N-Methylpyrrolidine has a boiling point of 81 °C, and...

Question

N-Methylpyrrolidine has a boiling point of 81 °C, and piperidine has a boiling point of 106 °C.

b. Tetrahydropyran has a boiling point of 88 °C, and cyclopentanol has a boiling point of 141 °C. These two isomers have a boiling point difference of 53 °C. Explain why the two oxygen-containing isomers have a much larger boiling point difference than the two amine isomers.

Accepted Answer

Hello everyone. Let's do this problem. It says the two nitrogen containing isomers below have a boiling point difference of 19 degrees Celsius. Meanwhile, the two oxygen containing isomers have a boiling point difference of 83.2 degrees Celsius. The two nitrogen containing isomers have a much smaller boiling point difference than the two oxygen containing isomers explain this observation. So we have di ethyl amine which has a boiling point of 55.5 degrees Celsius. And we have ethyl dimethyl amine with a boiling point of 36.5 degrees Celsius. Those are the two nitrogen containing isomers and the two oxygen containing isomers are one butanol with a boiling point of 117.7 degrees Celsius and oxy ethane with a boiling point of 34.5 degrees Celsius. So we know that boiling point is related to in intermolecular forces, right. So the higher the boiling point, the stronger the inner molecular forces we have. So let's talk a little bit more about these intermolecular forces. We know that dipole dipole forces are common in compounds that have polar bonds, right. And dipole dipole interactions happen with molecules that have net dipoles and those are weaker than hydrogen bonding in intermolecular forces. And recall that hydrogen bonding occurs when we have a hydrogen bonded to either a nitrogen oxygen or fluorine atom. And that interacts with a nitrogen oxygen or fluorine that has a lone pair, at least one lone pair. OK. So let's look more closely at the intermolecular forces that these four compounds have. So let me redraw these structures to show their molecular geometry. At least for the nitrogen containing groups, it makes it a lot more clear. So in diamine, we have the hydrogen and two ethel groups, I will draw those wedge and dash and we have a long pair on the top of that nitrogen, right. So this is actually going to be in a tetrahedral shape if we consider the electron geometry right and drawing the dipoles, we have dipoles between the nitrogen and hydrogen because there is an electron electron negativity difference there. So we have a partial positive charge on the hydrogen, partial negative charge on that nitrogen. And same with the carbon and nitrogen, right. Carbon is partially positive nitrogen is partially negative. So we have another dipole there and same with the other ethyl group, carbon, partial positive nitrogen, partial negative. And we have a dipole pointing upwards because of that long pair. So you can see that this compound has a net dipole pointing up, right. So this is going to have dipole dipole interactions. And So will ethyl die methyl a mean let's draw the ethyl group a wedged methyl and a dashed methyl. And again, that lone pair on top creating that tetrahedral electron geometry. And because those three substituent groups on the nitrogen are all carbons bonded to that nitrogen. Those carbons have a partial positive charge, nitrogen has a partial negative charge. And we have those three dipoles pointing towards nitrogen from carbons and that one dipole pointing up towards the lone pair. So again, we have a net dipole here as well. So why does diamine have a higher boiling point than ethyl di methylamine if they both have dial dial interactions? Well, we talked about hydrogen bonding, right? And how that is a stronger in intermolecular force and a stronger in your molecular force relates to a higher boiling point. So notice that diamine has a nitrogen to hydrogen bond and nitrogen also has a lone pair. So one molecule of diamine can interact with another molecule of diamine via hydrogen bonding interactions, right. So that is going to make it stronger in molecular forces then the other n containing isomer, right? Ok. Well, let's look at the o containing isomers and see what intermolecular forces those have. So one butanol has an oh bond that oxygen is more electron negative. So it has a negative charge and we are going to have a dipole pointing from that partial positive hydrogen towards the oxygen. Then we have the carbon oxygen dipole, partial positive carbon, partial negative oxygen. We will also have dipoles from these lone pairs. So this is going to be a net dipole down. So there will be dipole dipole interactions here. And what about oxy ethane again, two carbon oxygen bonds. So, partial positive carbon pointing towards partial negative oxygen, partial positive carbon, pointing towards partial negative oxygen and two lone pairs on that oxygen creating a net dipole pointing down. So these two also have dipole dipole interactions but do either one of these have hydrogen bonding. Yes, one butanol has that oxygen to hydrogen bond and that oxygen has lone pairs. So similar to diamine, this o containing compound also has hydrogen bonding. Ok. But the question is asking why is the difference between the two nitrogen compounds? So much less than the difference between the two oxygen containing isomers. Well, think about the electron negativity difference between nitrogen and hydrogen versus oxygen and hydrogen, right oxygen is more electron negative than nitrogen. So the oh bond is going to be more polarized than the nitrogen hydrogen bond. So because that oh bond is more polarized, it is going to be a more dramatic increase in the strength of the in intermolecular forces of this molecule than the nitrogen containing compound. So the correct answer for this problem is since an oh bond is more polarized than an nitrogen hydrogen bond, the presence of hydrogen bonding more dramatically increases the strength of intermolecular traction and the energy required to disrupt interactions when the oh bond is involved compared to when the nitrogen hydrogen bond is involved. So that is it for this problem? I hope this was helpful and thanks for watching.

Key Concepts

Hydrogen Bonding

Molecular Structure and Intermolecular Forces

Boiling Point and Molecular Interactions

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