9.3.1. Structure of Double Bond

Carbon-carbon double bond in alkenes consists of one strong sigma (σ) bond (bond enthalpy about 397 kJ mol–1) due to head-on overlapping of sp2 hybridised orbitals and one weak pi (π) bond (bond enthalpy about 284 kJ mol–1) obtained by lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms.

A carbon-carbon double bond has two components: the sigma bond and the pi bond. The sigma bond is formed when sp² hybridized orbitals from two carbon atoms overlap directly, creating a strong bond. The pi bond, however, is weaker because it forms from the sideways overlap of p orbitals from the carbon atoms, making it less stable. This distinction affects how alkenes react and behave.

Think of the sigma bond as a strong handshake between two people, where they hold onto each other firmly. The pi bond, in contrast, is like two people trying to hold hands from a distance; it's not as stable and can be easily broken.

The double bond is shorter in bond length (134 pm) than the C–C single bond (154 pm). You have already read that the pi (π) bond is a weaker bond due to poor sideways overlapping between the two 2p orbitals.

Double bonds are shorter than single bonds because the two bonds pull the connected atoms closer together, creating a stronger attraction. However, the pi bond's distance and strength mean it can easily break when the double bonded carbon reacts with other substances, showcasing the reactive nature of alkenes.

Imagine a double bond as a couple who are closely linked (the stronger bond of a handshake) compared to a single bond, where the couple holds hands but are further apart. When someone tries to interact with them, it’s easier to break the hand-holding (the pi bond) than to sever the bond of their close embrace (the sigma bond).

Thus, the presence of the pi (π) bond makes alkenes behave as sources of loosely held mobile electrons. Therefore, alkenes are easily attacked by reagents or compounds which are in search of electrons.

The weaker pi bond in alkenes makes them more reactive compared to alkanes. The loosely held electrons in the pi bond are readily available to react with other substances known as electrophiles, making alkenes prime candidates for addition reactions.

Think of the pi bond as an open invitation at a party, where guests (electrophiles) are eager to join the fun by grabbing those loosely held electrons. In contrast, the sigma bond is like a strict bouncer who only lets people in if they know them well, meaning that alkanes are less reactive and don’t have that same open invitation.

Strength of the double bond (bond enthalpy, 681 kJ mol–1) is greater than that of a carbon-carbon single bond in ethane (bond enthalpy, 348 kJ mol–1).

While double bonds are generally stronger than single bonds, they are not invincible. Alkenes can undergo reactions where the double bond is broken, which usually requires lower energy input compared to breaking the stronger single bonds found in alkanes. This contrast highlights the unique properties of alkenes in chemical reactions.

Imagine trying to chop through a thick rope (the strong single bond) versus a thin, flexible string (the double bond). The thick rope resists cuts and takes significant effort to break, while the thin string can easily be snipped, akin to how alkenes readily react and change compared to alkanes.

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