Lecture Series 8 Covered for FinalC HHHC HHHC HHC HHHC HHHC HHC HHC HHn-propanen-pentane0BondEnergyrchemical bonddistancevan der Waalsinteraction distanceEnergyFigure 2. A van der Waals potential compared to anormal bonding potential. Note that vdW minimum is at a larger internuclear separation thanfor the chemical bond. The depth of the vdW curve (i.e. the strength of the interaction) is exaggerated here. Lecture Series 8 Covered for FinalDescriptive Chemistry of Organic Molecules, Macromolecules, and PolymersRemember how I have stressed time and time again that I didn’t want you to have to memorize anything? Unfortunately, you will probably have to memorize some stuff here.I am not really certain how to enhance what is given to you in Chapter 23, other than to perhaps highlight the stuff that I feel is the most important. Let’s first try to make a little sense of some of the stuff that Oxtoby gives you. For example, in the first few pages, especially Figure 23-5 and the related discussion, it is stated that organic molecules that are larger are characterized by higher boiling points than smaller ones, provided that the types of organicmolecules being compared are very similar. For example, take the following two moleculesn-propane and n-pentane. It turns out that n-propane is a gas (propane stoves, for example),while n-pentane is a liquid that boils around 65oC. If we made n-octadecane, which is just like the two molecules at left, but contains 18 carbon atoms all arranged in a row, we would havea waxy solid that boils around 200 oC. So what we want to explain is, why does higher molecular weight (i.e. larger molecules) translate into higher boiling points? It turns out that molecules attract each other via weak interactions. A bonding interaction, or a Coulombic interaction, is a strong interaction.However, things that don’t bond to one another still attract each other, through what are called dispersion forces, or van der Waals interactions. How strong are these interactions? It dependson the system, of course, but theyare typically 100 times weaker than a chemical bond. These are the forces that hold a solution of pentane molecules together, or a solid of octadecane molecules together. After all, the solution doesn’t fly apart spontaneously! We compare a van der Waals potential energy curve of attraction with that of a chemical bond in Figure 1. Note that both curves have a repulsive part that is about equally steep. This makes sense. Consider a liquid of molecules held together by van der Waals attractions, and a solid of atoms held together by ionic or covalent chemical bonds. Neither material is particularly compressible. That is why the repulsive part of the potential appears the same for both systems. Recall that we discussed that we could eject an electron from an atom (ionization of the atom) by supplying energy – possibly the form of a photon. We could also supply energy in the form of heat, but we would have to get an atom pretty hot before it lost electrons. It turns out that, because van der Waals interactions are relatively weak, we only have to heat up a solution of molecules a relatively small amount to break it apart – or, in other words, to get it to boil. It turns out that larger molecules are characterized by stronger van der Waals attractions than small (but similarly structured) molecules. Why? Think of the followingexperiment. Take a small 1 cm2 square of glass and put it on top of a second square, with a drop of water sandwiched in between. Now pull (don’t slide) the two pieces apart off. They will stick a little bit, but you should be able to separate them without too much trouble. Now, take a large (1 m2) plate of glass, and put it on another plate of glass, once again sandwiching a small amount of water in between the two glass panes. Try to pull the two plates apart now. You probably won’t be able to do it! You may even break the glass before you get two plates separated. Obviously the interactions between the variouspieces of glass are the same, there are just more of them when the glass pieces are larger. This is the same situation for molecules. The van der Waals interactions per unit length of the molecule are nearly identical for two interacting propanes or two interacting pentanes, or two interacting octadecanes. However, the interactions add up for the longer molecules. Thus, a solution of octadecane is held together more strongly than is a solution of pentane than is a solution of propane. The net result is, larger molecules boil (and melt) at higher temperatures. The molecules we discussed above are n-alkanes. The ‘n-‘ prefix indicates that they are linear, and the ‘-ane’ suffix indicates that all carbon atoms are sp3 hybridized, which means that there are no double bonds. Obviously, there are many different types oforganic molecules, of which alkanes are the simplest. Even if we just stay with the two elements C and H, we can generate several different types of organic molecules. We havealready discussed linear alkanes. There are also branched alkanes. Once again, all carbon atoms are sp3 hybridized, but they are not bonded in a linear arrangement. A really painful thing that we are going to have to do now is to develop a nomenclature, so that when we refer to an organic molecule, the name that we call it tells us what itsC HHHC HHHC HC HHC HHC HC HHCHCHHHHCHHH1234567 83,6 dimethyloctaneFigure 3. C HHHC HHHC HC HHC HHC C HHCHCHHHHCHHH8765432 1H - C - HCHHH3-ethyl, 3,6-dimethyloctaneFigure 4. structure is. Consider the molecule shown in figure 3, which is named 3,6 dimethyl octane. To name this molecule, we count the longest continuous carbon network that we can, regardless of how the molecule is drawn. When we do this, we find that we come up with an uninterrupted chain of 8 carbon atoms. Since there are no double bonds, and the only atoms present are carbon and hydrogen, it is an alkane. The 8 carbon atom chain makes it an octane. If we number the carbon atoms as we count them, then we can locate the positions along the carbon backbone where the extra hydrocarbon branches originate. We find that a methyl group (1 carbon atom, 3 hydrogens) originates at carbon #3, and a second one originates at carbon #6.Since there are two methyl groups, and they are at positions 3 and 6, we call it 3,6-dimethyloctane. Let’s do one more – the molecule shown in