Lecture Series 8 Covered for Final Descriptive Chemistry of Organic Molecules Macromolecules and Polymers Remember 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 n propane the stuff that Oxtoby gives you For example in the first few pages especially Figure 23 5 H C C C H and the related discussion it is stated that organic molecules that are larger are H H H characterized by higher boiling points than n pentane smaller ones provided that the types of organic molecules being compared are very similar H H H H H For example take the following two molecules and n pentane It turns out that nH C C C C C H n propane propane is a gas propane stoves for example while n pentane is a liquid that boils around 65 H H H H H o C 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 have van der Waals a waxy solid that boils around interaction distance 200 oC 0 r 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 Bond weak interactions A bonding Energy chemical bond interaction or a Coulombic distance interaction is a strong interaction Figure 2 A van der Waals potential compared to a However things that don t bond to one another still attract each normal bonding potential Note that vdW minimum is at a larger internuclear separation than other through what are called dispersion forces or van der for the chemical bond The depth of the vdW Waals interactions How strong curve i e the strength of the interaction is are these interactions It depends exaggerated here on the system of course but they Energy H H H are 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 following experiment 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 various pieces 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 of organic 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 have already 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 its 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 H 1 this we find that we come up with an uninterrupted chain of 8 carbon H C H atoms Since there are no double 2 3 6 dimethyloctane bonds and the only atoms present H C H are carbon and hydrogen it is an alkane The 8 carbon atom chain H H H H H H makes it an octane If we number 3 4 5 6 7 8 carbon atoms as we count H C C C C C C C H the them then we can locate the positions along the carbon H H H H H H backbone where the extra hydrocarbon branches originate We find that a methyl group 1 C carbon atom 3 hydrogens originates at carbon 3 and a second one originates at carbon 6 Figure 3 Since there are two methyl groups and they are at positions 3 and 6 3 ethyl 3 6 dimethyloctane we call it 3 6 dimethyloctane Let s H do one more the molecule shown 8 in figure 4 Here we once again H C H H have a branched octane molecule 7 We name this molecule in the H C HH C H following way …