Chapter 18: Organic Chemistry
Structure and nomenclature
Organic compounds contain carbon, usually bonded to H, O, N, S, or halogens. Carbon forms four bonds, and its , , and hybrid states give tetrahedral, trigonal planar, and linear geometries. The IUPAC system names a compound from the longest carbon chain (parent), the principal functional group (suffix), and substituents (prefixes) with locants.
| Functional group | General formula | Suffix / prefix | Example |
|---|---|---|---|
| Alkane | CH | -ane | ethane |
| Alkene | C=C | -ene | ethene |
| Alkyne | C≡C | -yne | ethyne |
| Haloalkane | R–X | halo- | bromoethane |
| Alcohol | R–OH | -ol | ethanol |
| Ether | R–O–R′ | alkoxy- | methoxyethane |
| Aldehyde | R–CHO | -al | ethanal |
| Ketone | R–CO–R′ | -one | propanone |
| Carboxylic acid | R–COOH | -oic acid | ethanoic acid |
| Ester | R–COO–R′ | -oate | ethyl ethanoate |
| Amide | R–CONH₂ | -amide | ethanamide |
| Amine | R–NH₂ | -amine | methylamine |
| Nitrile | R–C≡N | -nitrile | ethanenitrile |
Isomerism
Structural isomers have the same molecular formula but different connectivity (chain, position, functional group). Stereoisomers have the same connectivity but different arrangements in space: geometric (cis–trans) isomers arise from restricted rotation about C=C, and optical isomers arise from a chiral centre (a carbon bonded to four different groups); the two enantiomers rotate plane-polarized light in opposite directions. Example: C₄H₁₀O has seven structural isomers: four alcohols (butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol) and three ethers (methoxypropane, ethoxyethane, 2-methoxypropane). Butan-2-ol is chiral.
Reaction types and mechanisms
| Type | Typical reactant | Example |
|---|---|---|
| Free-radical substitution | Alkanes + halogens, UV light | CH₄ + Cl₂ → CH₃Cl + HCl |
| Electrophilic addition | Alkenes + HBr, Br₂, H₂O/H⁺ | CH₂=CH₂ + HBr → CH₃CH₂Br |
| Nucleophilic substitution (, ) | Haloalkanes + OH⁻, CN⁻, NH₃ | CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻ |
| Elimination (E1, E2) | Haloalkanes + alcoholic KOH | CH₃CH₂Br → CH₂=CH₂ |
| Nucleophilic addition | Aldehydes and ketones + HCN | CH₃CHO + HCN → CH₃CH(OH)CN |
| Electrophilic aromatic substitution | Benzene + HNO₃/H₂SO₄; Br₂/FeBr₃ | C₆H₆ → C₆H₅NO₂ |
| Condensation / acyl substitution | Carboxylic acids + alcohols | CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O |
Markovnikov’s rule: in the addition of HX to an unsymmetrical alkene, hydrogen attaches to the carbon with more hydrogens, and the halogen to the more substituted carbon, because the more stable carbocation forms. So propene + HBr gives mainly 2-bromopropane. reactions occur in one step with inversion of configuration and are favoured for primary haloalkanes; reactions proceed through a carbocation and are favoured for tertiary haloalkanes.
Oxidation of alcohols with acidified potassium dichromate(VI): primary alcohols give aldehydes (distil off immediately) and then carboxylic acids (reflux); secondary alcohols give ketones; tertiary alcohols do not oxidize. Strong reducing agents (LiAlH₄) reduce carbonyl compounds and carboxylic acids to alcohols. Benzene is aromatic and stabilized by delocalized electrons, so it undergoes substitution rather than addition.
Acidity and basicity
Carboxylic acids are stronger acids than phenols, which are stronger than alcohols and water; electron-withdrawing groups (for example Cl) increase acidity. Amines are basic because of the lone pair on N; aliphatic amines are stronger bases than ammonia, and aromatic amines are weaker.
Polymers and biomolecules
Addition polymers (polyethene, PVC, polystyrene) form from alkenes; condensation polymers (nylon from a diamine and a diacid chloride or diacid; polyesters such as PET) lose a small molecule such as water. Carbohydrates (glucose, starch, cellulose), proteins (amino acids joined by peptide bonds), and lipids (triglycerides, fats, and oils) are the major biomolecules.
Spectroscopy and analysis
Infrared (IR): O–H (alcohol) broad near 3300 cm⁻¹; O–H (acid) very broad 2500–3300; C=O strong near 1700 cm⁻¹; C–H near 2900 cm⁻¹.
Mass spectrometry: the molecular ion peak M⁺ gives the molar mass, and fragments give structural clues.
NMR (proton): the number of signals shows the number of different hydrogen environments, chemical shift the environment, integration the ratio, and splitting the neighbours.
Chromatography: components are separated by their different affinity for the mobile and stationary phases; .
Common mistakes
Forgetting that the major product follows Markovnikov’s rule, and that not all additions do.
Writing curly arrows that start at an atom instead of at a bond or lone pair.
Confusing structural and displayed formulae, or missing a chiral centre.
Practice questions
Give the products of the reaction of propan-2-ol with warm acidified potassium dichromate(VI). [Propanone]
How many structural isomers does C₅H₁₂ have? [3]
Explain why chloroethanoic acid is a stronger acid than ethanoic acid.
Part Five: Information and Communication Technology
The A/L ICT syllabus of the National Institute of Education (NIE) is organized in units that run from the basic concepts of ICT and computer organization through data representation, digital circuits, operating systems, networking, systems analysis and design, databases, programming, web development, and the Internet of Things, to ICT in business, new trends, and a project (itguru.lk, n.d.-a, n.d.-b). The programming unit uses Python (itguru.lk, n.d.-a). The unit titles and time allocations come from secondary summaries of the NIE syllabus, so check the official NIE syllabus for the current requirements. General texts that support the ideas include Brookshear and Brylow (2019), Tanenbaum et al. (2021), Silberschatz et al. (2019), and Sommerville (2016).