Arrange the alkanes in each set in order of increasing boiling point. (a) 2-Methylbutane, 2,2-dimethylpropane, and pentane (b) 3,3-Dimethylheptane, 2,2,4-trimethylhexane, and nonane

Alkanes are organic compounds made up solely of carbon and hydrogen atoms arranged in a tree-like structure, meaning they don't contain any loops or rings. Their formula is generally written as C_nH_2n+2, where 'n' represents the number of carbon atoms. In their simplest form, the carbon atoms are connected by single bonds in a continuous, unbranched chain, with hydrogen atoms filling all other available bonds. This type of structure is known as a straight-chain or normal alkane.

However, alkanes can also have branches – when carbon chains diverge from the main carbon chain, creating a more complex molecular structure. This has significant effects on their physical properties, such as their boiling points. An important aspect of alkanes is that they are saturated hydrocarbons, meaning all available bonds are fully utilized (saturated) with hydrogen atoms.

The boiling point of an alkane is influenced by the molecular structure, including the degree of branching. Branching tends to lower the boiling point of an alkane. This occurs because more branching reduces the surface area of the molecule, which in turn reduces the van der Waals forces (a type of intermolecular force) between the molecules.

These forces are weaker in highly branched alkanes than in unbranched ones since the latter can align more closely together, making them harder to separate—and thereby increasing their boiling points. Essentially, as branching increases, the molecules can't pack together as efficiently, resulting in a lower boiling point for the substance.

Effect of Surface Area

With a smaller surface area, the branched alkanes have fewer interactions with neighbouring molecules, leading to less energy required for the molecules to escape from the liquid to the gas phase.

To compare the boiling points of alkanes, one must acknowledge both the length of the carbon chain and the degree of branching. Generally, a longer carbon chain means a higher boiling point due to increased van der Waals forces throughout the larger molecule. However, this trend is only true for alkanes with similar structures. When comparing different alkanes with varying degrees of branching, the trend can be counterintuitive.

For example, pentane, an unbranched alkane, has a higher boiling point than its isomers 2-methylbutane and 2,2-dimethylpropane, even though all have a similar molar mass. The key to understanding this lies in recognizing that 2,2-dimethylpropane, being the most branched, has the lowest boiling point among the three, while unbranched pentane has the highest.

Analyzing Molecular Arrangements

Therefore, analyzing the molecular arrangement provides insight into the energy necessary to overcome the attractive forces between the alkane molecules, which is a major contributing factor to the boiling points.