Organic Chemistry – Hydrocarbons (Alkanes) Study Notes

Context & Relevance

• Emergency professionals (firefighters/EMTs)
  • Must understand fire codes, arson investigation, hazardous-material handling.
  • Provide medical care ⇒ need infection-control knowledge.
  • Organic chemistry intersects with these duties because fuels, plastics, medicines, cleaning agents, etc., are carbon-based.

11.1 Organic Compounds – Core Ideas

• Organic chemistry = study of carbon compounds.
• General characteristics of an organic compound
  • Built from C & H; may also contain O, S, N, P, halogens (F, Cl, Br, I).
  • Formulas written C first, H second, then heteroatoms.
  • Found ubiquitously: gasoline, vegetable oil, shampoos, pharmaceuticals, plastics, perfumes.

Typical Properties of Organic vs. Inorganic

• Bonding
  • Organic: mostly covalent.
  • Inorganic: many ionic (some covalent).
• Melting/Boiling Points
  • Organic: usually low (e.g., propane: $-188\,^{\circ}\mathrm{C}$ m.p.; $-42\,^{\circ}\mathrm{C}$ b.p.).
  • Inorganic: usually high (e.g., NaCl: $801\,^{\circ}\mathrm{C}$ m.p.; $1413\,^{\circ}\mathrm{C}$ b.p.).
• Flammability
  • Organic: high (propane burns readily).
  • Inorganic: generally low (NaCl does not burn).
• Water Solubility
  • Organic: not soluble unless a polar group present.
  • Inorganic ionic: mostly soluble.

Quick Classification Practice (selected answers)

A. High m.p. ⇒ inorganic
B. Not water-soluble ⇒ organic
C. Formula containing only C & H ⇒ organic
D. $\text{MgCl}_2$ ⇒ inorganic
E. Burns easily ⇒ organic
F. Covalent bonds ⇒ organic

Hydrocarbons & Molecular Geometry

• Hydrocarbon = compound containing only C + H.
• Carbon always forms 4 bonds (tetra-valent).
• Saturated hydrocarbon (alkane) ⇒ only single C–C bonds.

Methane, $\text{CH}_4$

• Geometry: tetrahedral, $109^{\circ}$ bond angles.
• Common representations: space-filling, ball-and-stick, wedge-dash, expanded, condensed.

Ethane, $\text{C}<em>2\text{H}</em>6$

• Each C: tetrahedral; three C–H + one C–C.

Butane Shape Check

• All four carbons have tetrahedral geometry.

11.2 Alkanes – Nomenclature & Structure

• Alkane: saturated hydrocarbon, continuous C chain.
• IUPAC naming rules
  • Base name ends in “-ane.”
  • Greek prefixes for ≥5 C atoms (pent-, hex-, hept-, oct-, non-, dec-…).

First Ten Alkanes (IUPAC)

1 C – methane
2 C – ethane
3 C – propane
4 C – butane
5 C – pentane
6 C – hexane
7 C – heptane
8 C – octane
9 C – nonane
10 C – decane

Formula Formats

• Molecular: totals (e.g., pentane $\text{C}<em>5\text{H}</em>{12}$).
• Expanded structural: every bond drawn.
• Condensed: groups written together (e.g., butane $\text{CH}<em>3\text{CH}</em>2\text{CH}<em>2\text{CH}</em>3$).
• Line-angle (skeletal): each vertex/end = carbon; hydrogens implied.

Cycloalkanes

• Ring alkanes; general formula $\text{C}<em>n\text{H}</em>{2n}$ (two fewer H than open chain).
• Named with prefix “cyclo-” (e.g., cyclopentane).

11.3 Alkanes with Substituents

• Structural isomers = same molecular formula, different connectivity (e.g., butane vs. isobutane).
• Substituents
  • Alkyl groups: carbon branches; named by replacing “-ane” with “-yl” (methyl, ethyl…).
  • Halo groups: fluoro, chloro, bromo, iodo.

IUPAC Naming Steps for Branched Alkanes

1. Identify longest continuous carbon chain → base name.
2. Number chain from end nearest first substituent.
3. List substituents alphabetically with location numbers; use prefixes (di-, tri-) but alphabetize by root name.
4. Combine: positions–substituent(s)–parent alkane.
   • Example: compound with five-carbon chain, $\text{CH}_3$ on C-2, Cl on C-3 ⇒ 3-chloro-2-methylpentane.

Cycloalkanes with Substituents

• Single substituent: no number required (e.g., ethylcyclohexane).
• Multiple substituents: number ring to give lowest set of locants; first point of difference rule applies.

Drawing from a Name (illustrative)

• 3-bromo-1-chlorobutane
  1. Draw 4-carbon chain.
  2. Number left→right.
  3. Place Cl on C-1, Br on C-3.
  4. Add hydrogens to give four bonds per carbon.

11.4 Physical Properties & Uses of Alkanes

• Phase trends (room temperature)
  • $C<em>1−C</em>4$ → gases (cooking/heating fuels: methane, ethane, propane, butane).
  • $C<em>5−C</em>8$ → volatile liquids (gasoline components: pentane–octane).
  • $C<em>9−C</em>{17}$ → higher-b.p. liquids (kerosene, diesel, motor oils).
  • $C_{18+}$ → waxy solids (paraffin wax, petrolatum/Vaseline).
• Solubility & Density
  • Non-polar; insoluble in water; float (density < water).
• Flammability
  • Readily combust in O₂ releasing heat → utility as fuels.

General Combustion Reaction

$\mathrm{C<em>nH</em>{2n+2} + \left(\tfrac{3n+1}{2}\right) O<em>2 \;\longrightarrow\; n\,CO</em>2 + (n+1)\,H_2O + \text{energy}}$

Specific Example – Propane

$\mathrm{C<em>3H</em>8 + 5\,O<em>2 \;\longrightarrow\; 3\,CO</em>2 + 4\,H_2O + \text{heat}}$

Butane Combustion (camp stoves)

$\mathrm{2\,C<em>4H</em>{10} + 13\,O<em>2 \;\longrightarrow\; 8\,CO</em>2 + 10\,H_2O}$

Ethical, Environmental & Practical Notes

• Fire safety: understanding flammability and combustion products critical for hazard mitigation.
• Oil spills: nonpolar alkane layer impedes oxygen transfer, harming marine life.
• Paraffin wax coatings on produce: reduce moisture loss—industrial application of long-chain alkanes.

Recap & Study Reminders

• Memorize first ten alkane names & formulas.
• Practice drawing: condensed ↔ skeletal ↔ expanded.
• Apply IUPAC rules methodically for branched and cyclic systems.
• Link physical properties to molecular weight and intermolecular forces.
• Balance combustion equations: focus on C, then H, finally O.
• Relate structural isomerism to physical/chemical property variation (e.g., branching lowers boiling point).