Key Points

Carbon is tetravalent, meaning it forms four covalent bonds. It also exhibits catenation, the unique property of forming strong bonds with other carbon atoms to create long chains and rings.

Carbon atoms can be $sp^3$, $sp^2$, or $sp$ hybridized, leading to specific molecular geometries. For example, methane ($\text{CH}_4$) is tetrahedral ($sp^3$), ethene ($\text{C}_2\text{H}_4$) is trigonal planar ($sp^2$), and ethyne ($\text{C}_2\text{H}_2$) is linear ($sp$).

A single covalent bond is a sigma ($\sigma$) bond. A double bond contains one $\sigma$ bond and one pi ($\pi$) bond. A triple bond consists of one $\sigma$ bond and two $\pi$ bonds. Pi bonds are weaker and more reactive than sigma bonds.

Organic molecules can be represented by complete structural formulas (all atoms and bonds shown), condensed formulas (e.g., $\text{CH}_3\text{CH}_2\text{CH}_3$), or bond-line formulas where vertices and line ends represent carbon atoms.

The IUPAC name of an organic compound consists of a word root (number of carbon atoms in the parent chain), a suffix (denoting the principal functional group), and prefixes (denoting substituents).

A functional group is an atom or group of atoms that determines the chemical properties of a molecule. A homologous series is a series of compounds with the same functional group where successive members differ by a $-\text{CH}_2$ unit.

Isomers are compounds that have the same molecular formula but different structures and properties. The main types are structural isomerism (different connectivity) and stereoisomerism (different spatial arrangement).

Covalent bonds can break via homolytic cleavage, where each atom gets one electron, forming free radicals. In heterolytic cleavage, one atom takes both electrons, forming a carbocation and a carbanion.

Carbocations (e.g., $\text{CH}_3^+$) are positively charged, electron-deficient species, while carbanions (e.g., $\text{CH}_3^-$) are negatively charged, electron-rich species. The stability of carbocations follows the order: tertiary > secondary > primary.

A nucleophile is an electron-rich species (e.g., $\text{OH}^-$, $\text{NH}_3$) that donates an electron pair to form a bond. An electrophile is an electron-deficient species (e.g., $\text{H}^+$, $\text{NO}_2^+$) that accepts an electron pair.

The inductive effect is the permanent polarization of a $\sigma$ bond due to electronegativity differences. The resonance effect is the delocalization of $\pi$ electrons in conjugated systems, leading to increased stability.

Hyperconjugation is the delocalization of $\sigma$-electrons of a C-H bond into an adjacent empty or partially filled p-orbital. This effect, also known as 'no-bond resonance', stabilizes carbocations and alkenes.

Common methods for purifying organic compounds include crystallization (for solids), distillation (for liquids), sublimation, differential extraction, and chromatography, each based on differences in physical properties.

Chromatography is a powerful technique used to separate, identify, and purify components of a mixture. It works on the principle of differential distribution of components between a stationary phase and a mobile phase.

Lassaigne's test is used to detect nitrogen, sulfur, and halogens in an organic compound. The compound is fused with sodium metal to convert these elements into ionic forms ($\text{NaCN}$, $\text{Na}_2\text{S}$, $\text{NaX}$), which can be identified by chemical tests.

The percentage of carbon and hydrogen is determined by burning a known mass of the organic compound in excess oxygen. The resulting carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$) are collected and weighed.

Nitrogen is estimated by either the Dumas method, where it is collected as $\text{N}_2$ gas, or the Kjeldahl method, where it is converted to ammonia ($\text{NH}_3$) and estimated by titration.

The Carius method is used to estimate halogens and sulfur. The organic compound is heated with fuming nitric acid, precipitating halogens as silver halides ($\text{AgX}$) and sulfur as barium sulfate ($\text{BaSO}_4$), which are then weighed.