as much of a factor and it's just a very small effect compared to the hydrogen bonds. Choice B that it has much more carbon, but it doesn't have much more carbon. The cyclops has five carbons. The um butane thiol has four. That's not going to make a big difference in this again, compared to the magnitude of the hydrogen bonding force. So that's why choice B isn't correct. And finally, choice D it has a higher boiling point because it has the most electron element. Well, it does not fluorine is the most electron uh but as we saw in fluorine, those uh electron negativity differences causing those dipole forces cancel each other out. So choice D is just incorrect. So once again, why is there this higher boiling boiling point of cyclopentolate to one butane thiol and tetra fluoro methane. Choice C Cyclin has a much higher boiling point because it can exhibit hydrogen bonding. See you in the next video.

The boiling point of propanol is 97 °C ,much higher than that of either ethanethiol (37 °C) or chloroethane (13 °C) , even though all three compounds have similar MWs. Explain.

Key Concepts

Hydrogen Bonding

Molecular Weight vs. Boiling Point

Intermolecular Forces

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