Difference Between Sigma and Pi Bonds: Key Features, Strength & Occurrence
Pi bonds are a crucial part of chemical bonding and molecular structure. They help students understand how atoms connect in molecules, why certain substances are reactive, and the reasons behind the properties of double and triple bonds in chemistry.
What is Pi Bond in Chemistry?
- A pi bond in chemistry is a type of covalent bond formed when two parallel p orbitals on adjacent atoms overlap sideways.
- This sideways (lateral) overlap leads to electron density above and below the axis connecting the nuclei of the bonded atoms.
- Pi bonds are commonly found alongside sigma bonds in molecules with double bonds (like alkenes) and triple bonds (like alkynes).
- Concepts involving pi bonds appear in chapters related to covalent bonding, organic chemistry, and molecular hybridization, making pi bonding a foundational chemistry topic.
Molecular Formula and Composition
- Pi bonds do not have a separate molecular formula, as they are a part of a molecule’s overall bonding structure.
- For example, in ethylene (C2H4), there's a double bond between carbon atoms—consisting of one sigma bond and one pi bond—while in acetylene (C2H2), a triple bond means there are one sigma and two pi bonds.
- Pi bonds are always associated with elements having available p orbitals, usually carbon, nitrogen, and oxygen.
Preparation and Synthesis Methods
- Pi bonds are not 'prepared' like a chemical substance, but they form naturally when unsaturated molecules are created.
- During the formation of a double bond, the first bond formed is always a sigma bond.
- Once that is in place, if unhybridized p orbitals are available, a pi bond forms by the sideways overlap of these p orbitals.
- This is seen in the synthesis of alkenes via elimination reactions or during processes like hydrogenation (where a pi bond might break, converting a double bond to a single bond).
Physical Properties of Pi Bonds
Pi bonds impact the physical properties of molecules. They cause restricted rotation around the double or triple bond, lead to planar structures, and often make compounds more reactive than simple single-bonded (sigma only) molecules.
Pi bonds also influence properties such as boiling point, melting point, and molecular polarity by determining the overall shape and electron distribution.
Chemical Properties and Reactions
Molecules with pi bonds (like alkenes and alkynes) are generally more chemically reactive. Pi bonds are weaker than sigma bonds, so they are often broken during addition reactions.
For example, when ethylene reacts with bromine, the pi bond breaks, and two bromine atoms add across the double bond. Pi bonds also participate in reactions like polymerization and hydrogenation, which are important in industrial chemistry.
Frequent Related Errors
- Confusing pi bonds with sigma bonds and not recognizing that every double bond has one pi and one sigma bond.
- Believing that pi bonds are stronger than sigma bonds—they are actually weaker due to less overlap.
- Missing pi bonds in Lewis structures, especially in rings or when triple bonds are present.
- Thinking pi bonds can exist alone, when in reality a pi bond always needs a sigma bond first as the base bond.
Uses of Pi Bonds in Real Life
- Pi bonds are found in many important compounds, from the ethylene used to ripen fruits to the acetylene used in welding torches.
- The presence of pi bonds affects the reactivity and physical characteristics of plastics, rubbers, and pharmaceuticals.
- Many dyes and pigments rely on the electronic structures created by pi bonding across large, conjugated systems. Understanding pi bonds is also essential for organic synthesis and materials science.
Relation with Other Chemistry Concepts
Pi bonds are closely connected to topics such as sigma bond, hybridization, Lewis structures, and types of chemical bonds. Understanding how pi and sigma bonds combine helps explain molecular geometry and the reactivity seen in organic compounds.
Step-by-Step Reaction Example
1. Consider the addition of bromine (Br2) to ethene (C2H4).2. The pi bond breaks as electrons from the pi bond attack the bromine molecule.
3. A bromonium ion intermediate forms and then reacts with a second bromide ion.
4. The product is 1,2-dibromoethane, in which the double bond no longer exists—both carbon atoms now have single (sigma) bonds to bromine.
Lab or Experimental Tips
An easy way to identify a pi bond is to look for multiple bonds in a structure. Each double bond has one sigma and one pi bond, and each triple bond has one sigma and two pi bonds.
In live Vedantu classes, educators often use colored markers to highlight sidewise p-orbital overlaps and help students visualize which areas of a molecule contain pi bonds.
Try This Yourself
- Count the number of pi bonds in benzene (C6H6).
- Is there any pi bond in methane (CH4)? Why or why not?
- Draw the structure of oxygen (O2) and mark the pi bond.
Final Wrap-Up
We explored pi bonds—what they are, how they form, their role in chemistry, and why understanding them is essential for advanced topics. To study more about molecular structure and bonding or get live help for your queries, explore interactive resources and classes from Vedantu for a clear and complete understanding.
Difference Between Sigma and Pi Bonds
| Feature | Sigma (σ) Bond | Pi (π) Bond |
|---|---|---|
| Formation | Head-on overlap of orbitals | Sideways overlap of p orbitals |
| Location of Electron Density | Along bond axis | Above and below bond axis |
| Strength | Stronger | Weaker |
| Rotation Around Bond | Free rotation possible | Prevents rotation |
| Existence | Can exist alone | Exists only with sigma bond |
| Example | Single bond in ethane (C2H6) | Second bond in ethene (C2H4) |
Interlinked Chemistry Concepts
FAQs on Pi Bonds Explained: Definition, Formation & Examples
1. What is a pi bond in chemistry?
A pi bond in chemistry is a form of covalent bond created when two parallel p orbitals overlap sideways. Pi bonds occur alongside sigma bonds in double and triple bonds, increasing bond strength and influencing molecular shape.
Key points:
- Sideways overlap of p orbitals
- Found in double and triple bonds
- Weaker than sigma bonds
2. How are pi bonds formed?
Pi bonds are formed when two unhybridized p orbitals on adjacent atoms overlap sideways (laterally) rather than end-to-end. This overlap occurs above and below the plane of the atoms involved.
Key steps:
- Atoms form a sigma bond first.
- Unhybridized p orbitals then overlap side-by-side.
- The resulting pi bond is weaker due to less orbital overlap.
3. Are pi bonds present in double and triple bonds?
Yes, pi bonds exist in both double and triple bonds.
- Double bond: 1 sigma bond + 1 pi bond
- Triple bond: 1 sigma bond + 2 pi bonds
Pi bonds always occur in addition to a sigma bond and never alone.
4. How are sigma and pi bonds different?
Sigma and pi bonds differ in formation, strength, and spatial orientation.
Comparison:
- Sigma bond: Head-on (axial) overlap, stronger, always first bond between atoms
- Pi bond: Sideways (lateral) overlap of p orbitals, weaker, only present after a sigma bond is formed
5. Are pi bonds weaker than sigma bonds?
Yes, pi bonds are weaker than sigma bonds. This is because the sideways overlap in pi bonds is less effective than the head-on overlap in sigma bonds.
Effects:
- Lower bond dissociation energy
- Pi bonds are more reactive and easier to break
6. How do you determine the number of pi bonds in a molecule?
To determine the number of pi bonds in a molecule, check for double and triple bonds in its Lewis structure.
Quick guide:
- Each double bond contains 1 pi bond
- Each triple bond contains 2 pi bonds
- Sigma bonds are present in all single bonds and as one component of double/triple bonds
7. Why can't pi bonds form without a sigma bond already present?
Pi bonds require a pre-existing sigma bond because sigma bonds hold the atoms close enough for the p orbitals to overlap sideways and create a pi bond. Without a sigma bond, the atomic orbitals would be too far apart for effective pi bonding.
8. Why do pi bonds restrict rotation around double bonds?
Pi bonds restrict rotation because the sideways overlap of p orbitals must remain parallel for the pi bond to exist. Any rotation would break the pi overlap, making free rotation around double bonds impossible.
9. How does the presence of pi bonds affect molecule reactivity?
Pi bonds increase molecule reactivity because their electrons are more exposed and accessible to reactants. This makes double and triple bonds more reactive in addition and substitution reactions compared to single bonds.
10. Can pi bonds exist between different elements, not just carbon?
Yes, pi bonds can be formed between different elements, not just carbon. For example, oxygen (in O2) and nitrogen (in N2) molecules have pi bonds due to p orbital overlap.
11. What are some examples of pi bonds in organic molecules?
Examples of pi bonds in organic molecules include:
- Ethene (C2H4): Contains one pi bond in the C=C double bond
- Ethylene (acetylene, C2H2): Contains two pi bonds in the C≡C triple bond
- Benzene: Contains pi bonds as part of its aromatic ring structure
12. Why does the presence of a pi bond lower the bond dissociation energy compared to a single bond?
The presence of a pi bond lowers bond dissociation energy because the sideways overlap in pi bonds is weaker and more easily broken than the head-on overlap in sigma bonds. As a result, double and triple bonds (with pi components) require less energy to break than expected if both bonds were sigma type.
