Alkanes: Nomenclature, Isomers & Conformations – Detailed Lecture Notes

Orientation & Administrative Comments

• Lecturer continues the “Carbon Compounds – Part 2” topic begun the previous day.
• Emphasis on:
  • Detailed note-taking during lecture.
  • Close observation of molecular models: identical kits will be required in Laboratory 2 (model-making session is two weeks away).
  • Concepts covered today will appear in
  • Online nomenclature quizzes.
  • Tomorrow’s workshop.
  • The second lab write-up and viva.

Quick Temperature/Biological Aside

• Lecturer jokes about chilly lecture theatre: no chemistry content, but reminds students real-world conditions (temperature) can influence physical properties such as melting/boiling points.

IUPAC Nomenclature – Foundations

• Standard alkane name = prefix + suffix.

  • Prefix indicates carbon count.
  • met-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- …
  • Suffix -ane signifies no C=C double bonds.
  • If a double bond appears, suffix changes to -ene (not detailed today).

• Linear (“normal” or n-alkane) boiling points rise with chain length:

  • $\text{CH}<em>4$ (methane) < $\text{C}</em>2\text{H}<em>6$ < … < $\text{C}</em>6\text{H}<em>{14}$ (hexane, $\sim 70^{\circ}\text{C}$) < $\text{C}</em>{10}\text{H}_{22}$ (decane, $\sim 174^{\circ}\text{C}$).
  • Trend driven by increased London dispersion (van der Waals) forces.

Drawing Conventions

• Full/expanded formula: every atom written.
• Condensed formula: e.g. $\text{CH}<em>3\text{CH}</em>2\text{CH}_3$.
• Line-angle (skeletal) formula: vertices = carbons; hydrogens and carbons are implied, hetero-atoms shown explicitly.
  • Each vertex/terminus has enough hydrogens to give carbon four total bonds.
  • Quick mental check: each carbon should appear tetravalent.

Naming Branched Alkanes

1. Identify longest continuous carbon chain → becomes parent name.
2. Number parent chain from the end that gives lowest possible locant to the first substituent.
3. Name and number substituents; change parent alkane ending -ane → ‑yl for substituent names.
4. Assemble name: locant-substituent-parent.

Single substituent examples

• CH₃-CH(CH₃)-CH₂-CH₂-CH₃
  → parent = pentane (5 C). Methyl at C-2 → 2-methylpentane (NOT 4-methyl-).
• CH₃-CH₂-CH₂-CH₂-CH(CH₃)-CH₃
  → parent = hexane. Methyl at C-2 → 2-methylhexane (NOT 5-methyl-).

Multiple identical substituents

• Use prefixes di-, tri-, tetra- … (do not affect alphabetisation in more advanced rules).
  • Example: CH₃-CH(CH₃)-CH(CH₃)-CH₂-CH₃
    Parent = hexane; methyls at C-2 & C-4 → 2,4-dimethylhexane (NOT 3,5-di-; use smallest set of numbers).

Mixture of different substituents

• List in alphabetical order (ignore di/tri for alphabetisation).
  • CH₃-CH₂-CH(CH₃)-CH₂-CH( CH₂CH₃ )-CH₃
    – Ethyl at C-3, Methyl at C-5
    → 3-ethyl-5-methylheptane.

Carbon Classification (Reactively Important)

• Primary (1°): C attached to 1 other carbon (and 3 H).
  Example: terminal CH₃ in any chain.
• Secondary (2°): C attached to 2 other carbons (and 2 H).
• Tertiary (3°): C attached to 3 other carbons (and 1 H).
• Quaternary (4°): C attached to 4 carbons (no H).
• Useful for predicting stability/reactivity (e.g. carbocation stability, oxidation selectivity).

Constitutional (Structural) Isomerism

• Definition: Same molecular formula, different connectivity (bond sequence).
• Example with C₄H₁₀:
  • Normal butane: CH₃-CH₂-CH₂-CH₃.
  • Isobutane (2-methylpropane): (CH₃)₃C-H.
• Both used in lighter gas; separation impractical → sold as mixture.

Growth of isomer count

• $n=1 \;(\text{C})$ → 1 isomer.
• $n=5$ (pentane) → 3 isomers.
• $n=10$ (decane) → 75 isomers.
• $n=30$ → ≈ $4.1\times10^9$ isomers (billions) – vastly exceeds human population.
• Physical properties vary with branching:
  • More branching ⇒ lower surface area ⇒ lower b.p./m.p.
  • Table (qualitative):
  • n-pentane > isopentane > neopentane boiling points.

Conformational Analysis – Rotation About Single Bonds

• Conformation: 3-D arrangement obtained by rotation around σ (single) bonds; atoms are not re-connected.
• Molecules constantly interconvert in gas & liquid phases.

Ethane as Prototype

• Representations:
  • Sawhorse diagram – oblique view.
  • Newman projection – look straight down C-C bond.
• Key conformers:
  1. Staggered – H atoms 60° apart → lowest energy.
  2. Eclipsed – H atoms aligned → highest energy (torsional strain).
• Energy profile:
  • Barrier: $\Delta E_{\text{torsion}} \approx 12\,\text{kJ mol}^{-1}$ between staggered minima and eclipsed maxima.

Drawing Newman Projections (tips)

• Large circle = front carbon; dot or small circle behind often omitted.
• Bonds on front carbon drawn from circle’s center; rear-carbon bonds from circle’s edge.
• For eclipsed, rear substituents drawn directly behind front bonds.

Butane – Gauche vs Anti

• Replace one H on each carbon with CH₃.
• Conformers (looking down C₂–C₃ bond):
  • Anti (staggered, 180°) – CH₃ groups opposite → global minimum.
  • Gauche (staggered, 60°) – CH₃ groups 60° apart → higher than anti (steric clash), but still lower than eclipsed.
  • Eclipsed (CH₃/H overlap) – high energy.
  • Eclipsed (CH₃/CH₃ overlap) – maximum energy.
• Energy ordering:
  E{\text{anti}} < E{\text{gauche}} < E{\text{eclipsed (CH3/H)}} < E{\text{eclipsed (CH3/CH_3)}}

Cycloalkanes & Ring Strain

• Cycloalkane formula: $\text{C}<em>n\text{H}</em>{2n}$.
• Small rings deviate from ideal $109.5^{\circ}$ sp³ angle → angle strain + eclipsing → instability.

Qualitative strain ranking

• Cyclopropane (n=3) – angles ≈ $60^{\circ}$ → highly strained; “spring-loaded”, can undergo ring-opening.
  • Exists in nature (e.g. lipid molecules in some insects).
• Cyclobutane (n=4) – $\approx 90^{\circ}$; still strained.
• Cyclopentane (n=5) – can pucker ("envelope") to reduce eclipsing.
• Cyclohexane (n=6) – can adopt strain-free chair conformation (all angles ≈ $109.5^{\circ}$ and bonds staggered).

Cyclohexane Conformations

Chair vs Boat

• Chair
  • All C–C bonds staggered.
  • Zero angle strain; lowest energy.
• Boat
  • Some bonds eclipsed; “flagpole” H-H steric clash; higher energy.
• Interconversion (chair flip) proceeds via half-chair → twist-boat → boat → twist-boat → half-chair → opposite chair.

Axial vs Equatorial

• In chair, each carbon bears:
  • Axial substituent – vertical (alternating up/down around ring).
  • Equatorial substituent – roughly horizontal (slanted outwards).
• Drawing protocol (quick sketch method practiced in class):
  1. Draw two offset parallel lines.
  2. Join upper ends up, lower ends down → six-membered ring.
  3. Mark carbons 1–6.
  4. Add axial lines (straight up/down) alternating around ring.
  5. Add equatorial lines slightly outwards (approx 109.5° from ring bond).
• Substituent preference: bulky groups favour equatorial to minimise 1,3-diaxial interactions.

Laboratory & Assessment Tips

• Bring/assemble molecular model kits:
  • White (lecture slides sometimes green) = H.
  • Black/Grey = C.
  • Single-bond sticks permit free rotation – mimic conformational analysis.
• Marks will be awarded for:
  • Correct Newman & chair drawings (pay attention to front vs back bonds).
  • Identification of most/least stable conformers.
  • Correct use of di-, tri-, etc. in names.
• Permitted to bring personal lecture notes (including these) to modeling workshop.

Real-World Connections & Examples

• Lighters & blow-torches: mixture of n-butane + isobutane for cost/combustion performance.
• MasterChef-style culinary torches operate on same gas blend.
• Cyclopropane derivatives found in insect pheromones/defense chemicals.
• Structural & conformational principles critical for understanding drug shapes, polymer properties, lipid membrane behaviour, etc.

Key Numerical / Formula Summary

• General alkane: $\text{C}<em>n\text{H}</em>{2n+2}$ (acyclic).
  Cycloalkane: $\text{C}<em>n\text{H}</em>{2n}$.
• Ethane rotational barrier: $\approx 12\,\text{kJ mol}^{-1}$.
• Ideal tetrahedral angle: $109.5^{\circ}$.
• Cyclopropane internal angle: $60^{\circ}$ (huge angle strain).
• Hexane boiling point: $\sim 70^{\circ}\text{C}$; Decane: $\sim 174^{\circ}\text{C}$.

Checklist for Self-Study

• [ ] Can I name branched alkanes up to $C_{10}$ with correct numbering & prefixes?
• [ ] Can I classify carbons (1°,2°,3°,4°)?
• [ ] Can I draw sawhorse & Newman projections and label eclipsed vs staggered?
• [ ] Do I understand energy ordering of butane conformers (anti, gauche, eclipsed)?
• [ ] Can I sketch a correct cyclohexane chair, add axial/equatorial bonds, and perform a chair flip mentally?
• [ ] Do I grasp why cyclopropane is strained (angle + torsional)?
• [ ] Am I comfortable using the model kit to physically feel rotations and ring flips?