Chapter 4

Chem 243

Chapter 4

Alkanes: Nomenclature, Conformational Analysis, Synthesis

Alkanes are hydrocarbons, consisting of only carbon and hydrogen. Additionally, they have only carbon-carbon single bonds, hydrocarbons with multiple bonds will be considered later. The "-ane" suffix is commonly used in the names of alkanes. The text thoroughly describes the systematic methods for naming alkanes and you should develop strong skills in this area. The reason is for communication purposes. In the darker past of organic chemistry, compounds were given whatever names their discoverers wanted. As the number of compounds increased to ridiculously high numbers, it became necessary to begin naming them in a systematic manner so that you could easily draw the structure of a compound by being given its name. Later we will look at various symbolic additions to the names that will indicate more detailed structural features. This is important to learn, since you will encounter names of compounds throughout the course and will have to know what their structures are. Initially, you will be asked to name compounds given their structures and asked to draw structures given their names, but later you may be given the name of a compound and asked to describe a reaction mechanism or the product of a reaction. You will have to know what the structure of that molecule is in order to answer the question. It's a language you have to learn. This can make beginning organic chemistry quite boring (though I find that difficult to believe!) but it makes the later stages of organic chemistry easier to comprehend. Do all the problems you can on nomenclature, get good at it and get it out of the way early.

The textbook also includes the nomenclature of alkenes and alkynes since the systematic methods are analogous.

Structural Features of Alkanes

The primary factor determining the structures possible for alkanes and cycloalkanes is the sp3 hybridized orbitals on carbon. The geometry is tetrahedral and you will see how subtle structural differences can result from this geometry.

When you look at a molecular structure in a book, it is static, it doesn't change while you are looking at it. If it does, please see a physician. In fact, the molecules are changing shape continuously as long as their temperature is above absolute zero. We saw how they vibrate when we looked at IR spectroscopy. They can move from place to place (translation) and they can rotate in space. Another motion that occurs is an internal rotation, in which one part of the molecule rotates relative to another part of the molecule. We can understand this simply by considering the single bonds as "sticks" connecting two atoms, as in the ball-and-stick models. Often, if you make these models, it will be possible to rotate the atoms about the single bonds. As the rotation proceeds, the structure changes and remember, the properties of the molecule are dependent on structure, so we should have an infinite number of structures possible and the molecule is continually changing from one structure to another.

This is another kind of isomer called a conformational isomer or just conformer. Two conformers can be interconverted by rotating about single bonds, whereas to interconvert constitutional isomers, bonds have to be broken and re-formed.

So, now which of these infinite structures are we talking about when we talk about a particular molecule? Since molecular structure is a dynamic property, We usually refer to the most stable of all those structures. When we say "most stable" we mean the one whose energy is lowest. But keep in mind that sometimes it is very easy to interconvert these structures.

The first concept of "lowest energy" conformation means that we calculate the energy of each structure quantum mechanically and look for the one with the lowest energy, this one is most stable. Often, we can estimate the structure of the lowest energy conformation by building a model and rotating about the single bonds and using our knowledge of physics (positive and negative charges, for example) to decide which of two structures should be most stable. Though a molecule may be neutral, the nuclei and electrons still have their localized charges, these don't disappear. Also, remember that a chemical bond, sometimes represented by a line connecting two atomic symbols, is made up of a pair of electrons with a fat -2 charge. If you rotate about some single bond and two other bonds at other points in the molecule start getting closer to each other, their negative charges will naturally repel each other. If we force them to be close to each other, we have to put energy into the molecule to make that conformation. This energy is called torsional strain, the force we encounter when we try to rotate about a single bond and the other parts of the molecule repel each other. It is also called a torsional barrier. So putting more energy into the molecule makes it less stable. This concept will be seen in many different contexts throughout the course, so PLEASE get comfortable with it. It will help you solve a lot of problems.

When we continue rotating about a single bond and keep calculating the energy of each conformation and plot this energy as a function of rotation angle, we can get a graph of the energy as a function of conformation. In the text, you will see these plots for several molecules. The low energy points on these graphs are minimum energy values, lying in "valleys" and the peaks are high energy values.

Some of the minima are not at the same energy value, some are at highr energy levels. The one at the lowest energy level is the global energy minimum and corresponds to the most stable conformation. The other minima at higher energy levels are called local minima. The important thing to see here is that in the graphs, in order to change the structure from either kind of minimum, you have to go uphill, or increase the energy of the molecule, so the molecule has a tendency to stay at a minimum energy unless energy is added from some external source, like increasing the temperature or pressure.

Now if you look along the energy path from one minimum to another, you will see that you must reach one of the maximum (global or local) before you can interconvert between two conformational minima. This is called the energy barrier and will be seen frequently throughout the course, not only in relation to changing conformations, but in many other contexts. Become comfortable with this kind of description of a system, because it will be used to describe a number of dynamic processes. Systems will spontaneously go from higher to lower energy values, but in order to go from lower to higher energy, something has to be done to the system. And even if you go from higher to lower energy, you may have to overcome an energy barrier, so energy will have to beintroduced into your system to first get the molecule to the peak, from which it can then fall to a lower energy. I am going on and on about this because it is an IMPORTANT concept, so gain a good understanding of it.

Cycloalkanes

Now suppose we have one end of an alkane connected to another end of that same alkane. These structures are possible and are some of the most interesting compounds we will encounter.

In some cases, there will still be some rotation about single bonds in cyclic compounds and we will concentrate on the various conformations of the 6-membered ring, cyclohexane. Smaller rings do not allow much rotation, there is just no way to rotate around any of the ring bonds without bending the whole mess out of shape. Try this with models. Cyclopentane can move a little, but we generally won't consider it to change much.

Angle Strain

Remember when we formed a covalent bond, there was a loss of energy, the bonded pair of atoms was at a lower energy (more stable) than the unbonded pair of atoms. When this is not the case, no bond is formed. OK, so now let's look at how the bond was formed. We said it was formed by the overlap of two atomic orbitals to form two molecular orbitals (one bonding and one antibonding in most cases). The greater the overlap of the two atomic orbitals, the lower the energy of the molecular orbital. The lower the energy of the molecular orbital, the stronger the bond is. so now we can say that the greater the overlap of two atomic orbitals, the stronger the bond that is formed. If something prevents the maximum quantum mechanical overlap of two orbitals, the bond formed is not at its lowest possible energy and is consequently weaker.

If we bend a molecule out of shape, so that we don't allow full orbital overlap, we can still form some bonds, but they will not be quite as strong as they could be. This can be seen if we try to make molecules whose geometry does not coincide with the geometry of sp, sp2 or sp3 orbitals. This is what we call angle strain. The angle at which the atoms can have their orbitals overlap, is not the angle which gives optimum overlap. The energy difference between these two cases is the angle strain energy. remember, weaker bonds will be broken more easily and are thus more reactive.

Using these concepts study the various conformations of cycloalkanes, especially mono and di-substituted cycloalkanes and learn the vast number of structural conformations and configurations you can make.

Reactions of Alkanes

Now we get into some very simple reactions of alkanes. We do not go into many subh reactions because alkane chemistry is limited to the breaking of C-H or C-C bonds. So there are just a few reactions here, we will see others when we consider radical reactions later.

Organic Synthesis

The subject is introduced here but we will get into more detail after we have learned a greater variety of reactions. Organic synthesis is the transformation of one molecule into another by a series of reactions which selectively break and form bonds. This should not be intimidating to you. As we introduce more reactions,think of them in terms of reactants and products. Then consider the products of one reaction to be used as reactants of another reaction to form new products. Essentially you are stringing together a series of reactions in this way to convert one molecule into another. You will be asked to synthesize a given molecule, often from a limited number of possible reactants. If you know the reactions, you should have no problem with this kind of process. Read the material in this chapter to get yourself started thinking in that direction.

One high tech tool which might be of help is the use of the index card. If you write a generic reaction on each index card as you learn it, you will have a collection of cards which show reactants and products. Since these are in a generic form, they will show mostly the functional groups and the reagents (above and below the arrows) that can transform one functional group into another. Well, once you build up a collection of these cards, say a couple of million, you can look at a particular reactant and see how it can be converted into another molecule by going through several steps, each step being represented on a single card. By placing these cards in the proper sequence, you have outlined a scheme for synthesizing a target molecule from a given starting compound. Many students find this an easy way to study what otherwise would be a difficult subject. Try it, you'll like it!