This is another byproduct of the surface-area dependence of Van der Waals dispersion forces – the more rod-like the molecules are, the better able they will be to line up and bond. The Role Of Symmetry (or lack thereof) On Melting And Boiling Points Individually, each interaction might not be worth very much, but when you add them all up over the length of a chain, Van der Waals dispersion forces can exert tremendous effects. As the chain length increases, there will be regions where they can line up next to each other extremely well. On an intuitive level, you could compare these long molecules to strands of spaghetti – the longer the noodles, the more work it takes to pull them apart. So as you increase the length of the chain, you also increase the surface area, which means that you increase the ability of individual molecules to attract each other. Well, the key force that is acting here are Van der Waals dispersion forces, which are proportional to surface area. Here’s the question: How, exactly do intermolecular forces increase as molecular weight increases? Look at the dramatic increases in boiling points as you increase molecular weight in all of these series: Trend #2 – For molecules with a given functional group, boiling point increases with molecular weight. You could tell a similar tale for the similar amine and carboxylic acid isomers shown below.įor a previous discussion of the 4 intermolecular forces, see the previous article ( The Four Intermolecular Forces And How They Affect Boiling Points) 2. Moral of the story: among molecules with roughly similar molecular weights, the boiling points will be determined by the functional groups present. The result is that butane boils at the temperature at which water freezes (0° C), far below even that of diethyl ether. The only attractive forces between individual butane molecules are the relatively weak Van der Waals dispersion forces. Then think about butane, C 4H 10, which contains no polar functional groups. Still, the attractive forces in butanol pale in comparison to those of the salt sodium butoxide, which melts at an extremely high temperature (well above 260 ☌) and actually decomposes before it can turn into a liquid and boil. The greatly increased boiling point is due to the fact that butanol contains a hydroxyl group, which is capable of hydrogen bonding. Molecules of diethyl ether, C 4H 10 O, are held together by dipole-dipole interactions which arise due to the polarized C-O bonds.Ĭompare the boiling point of diethyl ether (35 ☌)with that of Its isomer 1-butanol (117 ☌). Ĭompare the different butane alcohol derivatives shown below. Trend #1: The relative strength of the four intermolecular forces. The Role Of Symmetry (or lack thereof) On Melting And Boiling Pointsġ.Trend #2 – For Molecules With A Given Functional Group, Boiling Point Increases With Increasing Molecular Weight.Trend #1: The Relative Strength Of The Four Intermolecular Forces.Boiling points increase as the number of carbons is increased.The influence of each of these attractive forces will depend on the functional groups present. The relative strength of the four intermolecular forces is: Ionic > Hydrogen bonding > dipole dipole > Van der Waals dispersion forces.There are 3 important trends to consider. The more they stick together, the more energy it will take to blast them into the atmosphere as gases. The key thing to consider here is that boiling points reflect the strength of forces between molecules. Figuring out the order of boiling points is all about understanding trends.
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