Alkenes are significantly more reactive than alkanes, a difference foundational to organic chemistry. Both are hydrocarbons, composed exclusively of carbon and hydrogen atoms. Alkanes are saturated, meaning their carbon atoms are linked only by single bonds, holding the maximum number of hydrogen atoms. Alkenes are unsaturated, containing at least one carbon-carbon double bond. This structural variation directly accounts for the difference in their chemical behavior.
The Stable Structure of Alkanes
Alkanes owe their chemical stability to their bonding arrangement. Every bond is a single bond, known as a sigma bond. Sigma bonds form from the head-on overlap of atomic orbitals, concentrating electron density squarely between the two atomic nuclei. This strong, centralized sharing results in high bond energy, making the bonds difficult to break.
The energy required to cleave these carbon-carbon or carbon-hydrogen sigma bonds is substantial, so alkanes do not readily interact with most chemical reagents. They are often described as paraffinic, reflecting their lack of affinity for reaction. Since their electrons are tightly held and localized, alkanes lack a readily accessible site for chemical attack, making them relatively unreactive at room temperature without significant external energy input, such as heat or ultraviolet light.
The Nature of the Alkene Pi Bond
The increased reactivity of alkenes stems from the carbon-carbon double bond. This double bond is not twice as strong as a single bond because it consists of two different connections: one strong sigma bond and one comparatively weaker pi bond. The sigma bond holds the two carbon nuclei together along the central axis. The pi bond forms from the sideways overlap of the remaining p-orbitals.
This lateral overlap is less efficient than the head-on overlap of a sigma bond, making the pi bond significantly weaker and requiring less energy to break. The electrons of the pi bond are not held directly between the nuclei. Instead, the electron density is distributed in two cloud-like regions located above and below the plane of the molecule, leaving the pi electrons exposed and highly accessible.
This exposed electron density creates a region of high negative charge around the double bond. This electron-rich area acts as a nucleophile, making it a prime target for electrophiles (electron-loving chemical species). The presence of this readily available, weak, and exposed set of electrons dramatically lowers the activation energy required for a reaction. The pi bond thus functions as the reactive site, making the alkene far more chemically dynamic than an alkane.
Manifestation of Reactivity: Addition vs. Substitution
The structural difference dictates the type of chemical reactions these hydrocarbons undergo. Alkanes, due to their robust sigma bonds, primarily react through substitution. This mechanism requires replacing one atom or group with another. This process often involves free radicals and necessitates harsh conditions, such as high heat or ultraviolet light, to initiate the cleavage of a strong sigma bond.
Alkenes favor a much faster, lower-energy process called an addition reaction. In this mechanism, the weak pi bond readily breaks, allowing two new single bonds to form across the two double-bonded carbon atoms. The added molecule attaches to the carbons, converting the double bond into a single bond. This reaction occurs quickly under mild conditions, often at room temperature and without the need for catalysts or light.
This difference is clearly demonstrated by the reaction with bromine water, an orange-brown solution. When bromine is added to an alkane, no immediate reaction occurs because the molecule cannot break the strong sigma bonds to substitute a hydrogen atom. Conversely, when bromine is added to an alkene, the solution is instantly decolorized. The bromine molecule rapidly adds across the double bond, consuming the reactant without high energy input.