Chemistry · General Principles Of Organic Chemistry · NEET
Look at each carbon's hybridisation. An sp2 carbon is trigonal planar, so it and the 3 atoms bonded to it lie in one plane. An sp carbon is linear, so it and its 2 neighbours lie on a straight line (also in any plane through that line). An sp3 carbon is tetrahedral (3D), so the atoms around it cannot all be in one plane. So: count is driven by sp2 and sp centres; every sp3 CH3 or CH2 tends to break flatness.
In ethene both carbons are sp2 (trigonal planar) and joined by a C=C double bond. The p orbitals of the pi bond must be parallel, which locks all 6 atoms (2 C + 4 H) into one flat plane. In ethane both carbons are sp3 (tetrahedral), so the hydrogens point into 3D space and all 8 atoms cannot lie in a single plane.
Ethyne is linear: all 4 atoms (2 C + 2 H) lie on one straight line, which is collinear. A straight line always fits inside a plane, so a collinear set is automatically coplanar too. So in ethyne all atoms are both collinear and coplanar.
Yes, for the atoms attached to it. An sp3 carbon is tetrahedral, so its 4 bonds point to the corners of a tetrahedron, which cannot all sit in one plane. In NEET coplanarity MCQs, spotting even one sp3 CH3 or a chair cyclohexane ring is enough to reject that option as fully coplanar.
Benzene has 6 sp2 carbons in a flat trigonal-planar ring with delocalised pi electrons, so all 6 C and 6 H lie in one plane (12 atoms coplanar). Cyclohexane has 6 sp3 carbons and adopts a puckered chair shape to reduce strain, so its atoms are not coplanar.
In which of the following molecules are all atoms coplanar? (Options are structures: (A) biphenyl (two benzene rings), (B) a cyclohexane-containing molecule, (C) a cyclohexane-containing molecule, (D) an alkene with CH3 / dinitrile groups.)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Coplanarity means a set of atoms all lie in the same single flat plane. It is decided by hybridisation: sp and sp2 centres keep atoms flat, while sp3 centres are tetrahedral and push atoms into 3D.
sp2 (trigonal planar) and sp (linear) give coplanar arrangements. sp3 (tetrahedral) does not, because four bonds cannot all fit in one plane.
All 12 atoms (6 carbons + 6 hydrogens) are coplanar because every carbon is sp2 and the ring is flat with delocalised pi electrons.
Yes. Ethyne (HC≡CH) is linear, so its 4 atoms are collinear, and any set of collinear atoms also lies in a plane, so they are coplanar.
NEET repeatedly asks how many atoms lie in one plane, which molecule is fully planar, or which atoms are coplanar. You solve these quickly by reading hybridisation (sp2/sp = flat, sp3 = not flat) instead of building models.
