When does a long carbon chain become more significant to the boiling point of a substance than polarity? [closed]








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I'm asked to sort the following compounds according to their boiling point (in an increasing manner): acetic acid, sodium chloride, and pentadecane (C15H32). Obviously, sodium chloride will have the highest boiling point because it's an ionic compound. But what about the acetic acid and the pentadecane? How does one proceed when you have to choose between a long hydrocarbon chain and a polar molecule? Is there a general rule or I should just guess in a case where the chain is smaller than that of a pentadecane?




Thanks for your question.
The B.P. of Acetic acid glacial is 118 C and Pentadecane is 270 C.


I think finding a general rule (or equation) for predicting the boiling point of an organic substance (its relation with factors like carbon chain length and polarity of its functional groups and...) might be difficult. But, in some cases we can guess in a number of *similar* substances, witch one may have a higher boiling point. Let me explain more about intermolecular forces and what may be this *similarity*.


The intermolecular forces are actually the electrostatic forces between positive and negative charges. These charges can appear simply by movement of electrons in a molecule. But, an instant later the new movement of electrons may decrease the charges or even naturalize them. These instant dipoles is considered to take place in every molecules and at different parts of it (with different magnitudes and directions at each instant). Although the dipoles are instant but an electrostatic force will appeared between molecules.
This type of intermolecular force is called dispersion forces(London forces). Naturally, it gets stronger by increasing the number of electrons in a molecule.


When element with high affinity to electrons (like Fluorine) are present in a molecules, permanent dipoles will appeared, among London forces. One may think that permanent dipole(dipole-dipole attraction) may be a stronger intermolecular bond than London force(for example, consider H-F and H-H). But, that's not generally correct.
Pentadecane is a compound with almost only London forces with such a high boiling point of 270 degree of centigrade. This indicates that the molecules needs high amount of kinetic energy, in order to could exit the liquid phase(to overcome the intermolecular forces and work on atmosphere pressure). The boiling point is even more than of water that has a strong hydrogen bond between its molecules.


So, generally we can't predict the trend in boiling points just by looking at types of intermolecular forces.
But, in some cases, when the substances are similar in some specifications, some assumptions can be made.
Consider the straight chain alkanes starting from methane. As more electrons results in stronger London forces, we may concluded that the boiling point must rise as the carbon chain become longer. Actually, in the experiment this is true.
Now, consider the boiling points of isooctane with eight carbon that is 99 degree and heptane with seven carbon that is 98.8 degree. Surprisingly, they are very close to each other. The branching leads to a decrease in isooctane(octane isomer) boiling point.
The intermolecular forces in this example was only London force. Adding dipole-dipole interactions (for example acetic acid in your question) makes problem more complex. You can find more information about this subject at the sites below.
<https://www.chemguide.co.uk/atoms/bonding/vdw.html>
<https://amrita.olabs.edu.in/brch=7&cnt=1&sim=111&sub=73>


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