Alkanes and Cycloalkanes

IUPAC nomenclature (International Union of Pure and Applied Chemistry) provides systematic rules for assigning a unique name to every organic compound. For linear alkanes, numerical prefixes are used: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C), hept- (7C), oct- (8C), non- (9C), dec- (10C) + suffix -ane.

For branched alkanes the rules are:

1. Identify the longest main chain (maximum number of C atoms). If two chains have the same length, choose the one with more branches.
2. Number the main chain starting from the end closest to the first substituent (first point of difference rule).
3. Identify and name the substituents (alkyl groups: —CH₃ methyl, —CH₂CH₃ ethyl, —CH(CH₃)₂ isopropyl, —C(CH₃)₃ tert-butyl, etc.).
4. Write the name: position(multiplier prefix)-substituent + main chain name. Substituents in alphabetical order (ignoring multiplier prefixes di-, tri-).
5. Separate numbers and letters with hyphens, numbers from each other with commas.

For cycloalkanes, add the prefix cyclo- to the corresponding alkane name (cyclopropane C₃H₆, cyclobutane C₄H₈, cyclopentane C₅H₁₀, cyclohexane C₆H₁₂). If there are substituents, number them giving priority to alphabetical order.

Real-world applications: IUPAC nomenclature is the international standard for scientific communication, chemical databases (CAS, PubChem), and regulatory documentation (REACH). An IUPAC name uniquely identifies a substance, avoiding ambiguity of common names.

The physical properties of alkanes depend mainly on London forces (dispersion), which are weak intermolecular forces due to instantaneous dipoles. The main characteristics:

• Boiling point: increases with increasing molecular mass (more electrons → larger dipole fluctuations). For isomers of equal mass, the more linear chain has a higher boiling point (greater surface area contact). n-pentane (C₅) boils at 36°C, 2,2-dimethylpropane (neopentane) at 9.5°C. Rule: for each added CH₂, the boiling point increases by about 20–30°C up to C₁₅, then gradually less.
• Solubility: alkanes are nonpolar and soluble only in nonpolar solvents (CCl₄, benzene, ether) — "like dissolves like". They are insoluble in water (polar solvent).
• Density: alkanes have density $approx 0.6-0.8\,\mathrm{g/mL}$, lower than water (1.0 g/mL), so they float (think of oil slicks on water).
• Physical state: C₁-C₄ gases, C₅-C₁₇ liquids, C₁₈⁺ solids (waxes). Methane (C₁) boils at −161°C, propane (C₃) at −42°C (used as liquefied fuel), octane (C₈) at 126°C.

Real-world applications: alkanes are the main components of petroleum and natural gas. Their separation by fractional distillation exploits different boiling points: LPG (C₃-C₄), gasoline (C₅-C₁₂), kerosene (C₁₂-C₁₆), diesel (C₁₅-C₂₅), lubricating oils (C₂₀-C₅₀), bitumen (C₇₀⁺).

Conformations are different spatial arrangements of atoms obtained by rotation around single bonds (C—C). They are not isomers, but conformers that interconvert rapidly at room temperature.

Ethane (CH₃—CH₃): rotation around the C—C bond produces:

• Eclipsed conformation: the H atoms of the two carbons are aligned (viewed along the C—C axis). Higher energy due to steric repulsion (torsional strain). Energy barrier: $approx 12\,\mathrm{kJ/mol}$.
• Staggered conformation: the H atoms are alternated (each H of one C falls between two H of the other C). Minimum energy (most stable). It is the preferred conformation at room temperature.

n-Butane: more complex due to rotation around the central C₂—C₃ bond:

• Anti: the two CH₃ groups are at 180° → minimum energy
• Gauche: the two CH₃ groups are at 60° → energy $approx 3.8\,\mathrm{kJ/mol}$ higher than anti
• Eclipsed: CH₃ groups at 0° (fully eclipsed) or 120° (partially eclipsed) → maximum energy

Cyclohexane: the 6-membered ring is not planar but assumes chair (most stable, no ring strain) and boat (less stable, steric strain between flag-pole H) conformations. In the chair conformation there are:

• 6 axial (a) bonds: vertical, alternating up/down
• 6 equatorial (e) bonds: tilted outward from the ring
• Ring flip exchanges axial and equatorial positions. Bulky substituents prefer the equatorial position (less steric hindrance).

Real-world applications: the chair conformation of cyclohexane is the basis of the stereochemistry of steroids (cholesterol, hormones) and many drugs. The equatorial preference of substituents influences reactivity: an axial —OH group is more accessible and more reactive than an equatorial one.

Radical halogenation is a substitution reaction in which a hydrogen atom of an alkane is replaced by a halogen (typically Cl₂ or Br₂) through a free radical mechanism. It requires activation by UV light or heat (≥ 250°C for chlorination).

Three-step mechanism:

1. Initiation: the halogen molecule splits into two radicals by homolytic bond cleavage: $mathrm{Cl_2 \xrightarrow{h u} 2,Clcdot}$. Sufficient energy is needed (UV light for Cl₂,$Delta H = 242\,\mathrm{kJ/mol}$; Br₂ requires less, 192 kJ/mol).
2. Propagation (2 stages): (a) the Cl• radical abstracts an H from the alkane, forming HCl and an alkyl radical R•: $mathrm{RH + Clcdot o Rcdot + HCl}$. (b) The R• radical reacts with Cl₂ to form RCl and regenerate Cl•: $mathrm{Rcdot + Cl_2 o RCl + Clcdot}$. Each propagation cycle produces one molecule of alkyl halide and regenerates the radical.
3. Termination: two radicals combine (R• + Cl• → RCl, R• + R'• → R—R', Cl• + Cl• → Cl₂), consuming the radicals and stopping the reaction.

Selectivity and radical stability: the rate of H abstraction depends on the type of carbon: tertiary (3°) > secondary (2°) > primary (1°) > methyl (CH₄). This is due to the greater stability of the tertiary radical (hyperconjugation effect: more alkyl groups donate electron density to the radical carbon, stabilizing it).

Bromination is much more selective than chlorination because the reaction is less exothermic, so the energy barrier more closely reflects radical stability (Hammond postulate). Bromination occurs almost exclusively at the tertiary position, while chlorination gives mixtures of products.

Real-world applications: radical chlorination of methane produces methyl chloride, dichloromethane, chloroform, and carbon tetrachloride (depending on the Cl₂:CH₄ ratio), all important industrial solvents. Selective bromination is useful in organic synthesis for introducing bromine at a specific position.