Organic Chemistry - Nomenclature, Stereochemistry, Mechanisms and Spectra
What the exam tests
Organic Chemistry spans IUPAC nomenclature, stereochemistry, reaction mechanisms, functional-group interconversion, and structure elucidation by IR and NMR.
Core rules
- Nomenclature priority for the suffix: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > ether > halide. The parent chain carries the highest-priority group and the lowest locants.
- Stereochemistry: assign R/S by CIP - rank substituents by atomic number, duplicate atoms at multiple bonds, and reverse the assignment if the lowest priority points toward you. E/Z uses CIP priority on each alkene carbon.
- Substitution/elimination: SN2 is concerted, backside, second-order, favoured by methyl/primary substrates and polar aprotic solvents; SN1 and E1 proceed through carbocations and are favoured by tertiary substrates and protic solvents.
- Aromaticity: Hückel 4n + 2 π electrons in a planar, cyclic, fully conjugated system.
- Directing effects: electron-donating groups (–OH, –NH₂, –R, halogens) direct ortho/para; strong withdrawers (–NO₂, –SO₃H, –CN, carbonyl) direct meta.
- Spectroscopy: IR C=O near 1700 cm⁻¹, O–H broad 3200–3600; ¹H NMR splitting follows n + 1 and integration gives proton ratios.
Worked example
C₄H₈O₂: IR 1740 cm⁻¹; ¹H NMR δ 2.05 (s, 3H), 4.12 (q, 2H), 1.26 (t, 3H). Degrees of unsaturation = (2×4 + 2 − 8)/2 = 1, so one C=O. The quartet/triplet pair is an ethyl group next to oxygen (–OCH₂CH₃). The 3H singlet on a carbonyl is an acetyl CH₃. Structure: ethyl acetate, CH₃COOCH₂CH₃.
Traps
Assigning R/S without reorienting; confusing SN1 with E1 products; forgetting that halogens are deactivating but ortho/para directing; ignoring integration in NMR; misreading a broad O–H as N–H.