4.1.2: Building Increasingly Complex Molecules

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You will soon realize that it is possible to build a rather amazing number of compounds using just hydrogen and carbon. For example imagine that we remove one hydrogen from a methane molecule; this leaves us with what is known as a methyl (-CH₃) group. We can combine two methyl groups by forming a C–C bond between them (you might want to convince yourself that each carbon atom is still making four bonds with neighboring atoms). The resulting molecule is known as ethane. The structure of ethane can be written in a number of ways, for example H₃C–CH₃, CH₃–CH₃or C₂H₆. As the number of atoms increases so does the number of different ways a molecule can be represented. It is for this reason that chemists have developed a number of rules that are rather strictly adhered to; these rules make it possible to unambiguously communicate the structure of a molecule to others.⁷¹ We will not spend much time on all of these various rules but there are web activities that you can do if you want to get an introduction and to practice them. These naming conventions are controlled by the International Union of Pure and Applied Chemistry, known as IUPAC and these rules can be found in the Compendium of Chemical Terminology.⁷²

The process of removing hydrogens and adding methyl groups can continue, essentially without limit, to generate a family of hydrocarbons⁷³ known as the alkanes; the rules that govern these molecules are simple: each hydrogen makes one and only one bond; each carbon must make four discrete bonds; and these four bonds are tetrahedral in orientation. The number of carbons is in theory unlimited and how they are linked together determines the number of hydrogens. (Can you see how two hydrocarbons with the same number of carbon atoms could have different numbers of hydrogens?)

Depending on how the carbons are connected it is possible to generate a wide variety of molecules with dramatically different shapes. For example there are cage-like, spherical, and long, string-like alkanes. Consider the four-carbon alkanes. There are butane and isobutane that have the formula C₄H₁₀ as well as others with four carbons but different numbers of hydrogens, for example: cyclobutane, methylcyclopropane, and tetrahedrane. Butane has a boiling point of -0.5ºC, and isobutane has a boiling point of -11.7ºC. Why are the boiling points of butane and isobutane, which have the same atomic composition (C₄H₁₀), different? The answer lies in the fact that they have different shapes. The roughly linear carbon chain of butane has a larger surface area than isobutane, which gives it more surface area through which to interact with other molecules via London dispersion forces. This idea, that the shape of a molecule and its composition, determine the compound’s macroscopic properties is one that we will return to repeatedly.

Questions to Answer

1. Why are the melting and boiling points of methane higher than the melting and boiling points of H₂?

2. How many different compounds can you draw for the formula C₅H₁₂?

3. What structures could you imagine for hydrocarbons containing five carbon atoms?

4. Is there a generic formula for an alkane containing n carbon atoms? How does forming a ring of carbons change your formula?

5. Which has the higher boiling point, a spherical or a linear alkane?

6. How do boiling points and melting points change as molecular weight increases?

Questions to Ponder 

1. Make a prediction as to the melting and boiling points of ethane, compared to methane. What assumptions are you making? How would you test whether those assumptions are valid?

2. Why does the shape of a molecule influence its behavior and its macroscopic properties?

References

⁷¹ Chemists are not being unnecessarily difficult; anatomists also have a very strict set of names for the various bones and nerves in the body, in part to avoid confusion during medical procedures.

⁷² http://www.iupac.org/ and http://goldbook.iupac.org/index.html

⁷³ Hydrocarbons contain only hydrogen and carbon. Be careful not to confuse them with carbohydrates, which contain carbon, hydrogen, and oxygen and include sugars. We will consider carbohydrates in more detail later on.