Organic Chemistry - Some Basic Principles and Techniques

The One Thing

The behaviour of an organic compound is determined by its carbon framework, functional groups, three-dimensional structure, and electron distribution. These features govern its naming, physical properties, purification, reactions, and reaction mechanisms.

Reactions, Processes and Experiments

Key Terms

• Catenation: The ability of carbon atoms to form stable bonds with one another, producing chains, branches, and rings.
• Tetravalency: The ability of carbon to form four covalent bonds because it has four valence electrons.
• Covalent bond: A bond formed by sharing electron pairs between atoms.
• Hybridisation: The mixing of atomic orbitals to form equivalent hybrid orbitals used in bonding.
• sp3 hybridisation: Hybridisation producing four sigma bonds with tetrahedral geometry and an approximate bond angle of 109.5°.
• sp2 hybridisation: Hybridisation producing three sigma bonds in a trigonal planar arrangement with bond angles of about 120° and one unhybridised p orbital.
• sp hybridisation: Hybridisation producing two sigma bonds in a linear arrangement with a bond angle of 180° and two unhybridised p orbitals.
• Sigma bond: A covalent bond formed by head-on overlap of orbitals along the internuclear axis.
• Pi bond: A covalent bond formed by sideways overlap of parallel p orbitals, present in double and triple bonds.
• Structural formula: A representation showing how atoms are connected and, in some cases, how bonds are arranged.
• Condensed formula: A shortened structural representation, such as CH3CH2OH for ethanol.
• Skeletal formula: A line representation in which carbon atoms are located at line ends and corners, while attached hydrogen atoms on carbon are usually omitted.
• Functional group: An atom or group of atoms responsible for the characteristic chemical reactions of an organic compound.
• Homologous series: A family of compounds with the same functional group and general formula, in which successive members differ by a –CH2– unit.
• IUPAC nomenclature: A systematic method of naming organic compounds using the longest chain, principal functional group, substituents, and bond positions.
• Isomerism: The existence of compounds with the same molecular formula but different arrangements of atoms or different spatial orientations.
• Structural isomerism: Isomerism caused by different connectivity of atoms, including chain, position, functional, and metameric isomerism.
• Stereoisomerism: Isomerism in which compounds have the same connectivity but differ in three-dimensional arrangement.
• Electrophile: An electron-deficient species that accepts an electron pair.
• Nucleophile: An electron-rich species that donates an electron pair.
• Free radical: A neutral species containing an unpaired electron.
• Homolytic fission: Bond breaking in which each atom receives one electron from the shared pair.
• Heterolytic fission: Bond breaking in which both bonding electrons go to one atom, producing ions.
• Carbocation: A positively charged carbon species with only six electrons around the carbon atom.
• Carbanion: A negatively charged carbon species possessing a lone pair of electrons.
• Inductive effect: Permanent displacement of sigma electrons along a carbon chain due to electronegativity differences.
• Resonance: Representation of a molecule or ion by two or more contributing structures when electrons are delocalised.
• Hyperconjugation: Delocalisation of sigma electrons of a C–H bond adjacent to a multiple bond or positively charged carbon.
• Purification: Removal of unwanted substances from an organic compound using physical or chemical methods.
• Crystallisation: Purification of a solid by dissolving it in a suitable hot solvent and obtaining pure crystals on cooling.
• Sublimation: Direct conversion of a solid into vapour and then back into a solid without passing through the liquid state.
• Distillation: Separation of liquids based on differences in boiling points through vaporisation and condensation.
• Fractional distillation: Separation of miscible liquids with close boiling points using repeated vaporisation and condensation in a fractionating column.
• Steam distillation: Separation of volatile, steam-insoluble organic substances from non-volatile impurities at temperatures below their normal boiling points.
• Differential extraction: Separation based on different solubilities in two immiscible solvents.
• Chromatography: Separation based on different distributions between a stationary phase and a mobile phase.
• Rf value: In paper or thin-layer chromatography, Rf = distance travelled by solute divided by distance travelled by solvent front.
• Qualitative analysis: Identification of elements or functional groups present in an organic compound.
• Quantitative analysis: Determination of the amount or percentage of each element present in a compound.
• Empirical formula: The simplest whole-number ratio of atoms of each element in a compound.
• Molecular formula: The actual number of atoms of each element in one molecule of a compound.
• Percentage composition: The percentage by mass of an element in a compound, calculated as (mass of the element in one mole of compound / molar mass of compound) × 100.

Easily Confused

• Structural formula and condensed formula: A structural formula shows the connections between atoms more explicitly; a condensed formula abbreviates the same structure, such as CH3CH2OH for ethanol.
• Condensed formula and skeletal formula: A condensed formula writes grouped atoms, whereas a skeletal formula uses lines, with carbon atoms at line ends and corners and most hydrogens on carbon omitted.
• Sigma bond and pi bond: A sigma bond forms by head-on overlap; a pi bond forms by sideways overlap and occurs in addition to a sigma bond in double and triple bonds.
• Homolytic and heterolytic fission: Homolytic fission gives one electron to each atom and forms free radicals; heterolytic fission gives both electrons to one atom and forms ions.
• Electrophile and nucleophile: An electrophile accepts an electron pair, whereas a nucleophile donates an electron pair.
• Carbocation and carbanion: A carbocation is positively charged and electron-deficient; a carbanion is negatively charged and has a lone pair.
• Inductive effect and resonance: The inductive effect involves displacement through sigma bonds, whereas resonance involves electron delocalisation represented by contributing structures.
• Crystallisation and sublimation: Crystallisation uses selective solubility and cooling; sublimation involves direct solid–vapour conversion.
• Simple and fractional distillation: Simple distillation is used for substantially different boiling points or removal of non-volatile impurities; fractional distillation separates miscible liquids with close boiling points using a fractionating column.
• Qualitative and quantitative analysis: Qualitative analysis identifies what is present; quantitative analysis determines how much is present.
• Empirical and molecular formula: The empirical formula gives the simplest whole-number ratio, whereas the molecular formula gives the actual number of atoms in a molecule.
• Addition, substitution, and elimination: Addition occurs across a multiple bond, substitution replaces an atom or group, and elimination removes atoms or groups to form a multiple bond.
• Melting point and boiling point as purity checks: A pure solid tends to have a sharp melting point, whereas a pure liquid tends to have a constant boiling point.

What Gets Asked

• Explain the unique chemistry of carbon: questions focus on tetravalency, catenation, strong carbon–carbon bonding, and the formation of single, double, and triple bonds. Marks are lost by omitting the role of carbon’s small size and high bond energy.
• Identify hybridisation and bonding: questions may require the geometry, bond angle, sigma bonds, pi bonds, or unhybridised p orbitals for sp3, sp2, and sp hybridisation. A common error is treating a double bond as two sigma bonds or a triple bond as three sigma bonds.
• Apply IUPAC nomenclature: questions require selection and numbering of the longest suitable chain containing the principal functional group, followed by correct substituent ordering. Marks are lost by giving the principal group an incorrect locant or using the wrong suffix, such as -ol, -al, -one, -oic acid, or -amine.
• Distinguish reaction mechanisms and intermediates: questions may ask for electrophiles, nucleophiles, free radicals, carbocations, carbanions, homolytic fission, heterolytic fission, or curved-arrow notation. The key slips are reversing electron donation and acceptance or using the wrong arrow type.
• Select a purification or separation method: questions may compare crystallisation, sublimation, simple distillation, fractional distillation, steam distillation, differential extraction, and chromatography. Marks are lost by assigning a method to the wrong physical property or forgetting that the stationary phase remains fixed while the mobile phase moves.
• Interpret analytical data and tests: questions may require empirical or molecular formula calculations, percentage composition, Rf values, combustion analysis, or Lassaigne’s tests. Specific errors include confusing empirical with molecular formula, omitting oxygen determination by difference, or failing to state Prussian blue for nitrogen, violet with sodium nitroprusside for sulphur, and silver nitrate after nitric-acid acidification for halogens.