10.4 Alkanes & Cycloalkanes

Alkanes & Cycloalkanes

Quick Answer: Alkanes are saturated hydrocarbons with only C–C and C–H single bonds. The PA-CAT Bulletin of Information (rev. 20240815) introduces organic chemistry with alkanes, cycloalkanes, and bond properties (Table 5). Master IUPAC naming, isomerism, ring strain, and combustion — these recur in synthesis and reaction-mechanism questions later.

Structure and Formulas

Alkanes (acyclic, saturated): general formula C_n H_2n+2. Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀. Each carbon is sp³ hybridized with tetrahedral 109.5° angles.

Cycloalkanes (single-ring, saturated): C_n H_2n. Cyclopropane C₃H₆, cyclobutane C₄H₈, cyclopentane C₅H₁₀, cyclohexane C₆H₁₂.

IUPAC Nomenclature (Worked Example)

Rules:

1. Find the longest continuous chain — that is the parent name.
2. Number the chain to give substituents the lowest set of locants (first point of difference rule).
3. Name substituents as alkyl groups (methyl, ethyl, propyl) with locants; list them alphabetically (ignoring multiplicative prefixes di-, tri-, tetra- when alphabetizing).
4. Use di-, tri-, tetra- for repeated substituents.

Worked example: Name the branched alkane

        CH₃
        |
CH₃–CH₂–CH–CH₂–CH–CH₃
        |       |
        CH₃     CH₃

1. Longest chain has 6 carbons → hexane.
2. Number from the left end to give substituents 2, 3, 5 (set {2,3,5}); numbering from the right gives {2,4,5}. Compare at first point of difference: 2 < 2 (tie), then 3 < 4 → left numbering wins.
3. Substituents: C2 has one methyl, C3 has one methyl, C5 has one methyl → three methyl groups total at 2, 3, 5 → 2,3,5-trimethylhexane.

If the substituents were an ethyl at C3 and a methyl at C2, you'd alphabetize ethyl before methyl → 3-ethyl-2-methylhexane.

Constitutional Isomerism

Constitutional (structural) isomers share a molecular formula but differ in connectivity. For C₆H₁₄ there are 5 isomers: n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane. More branching lowers boiling point because molecules pack less efficiently and have smaller surface area for van der Waals contact.

Physical Properties

• Nonpolar — only C–C and C–H bonds, no dipole.
• Insoluble in water, soluble in nonpolar solvents ("like dissolves like").
• Density < 1 g/mL — alkanes float on water.
• Boiling point rises with chain length (more surface area → stronger London forces) and falls with branching (compact shape → less contact).
• Melting point follows the same trend but depends also on crystal packing symmetry.

Cycloalkanes and Ring Strain

Ring strain has two sources: angle strain (bond angles deviate from 109.5°) and torsional strain (eclipsing H atoms). Total strain per CH₂ is highest in cyclopropane (60° angles, severe) and cyclobutane (slightly puckered). Cyclopentane has modest strain. Cyclohexane is essentially strain-free because it adopts a chair conformation with tetrahedral angles and staggered C–H bonds; ring flip interconverts two equivalent chairs, swapping axial and equatorial positions.

flowchart LR
    A[Cyclopropane 60 deg] -->|High angle strain| S[Strained]
    B[Cyclobutane ~90 deg] -->|Puckered, moderate| S
    C[Cyclopentane 108 deg] -->|Envelope, low| S2[Low strain]
    D[Cyclohexane chair 109.5 deg] -->|No angle strain, staggered| F[Strain-free]

Combustion of Alkanes

Complete combustion: C_n H_2n+2 + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O, strongly exothermic. For methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O, ΔH = −890 kJ/mol. Incomplete combustion (limited O₂) yields CO (lethal) or elemental carbon (soot): 2 CH₄ + 3 O₂ → 2 CO + 4 H₂O; CH₄ + O₂ → C + 2 H₂O.

Substitution Reactions (Preview)

Alkanes undergo free-radical halogenation (Cl₂ or Br₂ + hν) — a chain mechanism with initiation, propagation, termination steps. Reactivity order for H abstraction is 3° > 2° > 1° because radical stability follows the same order. Halogen selectivity: Br₂ is more selective than Cl₂.

PA-CAT Tips

• Always number to give the lowest locant set, not the lowest single locant.
• When two chains tie for longest, choose the one with more substituents as parent.
• Branching lowers BP; chain length raises it.

Cyclohexane Chair Conformation: Axial vs Equatorial Positions

Cyclohexane's chair conformation has six carbons with tetrahedral angles and all C–H bonds staggered, eliminating angle and torsional strain. Each carbon has one axial C–H bond (parallel to the ring's vertical C3 axis) and one equatorial C–H bond (projecting outward around the ring equator). A ring flip interconverts two equivalent chairs, swapping every axial substituent to equatorial and vice versa without breaking bonds.

Substituents prefer the equatorial position because an axial substituent experiences 1,3-diaxial interactions — gauche-like steric clashes with the two axial hydrogens on C3 and C5. The energy cost of an axial substituent is its A-value: methyl ≈ 7.6 kJ/mol, ethyl ≈ 7.9, isopropyl ≈ 9.2, tert-butyl ≈ 22 kJ/mol. The bulky tert-butyl group effectively locks the ring in one chair.

For cis-1,2-dimethylcyclohexane, one methyl is axial and one equatorial in each chair (cis means both up or both down; at adjacent carbons one up position is axial, the other equatorial), so the two chairs are degenerate. For trans-1,2-dimethylcyclohexane, both methyls are axial in one chair or both equatorial in the other; the diequatorial chair is favored by about 15 kJ/mol.

PA-CAT questions test: (1) identifying axial vs equatorial on a chair drawing, (2) predicting which chair of a substituted cyclohexane is more stable by summing A-values, (3) recognizing that a ring flip does not change cis/trans configuration — it only swaps axial and equatorial positions.