Characteristic Features of Pi Bonds and Their Reactivity
Chemistry · General Principles Of Organic Chemistry · NEET
A pi bond forms by sideways (lateral) overlap of two p orbitals, so its electron cloud sits above and below the bond axis, not between the atoms. This makes it weaker than a sigma bond (about 284 kJ/mol versus 397 kJ/mol in a C=C bond) and its exposed electrons make it the reactive part of double and triple bonds. Memory hook: "pi = exposed and loose, so it reacts first."
A C=C double bond has one strong sigma bond (blue, head-on overlap between the nuclei) and one weaker pi bond (red, sideways overlap forming lobes above and below the axis). The exposed pi electrons make the double bond reactive.
Your doubts, answered
Why is a pi bond weaker than a sigma bond?
A sigma bond forms by head-on (axial) overlap, so the orbitals overlap to a large extent directly between the two nuclei. A pi bond forms by sideways overlap of parallel p orbitals, which overlap to a much smaller extent. Less overlap means less shared electron density holding the atoms, so the pi bond is weaker. In a C=C bond the sigma part is about 397 kJ/mol while the pi part is only about 284 kJ/mol.
Why are pi bonds more reactive than sigma bonds?
The pi electron cloud lies above and below the line joining the two atoms, not tucked between them. These exposed, loosely held electrons are easy for an attacking reagent to reach. So electrophiles (electron-loving reagents) attack the pi bond first. This is why alkenes and alkynes readily undergo addition reactions, while alkanes (only sigma bonds) are unreactive.
Can atoms rotate around a pi bond?
No. Free rotation is possible around a single (sigma) bond, but not around a double or triple bond. Rotating the atoms would twist the two parallel p orbitals out of alignment and break the sideways overlap, destroying the pi bond. This fixed geometry is exactly why cis-trans (geometrical) isomerism exists in alkenes.
Where exactly are the pi bond electrons?
They are in the pi molecular orbital, which is two lobes of electron density lying above and below the plane that contains the two bonded atoms. The pi bond is NOT symmetrical about the bond axis, unlike a sigma bond. This off-axis position is the reason pi electrons are available for resonance and for attack by reagents.
In a double bond, which bond breaks first?
The pi bond breaks first because it is the weaker of the two. A double bond is one sigma plus one pi bond. During an addition reaction the weak pi bond opens up and the two carbons form new sigma bonds to the added groups, while the original sigma bond stays intact. A triple bond has one sigma and two pi bonds, so it can add across twice.
⚠️ The NEET trap ✗ A pi bond is stronger than a sigma bond because a double bond is stronger than a single bond. ✓ The double bond as a whole is stronger and shorter than a single bond, but within it the pi component alone is weaker than the sigma component. The extra strength comes from adding a pi bond on top of the sigma, not from the pi bond being strong by itself. 🧠 Double bond stronger overall, but the pi part alone is the weak, reactive one.
Real NEET questions
2019
The number of sigma (σ) and pi (π) bonds in pent-2-en-4-yne is
A · 10 σ bonds and 3 π bonds ✓
B · 8 σ bonds and 5 π bonds
C · 11 σ bonds and 2 π bonds
D · 13 σ bonds and no π bonds
Solution: Pent-2-en-4-yne is CH3-CH=CH-C≡CH. Sigma bonds: 4 C-C single/skeleton sigma bonds + 6 C-H sigma bonds = 10 sigma. Pi bonds: 1 from the C=C double bond + 2 from the C≡C triple bond = 3 pi. So 10 sigma and 3 pi bonds. Remember: every double bond has 1 pi, every triple bond has 2 pi.
2026
The carbocation C6H5-CH(+)-CH3 (1-phenylethyl cation) is stabilized by the interaction of the empty p orbital with:
A · filled σ and filled π orbitals ✓
B · empty σ and empty π* orbitals
C · empty σ* and filled π orbitals
D · empty σ* and empty π* orbitals
Solution: The empty p orbital on the positive carbon is stabilised two ways: by resonance with the filled pi orbitals of the benzene ring, and by hyperconjugation with the filled sigma (C-H) orbitals of the neighbouring CH3 group. Both donors are FILLED orbitals, so option A. This shows how exposed pi electrons donate into an empty orbital.
Solved General Principles Of Organic Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes, for the bonds you meet in NEET organic chemistry. Because sideways overlap is always less effective than head-on overlap, the pi component is weaker than the sigma component. This is why pi bonds are the site of reaction.
How many pi bonds are in a double and a triple bond?
A double bond has 1 sigma + 1 pi. A triple bond has 1 sigma + 2 pi. A single bond is a pure sigma bond with no pi bond.
Do pi bonds decide molecule shape?
No. Shape and bond angles are set by the sigma framework and hybridisation. The pi bond only sits on top of an already-formed sigma bond and does not change the geometry, but it does lock rotation.
Why do alkenes and alkynes react easily but alkanes do not?
Alkanes have only strong, buried sigma bonds and are unreactive. Alkenes and alkynes have exposed, weaker pi bonds that reagents can attack, so they readily undergo addition reactions.
