Why Alkanes Are More Reactive Than Alkenes

The world of organic chemistry often presents counterintuitive scenarios. One such puzzle is the assertion “Why Alkanes Are More Reactive Than Alkenes.” It seems paradoxical, given that alkenes possess a pi bond, a region of electron density seemingly ripe for reaction. However, the reality is nuanced. Alkanes, typically considered inert, can exhibit greater reactivity than alkenes under specific, often harsh, conditions. This article explores the reasons behind this unexpected behavior, delving into the unique conditions that unlock alkane reactivity.

The Misunderstood Reactivity of Alkanes Compared to Alkenes

The apparent inertness of alkanes stems from their strong sigma bonds. These bonds, formed by the direct overlap of atomic orbitals, are incredibly stable and require substantial energy to break. Alkanes consist solely of carbon-carbon and carbon-hydrogen single bonds, both of which are relatively non-polar. This lack of polarity means there are no significant partial charges within the molecule, making them less susceptible to attack by electrophiles or nucleophiles. Therefore, under typical laboratory conditions, where reactions are often driven by charge interactions, alkanes remain largely unreactive.

However, the stability of alkanes can be overcome under specific circumstances, most notably through radical reactions. These reactions, often initiated by heat or light, involve the formation of highly reactive species called free radicals. Free radicals are atoms or molecules with unpaired electrons, desperately seeking to pair up with another electron. This desire makes them incredibly aggressive and capable of abstracting hydrogen atoms from alkanes, initiating a chain reaction that can lead to a variety of products. The key to alkane reactivity lies in these radical mechanisms, which bypass the need for strong electrophilic or nucleophilic attack. The relative unreactivity in normal lab conditions can be outlined:

• Alkanes: Generally unreactive due to strong, non-polar sigma bonds.
• Alkenes: More reactive due to the presence of a pi bond and the possibility of electrophilic addition.

In contrast, alkenes are more susceptible to reactions involving electrophiles due to the presence of pi bond. Pi bonds are weaker than sigma bonds and represent a region of high electron density above and below the plane of the sigma bond. This electron density makes alkenes attractive to electrophiles, which are electron-seeking species. Electrophilic addition reactions are a common and characteristic reaction of alkenes, where the electrophile attacks the pi bond, forming new sigma bonds. This difference in reactivity is summarized below:

To further understand the nuanced reactions of both Alkanes and Alkenes, we suggest reading more at LibreTexts Chemistry. There, you can find detailed mechanisms and explanations of radical reactions and electrophilic additions, providing a deeper insight into the reactivity differences of these two important classes of organic molecules.