Lesson 1 of 13 · 7 min

Carbon's tetravalence and the shapes of organic molecules

NCERT § "Tetravalence of Carbon: Shapes of Organic Compounds"

Acetic acid has only two carbon atoms, yet they are not alike. One sits at the centre of a tetrahedron; the other lies flat in a plane. Same element, same molecule, two different shapes.

The story this chapter follows: A bottle of kitchen vinegar

Every kitchen shelf in India has a bottle of vinegar for pickles and chutneys: about 5 % acetic acid, CH₃COOH, in water. That one small molecule carries this whole chapter: how carbon bonds, how we draw and name it, why it is an acid at all, and how a chemist would purify it and prove what it is made of.

The lesson in notes

In short

Carbon is tetravalent and forms covalent bonds using hybrid orbitals: sp³ (tetrahedral, as in CH₄), sp² (trigonal planar, as in C₂H₄) and sp (linear, as in C₂H₂).

s character is 25% in sp³, 33% in sp² and 50% in sp; more s character holds electrons closer to the nucleus.

An sp-hybridised carbon is therefore more electronegative than sp², which is more electronegative than sp³.

More s character also means shorter and stronger bonds formed by that carbon.

A π bond forms by sideways overlap of unhybridised p orbitals; the two atoms joined by it and the atoms attached to them must lie in one plane, and rotation about the double bond is restricted.

π electrons lie above and below the bond axis and are loosely held, so multiple bonds are reactive centres attacked by electron-seeking reagents.