C–C and C–H bonds are described by molecular orbitals; calculations indicate that most of the electron density associated with these orbitals lies between the two nuclei. The C-H bonds have a length of 109 x 10^-12 m (109 pm), while the C-C bond is approximately 50% longer, 154 x 10^-12 m (154 pm). This is because the C-C bonding orbital is made from sp3 hybrid orbitals, which are larger than the 1s orbital that hydrogen uses to form bonds. These so-called σ (sigma) bonds have an interesting property; the atoms that they link can spin relative to each other without breaking the bond between them.

4.1 Heterogeneous compounds
4.2 Single bonds
4.3 Double & triple bonds
4.4 Nitrogen, Oxygen, & Fluorine
4.5 Molecular Shape
4.6 Ionic bonding

For a C–H bond, if the H spins it would be impossible to tell, since the H atom is radially symmetric around the C-H bond axis. But if the carbons in the C-C bond of ethane spin relative to each other, then it is possible to observe different arrangements by looking down the C-C bond axis.

For example:  and   are both representations of ethane (the C–C bond is not seen in this depiction, since you are looking straight down the C–C bond). They appear different because the arrangement of the atoms is different in space - but in fact at room temperature these two arrangements can easily interconvert by rotating around the C–C bond.

This raises another point to consider, namely that starting (and stopping) bond rotations requires energy (similarly, and something that we will consider further later on, there can be vibrations along the length of a bond, which again involves the absorption or release of energy). 89 In the case of the rotating bond, it turns out that as the bulk of the groups attached to the carbons increases, the energy required for the rotation around the C–C bond also increases.  Big bulky groups tend to bump into each other - occupying each other’s space - which raises the energy of any shape where the groups are too close. This tends to “lock” the molecule into specific orientations, which can influence the compound’s physical properties.

An example of how structure interferes with the formation of a molecule is a molecule containing 17 Cs and 36 Hs; while possible to draw, this molecule has never been synthesized - the atoms crowd each other, particularly at the periphery. It is possible, however, to synthesize molecules with the same number of Cs but fewer Hs - can you figure out why?

Collapsing real structures down to 2-dimensional representations:

Now, an obvious problem with complex three dimensional molecules, even those made up only of hydrogen and carbon, is how to convey their structure when they must be depicted in two dimensions, like when you are writing on paper. Research indicates that students (that is, most people) have a tough time with this task, which is why we will describe various approaches here.

Before we begin, we need to have some rules. Let us use the set of possible molecules that contain 5 carbon atoms and 12 hydrogen atoms - these are generically known as “pentanes”. You can begin with a piece of paper (and a pencil); how many different molecules can you draw with the composition of C₅H₁₂? Clearly C₅H₁₂ does not uniquely define the structure of the molecule – it is better to use their distinct names (pentane, isopentane, and neopentane). Each of the different molecules you have drawn have the same molecular formula but they have different shapes and, it turns out, different properties. For example, pentane has a boiling point of 308 K, whereas the boiling points of isopentane and neopentane are 301 K and and 283 K, respectively. Their shapes, rather than their elemental composition, influences the strength of the attractions between the individual molecules, which in turn influences their boiling points. We call these kinds of related compounds structural isomers - that is they have the same composition (e.g. C₅H₁₂) but their constituent atoms are connected differently to give different structures and shapes.

It is common to use a number of different types of representations to represent molecules. One way is through what are known as text (or linear) formulas. In this scheme, pentane is written
CH₃-CH₂-CH₂-CH₂-CH₃, which can also be written as CH₃-[CH₂]₃-CH₃). This captures some of the structural subtleties of pentane, but not all - it does not illustrate the fact that the molecule is not strictly linear, nevertheless we can already anticipate complications - how would we write isopentane? The most obvious way would be ((CH₃)2CHCH₂CH₃). Neopentane is written as ((CH₃)4C)[does that make sense? - try deciphering them – we will return to this point later on in this chapter).

One important skill you will need to master (or at least remember) is that “short-hand” structures (such as Lewis structures) can provide information about the 3D structure of the molecule - and as we will see - this will allow us to predict chemical and physical properties?  This one more representation that you will often see used which leaves out even more information.

In the line structure, the only things that are shown are the bonds between carbons! So for example for the pentanes (C₅H₁₂) we can draw structures that omit all the symbols for atoms, and all the C-H bonds. These structures should be used with caution - it is very easy to forget atoms or bonds when they are not in the representation. But what these line structures do show very clearly is how the carbon atoms are connected, which can be very helpful at times.

4.1 Heterogeneous compounds
4.2 Single bonds
4.3 Double & triple bonds
4.4 N, O, & F
4.5 Molecular Shape
4.6 Ionic bonding

Question to answer:

• How many different compounds can you draw for C₆H₁₄?
• Draw out the full Lewis structure, the condensed formula, and the line formula.
• What are the advantages and disadvantages of each type of structure?
  

Questions for later :

• When you think about rotating around a C-C bond (say in ethane), there are more and less stable orientations - which orientation do you think is the most stable and why?
• Now imagine a butane molecule (C₄H₁₀) looking along the C2-3 bond. You would see one methyl groups and 2 Hs bonded to the two Cs.
• How would that influence rotation around the C-C bond we have been considering?