Short Answer

The bond that restricts rotation in molecules is the pi bond. Because a pi bond is formed by sideways overlap of parallel p orbitals, its electron cloud lies above and below the internuclear axis. This special arrangement prevents the bonded atoms from freely rotating without breaking the overlap.

Sigma bonds, on the other hand, allow free rotation because they are formed by head-on overlap. But when a pi bond is present—as in double or triple bonds—the rotation becomes restricted, giving the molecule a fixed shape.

Detailed Explanation :

Bond That Restricts Rotation in Molecules

In molecules, the bond responsible for restricting rotation is the pi (π) bond. Rotation is restricted because the pi bond is formed from the sideways overlap of unhybridized p orbitals, which creates electron density above and below the bond axis. Any attempt to rotate the atoms would break this delicate sideways overlap, making rotation impossible without breaking the pi bond.

This restriction is especially important in molecules with double and triple bonds. While the sigma bond provides the main strength in a covalent bond, the pi bond adds rigidity and defines the molecule’s shape so that rotation becomes energetically unfavorable. Understanding this concept is essential in organic chemistry because it explains why certain molecules have fixed geometries and cannot freely rotate.

1. Why Pi Bonds Restrict Rotation

A pi bond forms when p orbitals overlap sideways.
This creates a bonding region:

• Above the internuclear axis
• Below the internuclear axis

This electron cloud is continuous and must remain aligned for the pi bond to exist.

If rotation occurs:

• The p orbitals would no longer remain parallel.
• Sideways overlap would be destroyed.
• The pi bond would break.

Because breaking a pi bond requires energy, natural rotation does not happen.
This is why molecules with pi bonds are rigid.

2. Sigma Bonds Allow Rotation, Pi Bonds Do Not

To understand why pi bonds restrict rotation, it helps to compare them with sigma bonds.

Sigma Bonds (σ)

• Formed by head-on overlap
• Electron density lies directly between nuclei
• Allows free rotation because rotation does not destroy overlap

Example:
Ethane (C₂H₆) rotates freely around its C–C single bond.

Pi Bonds (π)

• Formed by sideways p-orbital overlap
• Electron cloud lies above and below the bond axis
• Rotation destroys overlap → rotation is restricted

Example:
Ethene (C₂H₄) cannot rotate around the C=C bond.

Thus, pi bonds only allow rotation if they are broken, which requires significant energy.

3. Rotation Restriction in Double and Triple Bonds

Double Bond (One σ + One π bond)

• Sigma bond allows rotation.
• Pi bond prevents rotation.
• Therefore, double bonds are rigid.

This leads to geometric isomerism (cis-trans or E/Z isomers) in alkenes.

Example:

• Cis-2-butene and trans-2-butene exist because rotation is not possible.

Triple Bond (One σ + Two π bonds)

• Two pi bonds create even more restriction.
• However, because of linear geometry (sp hybridization), rotation is still not possible.

Thus, both double and triple bonds restrict rotation.

4. Why Rotation Restriction Is Important

Rotation restriction due to pi bonds affects many chemical and physical properties:

(a) Geometric Isomerism

Molecules like alkenes show cis and trans forms because atoms cannot rotate around the double bond.

(b) Fixed Molecular Shape

Pi bonds create rigidity that helps molecules maintain specific shapes important for:

• Biological activity
• Drug design
• Polymer properties

(c) Reactivity

Pi bonds are more reactive because their electron cloud is exposed.
This makes them targets in addition reactions.

(d) Structural Stability

Restricted rotation strengthens molecular frameworks in aromatic rings and conjugated systems.

5. Additional Examples of Restricted Rotation

1. Benzene Ring
   • Alternating pi bonds form a delocalized system
   • No free rotation of C–C bonds
2. Amide Bond (–CONH–)
   • Partial double bond character
   • Strong pi-bond-like behavior
   • Rotation restricted, making peptides rigid
3. Alkenes and Alkynes
   • All multiple bonds restrict rotation

These examples show that anywhere a pi bond exists, rotation becomes limited.

6. Why Pi Bonds Are Easily Broken in Reactions

Although pi bonds prevent rotation, they are weaker than sigma bonds.
Because they lie outside the internuclear axis:

• They are easier to attack by reagents
• They often break during addition reactions

But under normal conditions (without breaking the pi bond), rotation cannot occur.

Conclusion

The bond that restricts rotation in molecules is the pi bond. This is because pi bonds are formed by the sideways overlap of p orbitals, which creates electron density above and below the bond axis. Rotating the atoms would destroy this overlap, so pi bonds lock molecules into fixed positions. Sigma bonds allow rotation, but once a pi bond is present—as in double or triple bonds—rotation becomes restricted, giving molecules rigidity and allowing geometric isomerism.