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 are not shown. • each C forms 4 bonds. Any “missing” bond not drawn out in a skeletal structure would be a C –H bond. Isomerism

2026 Organic Chemistry Review NJC SH2 H2 Chemistry 4 Cis-trans isomerism Requirement : 2 different groups attached to each C in the restricted bond (C=C) We must show the trigonal planar arrangement around the C=C for cis and trans isomers. C C H Br Br H C C H Br H Br cis trans Note: C=C in a ring cannot exhibit cis-trans isomerism Optical isomerism Requirement : A chiral carbon (C atom bonded to 4 unique groups) We must show the 3-D tetrahedral arrangement around the chiral C for optical isomers. Optical activity of organic molecules Racemic mixture Equal amounts of the 2 enantiomers -Rotation of plane of plane -polarised light by each enantiomer cancels out completely 50 : 50 ratio Meso compound A compound with 2 chiral carbons AND internal plane of symmetry -Rotation of plane of plane-polarised light by each chiral carbon cancels out completely by the other. Organic Molecule Chiral Carbon Racemic mixture Optically inactive Meso compound Optically inactive Not meso/ racemic Optically active No Chiral carbon Optically inactive Note : Alkenes with four different substituents across C=C also exhibit cis-trans isomerism. Another naming convention (E / Z) is used instead. **E/Z naming is not in H2 syllabus.

2026 Organic Chemistry Review NJC SH2 H2 Chemistry 5 Reaction Mechanisms for H2 Chemistry A chemical equation shows the reactants and products of a chemical reaction in their stoichiometric ratios. A reaction mechanism shows us the detailed reaction process which is not reflected in a chemical equation. Functional Group Reactive Site Species reacting with the C atom Reaction Type Alkane C C H H H H H H Non-polar C−H bond Free Radical Free Radical Substitution Alkene C C H H H H  electrons in C=C act as source of electrons e− rich (-ve) C attracts electrophile (+ve) Electrophile (electron deficient) Electrophilic Addition (break C=C  bond) Arene Delocalised  electron cloud in ring results in a region of high electron density e− rich (-ve) C attracts electrophile (+ve) Electrophile (electron deficient) Electrophilic Substitution Halogenoalkanes C C H H H H Br H + C in the polar C−Halogen bond e− deficient (+) C attracts nucleophile (-ve) Nucleophile (electron rich) Nucleophilic Substitution Carbonyl Compounds C C H H H O H + C in the polar C=O bond e− deficient (+) C attracts nucleophile (-ve) Nucleophile (electron rich) Nucleophilic Addition (break C=O  bond) Note: Benzene rings prefer to undergo substitution rather than addition due to the resonance stabilisation from delocalisation of the -electron cloud in benzene. Addition reaction would disrupt the aromatic stability and hence it is not favourable. + - + -

2026 Organic Chemistry Review NJC SH2 H2 Chemistry 6 To describe the mechanism of a known organic reaction (i) Name the type of reaction mechanism undergone. (ii) Show reaction step-wise using equations – include intermediates or transition states. Indicate the slow step where necessary. (iii) Indicate + and - on the polar bond undergoing reaction (if any). (iv) Show movement of arrows: Electron flow from electron donor (bond or lone pair electrons) to electron acceptor (atom or existing bond) Breaking bonds (arrow start from the covalent bond) Homolytic (Half arrow to each atom) and produce radicals Cl Cl 2 Cl Heterolytic (Full arrow to the more electronegative atom) I Cl I + + Cl − + - Forming bonds (Arrow start from lone pair electrons to the destination atom) H C H H OH−.. H C H H OH+ Breaking and forming bonds C H H H  Br Br C H H H Br Br C CH H H H H Br C CH H H H H + Br−+ − + A  bond is formed when an electron pair moves to an existing sigma bond FYI

2026 Organic Chemistry Review NJC SH2 H2 Chemistry 7 Mechanism: Free Radical Substitution (Alkane) • Free radicals are atoms or groups of atoms which have a single unpaired electron. • Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. (homolytic fission) E.g. Describe the mechanism of free radical substitution between propane, C3H8 and Cl2 (g), UV light to give 2−chloropropane Free Radical Substitution Stage 1: Initiation Cl Cl 2Cl Stage 2: Propagation CH3CH2CH3 + Cl• → CH3CHCH3 + HCl CH3CHCH3 + Cl2 → CH3CHClCH3 + Cl• Stage 3: Termination 2 Cl• → Cl2 Cl• + CH3CHCH3 → CH3CHClCH3 CH3CHCH3 + CH3CHCH3 → CH3CHCH3 CH3CHCH3 UV light • • • • • Important Note: Initiation stage • must use half arrows to show homolytic fission • Indicate “UV” above the reaction arrow Termination stage • 2 radical