Alkanes - Properties and Trends

What's an Alkane?

An alkane is an organic compound made strictly of carbons single bonded to each other or hydrogens. No functional groups are present in an alkane.

Melting and Boiling Points

The graph above plots the melting point temperatures of the first 32 alkanes as squares. The boiling point temperatures are diamonds. Inspection of the graph shows there is a direct relationship between the number of carbons in an alkane and its melting/boiling point temperature. That is, the more carbons, the higher the temperature. For boiling point, this is a direct result of increased intermolecular (van der Waals) forces that are present in a larger molecule. The increase in van der Waals forces is a consequence of two causes. The first is the increased surface area of a large molecule, which provides greater possibility for interaction between molecules. Secondly, with a growing carbon chain, there is an increase in the number of electrons that can interact between molecules, in addition to an increased molecular mass. For melting point, the trend line does not appear as smooth as the boiling point trend line. This is due to an important difference between odd and even numbered carbon chains. Even numbered carbon chains pack tighter (closer) when solid, therefore requiring more energy to separate them and enter the liquid phase. Conversely, odd numbered carbon chains do not pack as tightly together in the solid phase and can be separated easier, thus making a lower melting point.

# Carbons (Name) Melting Point (°C) Boiling Point (°C)
1 (methane) -182.5 -161.6
2 (ethane) -181.76 -89
3 (propane) -187.7 -42.1
4 (butane) -138.4 -0.5
5 (pentane) -129.8 36.1
6 (hexane) -95 69
7 (heptane) -90.61 98.42
8 (octane) -57 125.52
9 (nonane) -53 151
10 (decane) -27.9 174.1
11 (undecane) -26 196
12 (dodecane) -9.6 216.2
13 (tridecane) -5 234
14 (tetradecane) 5.5 253
15 (pentadecane) 9.9 269
16 (hexadecane) 18 287
17 (heptadecane) 21 302
18 (octadecane) 29 317
19 (nonadecane) 33 330
20 (icosane) 36.7 342.7
21 (henicosane) 40.5 356.5
22 (docosane) 42 369
23 (tricosane) 49 380
24 (tetracosane) 52 391.3
25 (pentacosane) 54 401.9
26 (hexacosane) 56.4 412.2
27 (heptacosane) 59.5 422
28 (octacosane) 64.5 431.6
29 (nonacosane) 63.7 440.8
30 (triacontane) 65.8 449.7
31 (hentriacontane) 67.9 458
32 (dotriacontane) 69 467

Hybridization

Since all of the carbons in an alkane are single bonded to hydrogen and other carbons, all carbons are sp3 hybridized. This is true regardless of the number of branches (if any) or the number of hydrogen/carbons to which the carbon is bonded. Every carbon makes four covalent bonds, therefore sp3 is the only possibility.

Trends in Entropy Values

The graph shows the relationship between entropy values and the number of carbons in a straight chain alkane. Interestingly, this graph nicely illustrates the two most important factors affecting the entropy value of a substance. The easiest trend can be ascertained from a quick glance at the data, showing an increase in entropy as the number of carbons increases. Why does this happen? Recall that entropy, while often described as a measure or "disorder", is best thought of as a measure of the number of ways in which matter/energy can be dispersed in a system. It is in this definition that an explanation can be derived. Consider the two molecules below, methane on top and heptane below:


Methane is the simplest alkane, and while it can experience vibrational motion between its atoms (and thus disperse energy), it cannot contort translationally the same way as heptane. That is, the carbon chain in heptane does not need to be "straight." It could also look like the following examples below:


These various contortions greatly increase the number of ways energy can be dispersed by this molecule. It is for this reason that larger molecules (alkanes, organic, or otherwise), tend to have greater entropy values than smaller ones.

A closer inspection of the graph reveals that there is not a increase from every alkane to the next. There is a decrease in entropy from butane to pentane before continuing upward again through dodecane and (presumably) beyond. Why the drop from butane to pentane? This can be answered by looking into the difference between the first four alkanes and the next eight. The answer cannot be found from this graph, but rather an investigation of states of matter. Methane, ethane, propane, and butane are all gases at room temperature. Pentane, and the seven others displayed in this graph, are liquids. There is a drop in entropy when the alkanes change from gases to liquids at room temperature. The reason is once again linked to dispersal of energy. Gases have the greatest freedom of movement and as a result their particles move freely in their container. This allows for the energy contained within the molecules to easily be dispersed throughout the container. Liquids, however, have significantly less freedom of movement. In fact, liquids are far more similar to solids in this respect than gases. As a result, liquids are poor at dispersing energy throughout their container because they are very restricted in their movement about the container. This explains the drop from butane to pentane, but the increasing length of the carbon chain continues to increase the entropy value for the remaining alkanes.

It may be asked about what will happen when the alkanes move from liquids to solids, as they do from heptadecane (liquid) to octadecane (solid). The values are not shown here and are in fact difficult to find. However, based on these explanations it is sound to assume that there would be a drop from heptadecane to octadecane (although perhaps not as steep since solids have a similar lack of freedom of movement like liquids) and a steady increase onward to the higher alkanes.