Organic Chemistry: Some Basic Principles and Techniques: Complete Concepts and Practice

Big picture

Carbon forms chains, branches and rings, producing an enormous variety of compounds. Organic chemistry explains their structures and reactions through bonding and electron distribution. A useful answer connects a name or formula with a definite arrangement of atoms rather than treating it as a word to memorise.

The same principles guide practical work: a sample must be separated, purified and analysed before its composition and structure can be established. This chapter therefore moves from drawing and naming molecules to predicting reactivity and interpreting laboratory evidence, with calculations based on conserved atoms and masses.

Learning goals

• Represent carbon compounds and apply systematic nomenclature.

• Distinguish structural isomerism and stereoisomerism.

• Explain electronic effects, bond fission and reaction intermediates.

• Identify electrophiles, nucleophiles and major reaction types.

• Choose purification methods and interpret qualitative tests.

• Calculate composition from combustion and nitrogen-analysis data.

Core ideas

1. Carbon bonding and representations

Carbon usually forms four covalent bonds. Catenation is its ability to bond to itself, building chains and rings. An sp³ carbon has approximately tetrahedral geometry, sp² approximately trigonal planar and sp approximately linear. A single bond contains one sigma bond; a double bond contains one sigma and one pi bond; a triple bond contains one sigma and two pi bonds. Pi bonding restricts free rotation.

For CH₂=CH–CH₃, the double-bond carbons are sp² and the methyl carbon sp³. Count sigma bonds as six C–H plus two C–C connections = 8 sigma bonds, with 1 pi bond. In bond-line representations, each unlabelled endpoint or corner represents carbon, and enough attached hydrogens are implied to complete normal valency. Heteroatoms and hydrogens attached to them are shown explicitly. Check valency before counting a molecular formula from a skeletal drawing.

2. Classification and functional groups

Acyclic compounds have open chains; cyclic compounds have rings. Homocyclic rings contain only carbon atoms, while heterocyclic rings include other atoms. Aromatic compounds have special cyclic delocalisation; not every ring is aromatic. Benzene is aromatic, while cyclohexane is alicyclic.

A functional group is an atom or group associated with characteristic reactions. Alcohols contain –OH, aldehydes –CHO, ketones an internal carbonyl, carboxylic acids –COOH, amines nitrogen-based amino groups, and halo compounds carbon–halogen bonds. Members of a homologous series share a functional group and a general structural pattern, with successive members differing by CH₂. For example, ethanol and propan-1-ol have similar functional chemistry but different physical properties and carbon counts.

3. Systematic nomenclature

Choose a parent containing the principal functional group and relevant multiple bonds. Number to give the principal group its required lowest locant, then apply the appropriate multiple-bond and substituent rules. Name substituents with locants, arrange different substituent names alphabetically and use di-, tri- or tetra- for repeated groups without counting those multiplicative prefixes for alphabetisation.

Worked naming: CH₃–CH(CH₃)–CH₂–CH₂OH has a four-carbon parent containing OH. Number from OH: HOCH₂ is carbon 1 and the methyl branch is on carbon 3. The name is 3-methylbutan-1-ol. Numbering from the other end would incorrectly give the alcohol locant 4.

For the common groups here, carboxylic acid takes priority over aldehyde, ketone, alcohol and amine. Groups not selected as the principal suffix may appear as prefixes. Use commas between numbers and hyphens between numbers and words. A cyclic parent uses cyclo-, as in cyclopentane; substituted benzene names must identify relative positions unambiguously.

4. Isomerism

Isomers have the same molecular formula but differ in arrangement. Chain isomerism changes the carbon skeleton: pentane and 2-methylbutane both have C₅H₁₂. Position isomerism changes a group's position, as in propan-1-ol and propan-2-ol. Functional isomerism changes the functional group: ethanol and methoxymethane both have C₂H₆O.

Metamerism changes carbon groups on either side of a polyvalent functional linkage, as in certain ethers. Stereoisomers retain connectivity but differ spatially. Restricted rotation can allow geometric isomers when each double-bonded carbon bears two different substituents. But-2-ene meets this condition; propene does not because its terminal carbon has two hydrogens. Optical isomerism involves non-superimposable mirror-image arrangements and is developed more fully in later study. Always draw structures before claiming that differently written formulas represent different isomers.

5. Bond fission and reactive species

Homolytic fission gives one bonding electron to each fragment, producing radicals: Cl₂ → 2Cl• under suitable irradiation. Heterolytic fission gives both bonding electrons to one fragment, producing ions. A carbocation is electron-deficient and positively charged at carbon, a carbanion has a negatively charged carbon with a lone pair, and a free radical has an unpaired electron.

Electrophiles accept an electron pair; examples include H⁺ and electron-deficient carbocations. Nucleophiles donate an electron pair; examples include OH⁻, CN⁻ and NH₃. A neutral species can be nucleophilic if it has an available lone pair. Curved arrows in mechanisms represent electron-pair movement from donor toward acceptor, not motion of positive charge. Half-headed arrows represent individual-electron movement in radical mechanisms.

6. Electronic effects and stability

The inductive effect is permanent polarisation transmitted through sigma bonds and decreases with distance. Electron-withdrawing groups show −I action, while alkyl groups commonly show +I action. Chlorine withdraws electron density through sigma bonds; this can stabilise a nearby negative conjugate base and affect acidity.

Resonance describes delocalisation using several contributing structures with the same atom positions but different electron arrangements. The real molecule is a resonance hybrid, not rapidly switching isolated drawings. In a carboxylate ion, negative charge is delocalised over two oxygens, helping explain carboxylic-acid acidity relative to alcohols. The electromeric effect is temporary complete displacement of pi electrons under an attacking reagent and disappears when that influence is removed.

Hyperconjugation involves interaction of suitable sigma bonds with an adjacent empty or partially occupied orbital or pi system. It helps stabilise alkyl-substituted carbocations and alkenes. For simple alkyl carbocations, tertiary is generally more stable than secondary, then primary, then methyl. Simple alkyl carbanions generally show the reverse trend because alkyl donation intensifies negative charge. Resonance and other effects can override these simple orders, so state the structural conditions.

7. Reaction types

In substitution, one group replaces another; CH₃Cl + OH⁻ → CH₃OH + Cl⁻ is balanced in atoms and charge. In addition, groups add across a multiple bond: CH₂=CH₂ + H₂ → CH₃CH₃ with a suitable catalyst. In elimination, a small molecule is removed while a multiple bond forms: CH₃CH₂OH → CH₂=CH₂ + H₂O under suitable dehydrating conditions. Rearrangement changes the carbon framework or position of a group during a reaction.

Classify by the overall structural change before discussing mechanism. A reaction can involve several mechanistic steps even when its overall category is simple. Conditions matter: aqueous hydroxide can favour substitution in suitable haloalkanes, while alcoholic base can favour elimination. A reagent's name without medium and conditions may not fully determine the product.

8. Purification and physical separation

Sublimation separates a sublimable solid from non-sublimable impurities. Crystallisation uses different solubilities, commonly dissolving an impure solid in minimum hot solvent and cooling to obtain purer crystals. Insoluble impurities are removed by hot filtration; washing with a small amount of cold solvent reduces loss.

Simple distillation separates a volatile liquid from nonvolatile material or liquids with sufficiently different boiling points. Fractional distillation improves separation of liquids with closer boiling points through repeated vaporisation and condensation. Reduced-pressure distillation allows high-boiling or heat-sensitive liquids to distil at lower temperatures. Steam distillation suits steam-volatile, water-immiscible substances under appropriate conditions.

Differential extraction distributes a solute between immiscible solvents; the chosen solvent must dissolve the desired compound preferentially. Chromatography separates components through differing interactions with stationary and mobile phases. In paper chromatography, Rf = distance travelled by solute ÷ distance travelled by solvent front. If these are 3.2 cm and 8.0 cm, Rf = 0.40, a dimensionless ratio. Compare Rf only under matched experimental conditions.

9. Qualitative elemental analysis

Carbon and hydrogen can be detected by oxidation to carbon dioxide and water, tested through appropriate indicators. In Lassaigne's test, sodium fusion converts covalently bound nitrogen, sulphur and halogens into ionic forms suitable for aqueous testing. Nitrogen can form sodium cyanide, sulphur sodium sulphide and halogens sodium halides.

Nitrogen testing can give a Prussian-blue product after the required iron-salt treatment. Sulphur gives a violet colour with sodium nitroprusside. For halogens, nitric-acid treatment removes interfering cyanide and sulphide before silver nitrate is added. AgCl is white and dissolves in ammonia; AgBr is pale yellow with more limited solubility, while AgI is yellow and insoluble in ordinary ammonia conditions. If nitrogen and sulphur occur together, thiocyanate formation can produce a blood-red iron complex. These are supervised laboratory procedures, not casual home experiments.

10. Quantitative analysis

Combustion data give carbon from CO₂ and hydrogen from H₂O. Percentage carbon = (12/44) × mass CO₂ ÷ sample mass × 100. Percentage hydrogen = (2/18) × mass H₂O ÷ sample mass × 100. For a 0.300 g sample producing 0.440 g CO₂ and 0.180 g H₂O, carbon mass = 0.120 g and hydrogen mass = 0.020 g. The percentages are 40.00% carbon and 6.67% hydrogen. If only C, H and O are present, oxygen by difference is 53.33%.

Dumas analysis measures nitrogen gas after suitable processing; correct its volume for conditions before using gas relationships. Kjeldahl analysis converts suitable organic nitrogen to ammonium, liberates ammonia and determines it through acid–base measurement; it is not suitable for every nitrogen functional type, including ordinary application to nitro and azo groups. If liberated ammonia consumes 12.0 mL of 0.100 mol L⁻¹ monoprotic acid from a 0.250 g sample, nitrogen amount is 0.0120 × 0.100 = 0.00120 mol, mass = 0.0168 g, and nitrogen percentage = 6.72%.

Halogens can be determined from silver-halide mass in the Carius method, and sulphur from barium sulphate after oxidation. For 0.250 g sample yielding 0.287 g AgCl, using Ag = 108 and Cl = 35.5, chlorine mass = 0.287 × 35.5/143.5 = 0.0710 g, giving 28.4% chlorine. State the atomic masses and assumptions used so numerical differences can be reconciled.

A reliable approach to questions

1. Draw connectivity and verify every atom's valency.

2. Choose the principal group and parent before numbering a name.

3. Track electron donation and withdrawal before predicting stability.

4. Match a separation method to volatility, solubility or adsorption differences.

5. Convert measured products into elemental masses, then divide by original sample mass.

Connections that matter

• Chemical bonding explains hybridisation, sigma bonds and pi bonds.

• Equilibrium connects electronic stabilisation with acidity and basicity.

• Stoichiometry supplies mass conservation and mole calculations in analysis.

Common mistakes

• Calling resonance structures separate molecules ignores fixed nuclear positions.

• Equating electrophile with positive ion excludes neutral electron-deficient reagents.

• Numbering branches before the principal group can produce an incorrect name.

• Dividing elemental mass by combustion-product mass gives the wrong percentage basis.

Revision checklist

• I can draw and name compounds from condensed or bond-line formulas.

• I can distinguish isomerism types with original structures.

• I can explain reactive species and electronic effects.

• I can choose purification methods and interpret tests.

• I can calculate elemental percentages from experimental data.