NJC Organic Chem 2026

2026 Organic Chemistry Review NJC SH2 H2 Chemistry 1 Organic Chemistry Review Hybridisation • Hybridisation is a model used to explain the observed shapes of molecules about the central atom. • Hybridisation model shows mixing of pure atomic orbitals in an atom to generate a set of degenerate hybrid orbitals (hybrid orbitals are of same energy level). General shape of sp3 / sp2 / sp hybrid orbital In the formation of covalent bonds, only the front lobe is used to overlap with an orbital of another atom as the front lobe is bigger and results in a more effective overlap. • No. of atomic orbitals mixed = No. of hybrid orbitals formed Hybrid orbitals take part in sigma (σ) bonding only. Pi bonds ( ) are formed by unhybridised p orbitals. Arrangement of hybrid orbitals around the central atom Total no. of electron regions Hybridisation of central atom Formation of hybrid orbitals and arrangement of hybrid orbitals around the central atom Let’s practice! 4 sp3 3 sp2 Note: There is one unhybridised p orbital for  bonding 2 sp Note: There are two unhybridised p orbitals for  bonding Note: For C atom, hybrid of 2s and 2p orbitals produces 2sp / 2sp2 / 2sp3 hybrid orbitals where “2” represents the principal quantum number (e.g. 2nd electron shell) front lobe back lobe

2026 Organic Chemistry Review NJC SH2 H2 Chemistry 2 O and N atoms can undergo hybridisation similar to carbon. The type of hybridisation depends on the number of electron regions (bond pair regions + lone pair regions) around the atom. Number of electron regions Hybridisation Examples 4 sp3 C OH H H H  sp3 C and sp3 O C NH H H H H  sp3 C and sp3 N 3 sp2 C O H H  sp2 C and sp2 O C N H H H  sp2 C and sp2 N 2 sp C NH  sp C and sp N Drawing of orbitals overlap in formation of sigma and pi bond C C H H H H Central atoms Both C are sp2 hybridised (3 e− regions) Bonds Orbitals involved Drawing of overlap of orbitals to form bond C−H 2sp2 hybrid orbital of C head-on overlap with 1s orbital of H C=C sigma bond : 2sp2 hybrid orbital of C head-on overlap with 2sp2 hybrid orbital of C pi bond : unhybridised 2p orbital of C side-way overlap with unhybridised 2p orbital of C

2026 Organic Chemistry Review NJC SH2 H2 Chemistry 3 Various representations of organic compounds Type of Formulae Representation Characteristics Molecular Formula C3H6O3 • does not show how the various atoms in the organic compound are connected to one another Condensed Formula CH3CH(OH)CO2H • shows the order of arrangement of atoms • bonds are not displayed Displayed / Full structural Formula • shows ALL atoms and bonds in the molecule. Skeletal Formula • Carbon atoms in a straight chain are drawn in a zigzag manner • each end of a line represents a carbon if other symbol of an atom is not written • C–H bonds at each C