Organic Chemistry - Some Basic Principles and Techniques

The One Thing

Organic chemistry is systematic because the structure of carbon compounds determines their properties, reactions, nomenclature, isomerism, and methods of purification and analysis. Carbon’s tetravalency, catenation, functional groups, and three-dimensional arrangements provide the basis for understanding this diversity.

Reactions, Processes and Experiments

Key Terms

• Organic compound: A compound mainly containing carbon, usually bonded with hydrogen and sometimes with oxygen, nitrogen, sulfur, phosphorus, or halogens.
• Catenation: The ability of carbon atoms to bond with one another to form straight chains, branched chains, and rings.
• Tetravalency: The ability of carbon to form four covalent bonds because it has four valence electrons.
• Covalent bond: A bond formed by the mutual sharing of one or more pairs of electrons between atoms.
• Hybridisation: The mixing of atomic orbitals to form equivalent hybrid orbitals used in bonding.
• sp3 hybridisation: Formation of four tetrahedrally arranged hybrid orbitals from one s and three p orbitals; the bond angle is approximately 109.5°.
• sp2 hybridisation: Formation of three trigonal-planar hybrid orbitals from one s and two p orbitals; the bond angle is approximately 120°.
• sp hybridisation: Formation of two linear hybrid orbitals from one s and one p orbital; the bond angle is 180°.
• Sigma bond: A covalent bond formed by head-on overlap along the internuclear axis; it generally permits rotation when no other structural factor restricts it.
• Pi bond: A covalent bond formed by sideways overlap of parallel p orbitals and present in double or triple bonds.
• 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 for naming compounds according to internationally accepted rules.
• Parent chain: The longest continuous carbon chain selected as the basis for naming an organic compound.
• Substituent: An atom or group replacing a hydrogen atom in the parent hydrocarbon chain.
• Isomerism: The existence of compounds with the same molecular formula but different arrangements or spatial orientations of atoms.
• Structural isomerism: Isomerism caused by different connectivity of atoms.
• Chain isomerism: Structural isomerism caused by different arrangements of the carbon skeleton.
• Position isomerism: Structural isomerism caused by different positions of a functional group, substituent, or multiple bond on the same carbon skeleton.
• Functional isomerism: Structural isomerism in which compounds have the same molecular formula but different functional groups.
• Metamerism: Structural isomerism caused by different alkyl groups attached to the same polyvalent functional group.
• Tautomerism: A dynamic equilibrium between structures differing in the position of a hydrogen atom and a double bond.
• Stereoisomerism: Isomerism in which compounds have the same connectivity but differ in three-dimensional arrangement.
• Geometrical isomerism: Stereoisomerism caused by restricted rotation around a double bond or within a ring, producing cis-trans or E-Z forms.
• Optical isomerism: Stereoisomerism associated with chiral molecules that rotate plane-polarised light.
• Chiral carbon: A carbon atom attached to four different atoms or groups, making the molecule potentially non-superimposable on its mirror image.
• Resonance: Representation of a molecule or ion by two or more contributing structures when electrons are delocalised.
• Inductive effect: Permanent displacement of sigma electrons along a carbon chain because of electronegativity differences.
• Electromeric effect: Temporary complete transfer of pi electrons of a multiple bond to one atom in the presence of an attacking reagent.
• Hyperconjugation: Delocalisation of sigma electrons of a C-H bond adjacent to a multiple bond, carbocation, or radical centre.
• Electrophile: An electron-deficient species that accepts an electron pair to form a covalent bond.
• Nucleophile: An electron-rich species that donates an electron pair to form a covalent bond.
• Free radical: A neutral species containing an unpaired electron, usually formed by homolytic bond cleavage.
• Homolytic fission: Bond breaking in which each bonded atom receives one electron, producing free radicals.
• Heterolytic fission: Bond breaking in which both bonding electrons go to one atom, producing ions.
• Purification: Removal of unwanted substances from an organic compound.
• Crystallisation: Purification based on differences in solubility in a suitable solvent.
• Sublimation: Purification of a solid that changes directly into vapour on heating and returns to solid on cooling.
• Distillation: Separation of liquids based on differences in boiling points.
• Fractional distillation: Separation of miscible liquids with close boiling points using repeated vaporisation and condensation in a fractionating column.
• Steam distillation: Separation of volatile, water-immiscible substances that decompose near their 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: Determination of which elements or functional groups are present.
• Quantitative analysis: Determination of the amount or percentage of each element present.
• 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.
• Molar mass: The mass of one mole of a substance, expressed in grams per mole.
• Degree of unsaturation: The total number of rings and pi bonds in a molecule, also called the index of hydrogen deficiency.

Easily Confused

• Homolytic fission vs 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.
• Sigma bond vs pi bond: A sigma bond forms by head-on overlap, whereas a pi bond forms by sideways overlap and occurs in addition to a sigma bond in double and triple bonds.
• Structural isomerism vs stereoisomerism: Structural isomerism involves different atom connectivity; stereoisomerism involves the same connectivity but different three-dimensional arrangement.
• Chain isomerism vs position isomerism: Chain isomerism changes the carbon skeleton; position isomerism changes the position of a functional group, substituent, or multiple bond on the same skeleton.
• Simple distillation vs fractional distillation: Simple distillation is used for liquids with considerably different boiling points; fractional distillation is used for liquids with close boiling points.
• Empirical formula vs molecular formula: The empirical formula gives the simplest whole-number ratio, whereas the molecular formula gives the actual number of atoms in a molecule.
• Electrophile vs nucleophile: An electrophile accepts an electron pair; a nucleophile donates an electron pair.
• Qualitative analysis vs quantitative analysis: Qualitative analysis identifies the elements or functional groups present; quantitative analysis determines their amounts or percentages.
• Geometrical isomerism vs optical isomerism: Geometrical isomerism results from restricted rotation and gives cis-trans or E-Z forms; optical isomerism is associated with chiral molecules and plane-polarised light.

What Gets Asked

• Identify or complete a systematic name: Select the parent chain, identify the principal functional group, number the chain, name substituents, arrange prefixes alphabetically, and add the correct suffix. Marks are lost by ignoring the lowest locant rule for the principal functional group or multiple bond before numbering substituents.
• Assign hybridisation and bonding: State whether carbon is sp3, sp2, or sp hybridised and distinguish sigma and pi bonds. Marks are lost by assigning tetrahedral, trigonal-planar, or linear geometry incorrectly, or by omitting the sigma bond present in double and triple bonds.
• Classify isomerism: Determine whether examples show chain, position, functional, metamerism, tautomerism, geometrical, or optical isomerism. Marks are lost by confusing different connectivity with different spatial arrangement.
• Explain electronic effects and stability: Apply inductive effect, resonance, electromeric effect, and hyperconjugation to explain stability or reaction behaviour. Marks are lost by reversing the signs of the inductive effects: electron-withdrawing groups show a -I effect, whereas alkyl groups generally show a +I effect.
• Select a purification method: Match crystallisation, sublimation, distillation, fractional distillation, steam distillation, differential extraction, or chromatography to properties such as solubility, boiling point, volatility, polarity, and thermal stability. Marks are lost by using simple distillation for liquids with close boiling points or by overlooking decomposition near the boiling point.
• Calculate composition or formula: Use the carbon and hydrogen combustion equations, percentage formulae, Molecular formula = (Empirical formula)n, or DBE = (2C + 2 + N - H - X) / 2. Marks are lost by including oxygen or sulfur in the DBE expression, or by confusing empirical formula mass with molecular mass.