Question

Explain why alkanes are relatively unreactive.

Short answer

Alkanes are relatively unreactive due to their stable molecular structure, strong and nonpolar C-H and C-C bonds, and the lack of readily accessible reactive sites. They neither have significant dipole moments nor contain double or triple bonds or functional groups, making them less prone to react with polar reagents. Although alkanes can participate in combustion, substitution, and free-radical reactions, these processes usually require high activation energy or a catalyst to overcome the energy barrier created by their strong bonds.

Step-by-step solution

Introduction to Alkanes

Alkanes are a class of organic compounds comprising only carbon and hydrogen atoms, arranged in single bonds (C-H and C-C). They are also known as saturated hydrocarbons because they contain the maximum possible hydrogen atoms for the carbon atoms present. Alkanes have the general molecular formula \(C_nH_{2n+2}\), where n represents the number of carbon atoms.

Molecular Structure and Bonding

Alkanes are built up of carbon atoms, which are sp3 hybridized. Each carbon atom forms four single sigma bonds, either with other carbon atoms or hydrogen atoms. The C-H and C-C bonds are strong and have bond energies of roughly 413 kJ/mol and 348 kJ/mol, respectively. This results in a stable and low-energy molecular arrangement. Moreover, alkanes are nonpolar due to the small electronegativity difference between carbon and hydrogen atoms. As a result, there isn't a significant charge separation in the C-H bond, which means that alkanes do not possess any strong dipole moment, making their molecules relatively unreactive towards polar reagents.

Lack of Reactive Sites

Alkanes are composed entirely of single bonds, meaning they lack readily accessible reactive sites, such as double or triple bonds between carbon atoms, which are more susceptible to reaction. They also lack functional groups (like -OH, -COOH, -NH2, etc.), which are more chemically active due to localized electron density or the presence of lone pair electrons.

Reactions of Alkanes

Despite their low reactivity, alkanes can undergo a few types of reactions, most notably combustion, substitution, and free radical reactions. However, these reactions usually require high activation energy or the presence of a catalyst to overcome the energy barrier produced by the strong C-H and C-C bonds.

Conclusion

Alkanes are relatively unreactive due to their molecular structure and bonding characteristics. They contain strong, nonpolar carbon-hydrogen bonds, which are not easily broken and do not react readily with polar reagents. Furthermore, they lack reactive sites such as double or triple bonds, or functional groups, which would make them more chemically active. Their reactivity is mostly limited to combustion, substitution, and free-radical reactions, often requiring the input of high energy or a catalyst to proceed.