Nomenclature, Structure of Double Bond — Definition

Definition

Imagine a world of organic molecules, built primarily from carbon and hydrogen atoms. Among these, we encounter different families based on how these atoms are connected. Alkanes, for instance, are like simple chains where every carbon is linked to another by a single bond. Now, picture a slight twist: what if two carbon atoms decide to share not just one, but two pairs of electrons, forming a 'double bond'? This is precisely what defines a family of hydrocarbons called alkenes.

Alkenes are unsaturated hydrocarbons, meaning they contain fewer hydrogen atoms than the maximum possible for a given number of carbon atoms, due to the presence of at least one carbon-carbon double bond.

Their general formula is $C_nH_{2n}$, where 'n' represents the number of carbon atoms. For example, ethene ($C_2H_4$) is the simplest alkene, with two carbon atoms connected by a double bond, and each carbon also bonded to two hydrogen atoms.

Propene ($C_3H_6$) has three carbons, with a double bond between two of them.

The carbon-carbon double bond is not just a stronger connection; it's fundamentally different in its structure. Each carbon atom involved in the double bond undergoes a special kind of atomic orbital mixing called $sp^2$ hybridization.

This means one $s$ orbital and two $p$ orbitals on each carbon combine to form three new, identical $sp^2$ hybrid orbitals. These three $sp^2$ orbitals arrange themselves in a flat, trigonal planar geometry, with bond angles of approximately $120^circ$.

The remaining unhybridized $p$ orbital on each carbon atom lies perpendicular to this plane.

Now, how does the double bond form? One of the bonds, called a sigma ($sigma$) bond, is formed by the direct, head-on overlap of one $sp^2$ hybrid orbital from each carbon atom. This is a very strong bond.

The second bond, called a pi ($pi$) bond, is formed by the sideways overlap of the two unhybridized $p$ orbitals, one from each carbon. This pi bond is weaker than the sigma bond and lies above and below the plane of the sigma bond framework.

This unique arrangement of sigma and pi bonds is crucial because the pi bond 'locks' the two carbon atoms in place, preventing free rotation around the double bond axis. This restricted rotation is a key concept that leads to different spatial arrangements of atoms, known as geometrical isomerism, which we'll explore in related topics.

Naming these alkenes follows a systematic approach called IUPAC nomenclature. The core idea is to find the longest continuous carbon chain that *contains* the double bond, number it such that the double bond gets the lowest possible number, and then indicate the position of the double bond and any substituents.

The 'ane' suffix of alkanes is replaced with 'ene' for alkenes. For instance, a two-carbon alkene is ethene, a three-carbon alkene is propene, and so on. Understanding this structure and nomenclature is foundational to comprehending the reactivity and properties of alkenes.