Figure 3.3. Pi (π) Bond
Electron density exists above and below the plane of the molecule, restricting rotation about a
double bond.It is important to remember that a π bond cannot exist independently of a σ bond. Only after the
formation of a σ bond will the p-orbitals of adjacent carbons be parallel and in position to form the π
bond. The more bonds that are formed between atoms, the shorter the overall bond length.
Therefore, a double bond is shorter than a single bond, and a triple bond is shorter than a double
bond. Shorter bonds hold atoms more closely together and are stronger than longer bonds; shorter
bonds require more energy to break.
While double bonds are stronger than single bonds overall, individual π bonds are weaker than σ
bonds. Therefore, it is possible to break only one of the bonds in a double bond, leaving a single
bond intact. This happens often in organic chemistry, such as when cis–trans isomers are
interconverted between conformations. Breaking a single bond requires far more energy.
KEY CONCEPT
A double bond consists of both a σ bond and a π bond; a triple bond consists of a σ bond and
two π bonds. π bonds are weaker than σ bonds, but the strength is additive, making double
and triple bonds stronger overall than single bonds.As discussed previously, double and triple bonds do not freely rotate like single bonds. As such,
double bonds in compounds make for stiffer molecules. Partial double-bond character in structures
with resonance also restricts free rotation, resulting in more rigid structures. Proteins exhibit this