Families of Hydrocarbons & Nomenclature – Comprehensive Study Notes

Classification of Organic Compounds

• Two broad families

  • Hydrocarbons: molecules that contain only C and H atoms.

  • Substituted hydrocarbons: one or more H atoms replaced by another atom/group (functional groups).

    • Examples include halogenated compounds, alcohols, etc. (see module p. 9).

Hydrocarbons: Overview & Natural Source

• Natural source: petroleum (crude oil).

• Two structural categories

  • Aliphatic (open-chain)

    • May be saturated (single bonds) or unsaturated (double/triple bonds).

  • Cyclic (closed-chain)

    • Alicyclic: ring structures structurally similar to open-chain but forming a ring.

    • Aromatic: contain one or more benzene rings.

Aliphatic Hydrocarbons – Terminology

• Saturated: all C–C single bonds, maximum H content.

• Unsaturated: at least one C=C or C≡C, lower H content.

• Aliphatic: not linked in rings.

• Cyclic: carbon chain closes to form a ring.

• Alicyclic: possesses features of both aliphatic & cyclic.

• Aromatic: possess benzene-type conjugated rings.

Molecular Representations

• Molecular formula: exact atom counts; e.g. glucose $C<em>6H</em>{12}O_6$.

• Empirical formula: simplest ratio of atoms.

• Structural formula: shows all atoms & bonds as lines.

• Condensed formula: compact structural information (e.g. $CH<em>3CH</em>2CH_3$).

• Bond-line / skeletal formula: zig-zag lines show C-C framework; vertices/endpoints = C, hydrogens implied.

Carbon-Chain Prefixes (Root Names)

• 1 C meth-

• 2 C eth-

• 3 C prop-

• 4 C but-

• 5 C pent-

• 6 C hex-

• 7 C hept-

• 8 C oct-

• 9 C non-

• 10 C dec-

• 11 C undec-

• 12 C dodec-

• 13 C tridec-

• 14 C tetradec-

• 15 C pentadec-

• 16 C hexadec-

• 17 C heptadec-

• 18 C octadec-

• 19 C nonadec-

Alkyl Groups

• Produced by removing one H from a straight-chain alkane.

• Naming: replace “-ane” with “-yl”.

  • $CH<em>4 \rightarrow CH</em>3^-$ → methane ⇒ methyl.

Alkanes (Paraffins)

• General formula $C<em>nH</em>{2n+2}$ (straight chain).

• Saturated aliphatic hydrocarbons; all C–C single bonds.

• Sample series

  • Methane $CH_4$

  • Ethane $C<em>2H</em>6$

  • Propane $C<em>3H</em>8$

  • Butane $C<em>4H</em>{10}$

  • Pentane $C<em>5H</em>{12}$

  • Hexane $C<em>6H</em>{14}$

IUPAC Rules for Naming Alkanes

1. Identify longest continuous carbon chain → parent name.

2. Number chain from end nearer first substituent.

3. Name each substituent (alkyl/halo etc.).

4. List substituents alphabetically (ignore prefixes di-, tri- when alphabetizing; “ethyl” before “methyl”).

5. Indicate position numbers; separate numbers with commas, numbers & letters with hyphens (no spaces).

6. Use prefixes di-, tri-, tetra- when identical groups repeat; repeat location numbers (e.g. 2,2-dimethylhexane).

Examples

• 2-methylpentane

• 3-methylpentane

• 2,2-dimethylhexane

• 4-ethyl-4-methyloctane

• 3-ethyl-2-methylheptane

Structural (Constitutional) Isomerism

• Isomers: different compounds sharing same molecular formula but different connectivity.

  • $C<em>4H</em>{10}$ → butane vs. 2-methylpropane.

  • $C<em>5H</em>{12}$ has pentane, 2-methylbutane, 2,2-dimethylpropane.

• Result: distinct physical/chemical properties.

Substituted Alkanes (Haloalkanes)

• Treat halogens as substituents; names end with “-o”: fluoro, chloro, bromo, iodo.

• Apply same numbering/alphabetization rules.

• Example: 6-chloro-5-ethyl-2,2,4-trimethyloctane.

Cycloalkanes

• General formula $C<em>nH</em>{2n}$ (one ring, no multiple bonds).

• Named by adding “cyclo-” prefix to alkane name.

• Common rings: cyclopropane, cyclobutane, cyclopentane, cyclohexane.

• Any single substituent: no locant required (assumed C-1).

• Multiple substituents: number to give lowest set of numbers; halogens before alkyls alphabetically.

  • Chlorocyclobutane (single Cl)

  • 1-bromo-2-ethylcyclopentane

  • 1,3-dimethylcyclohexane

  • 1,2,4-trimethylcyclohexane

Reading & Naming Skeletal (Line) Structures

• Each vertex/end = carbon; hydrogens implied to complete valency.

• Branch endpoints = carbon atoms of substituents.

• Example provided: 3-methylheptane.

• Students practise by assigning parent chain, numbering, naming.

Alkenes (Olefins)

• Unsaturated; contain at least one C=C.

• General formula $C<em>nH</em>{2n}$.

• Naming (IUPAC)

  1. Parent = longest chain containing C=C.

  2. Change “-ane” → “-ene”.

  3. Number so first double-bond carbon has lowest number; indicate position (if chain ≥4).

  4. Indicate substituent positions.

• Examples & formulas

  • Ethene $C<em>2H</em>4$

  • Propene $C<em>3H</em>6$

  • 1-butene vs. 2-butene $C<em>4H</em>8$

  • 3,3-dimethyl-1-butene

  • 4-methyl-2-pentene

Alkynes (Acetylenes)

• Unsaturated; contain at least one C≡C.

• General formula $C<em>nH</em>{2n-2}$.

• Naming rules identical to alkenes; suffix “-yne”.

• Examples

  • Ethyne (acetylene) $C<em>2H</em>2$

  • Propyne $C<em>3H</em>4$

  • 1-butyne $C<em>4H</em>6$, 2-butyne.

  • 2,4-pentyne $C<em>5H</em>6$.

  • 1-, 2-, 3-hexyne series with structures illustrated.

Physical Properties Across Homologous Series

• Increasing chain length ↑ molecular mass →

  • ↑ boiling point.

  • ↑ viscosity (thicker liquids; e.g. honey > water).

  • ↓ volatility (harder to evaporate).

• Phase at room temperature (alkanes/alkenes)

  • $C<em>1–C</em>4$ gases.

  • Mid-weight (≈$C<em>5–C</em>{17}$) liquids.

  • Heavier $C_{18+}$ solids/tars.

Representative Boiling Points

• Methane: $-161.5\,^{\circ}\text{C}$

• Ethane: $-88.6\,^{\circ}\text{C}$

• Propane: $-42.1\,^{\circ}\text{C}$

• Butane: $-0.5\,^{\circ}\text{C}$

• Pentane: $36.1\,^{\circ}\text{C}$

Solubility ("Like Dissolves Like")

• Polar solvents dissolve polar solutes (e.g. water, alcohol).

• Hydrocarbons = non-polar → dissolve in non-polar solvents; generally insoluble in water.

Flammability

• Metric of how readily a substance burns; gaseous/light hydrocarbons particularly flammable (fuel use).

Chemical Reactivity – Combustion of Alkanes

• Complete combustion: hydrocarbon + $O<em>2$ → $CO</em>2 + H_2O$ + energy (heat/light).

• Basis for use as fuels (LPG, gasoline, etc.).

Aromatic Hydrocarbons

• Contain benzene ring(s); general core $C<em>6H</em>6$ for benzene.

• Common aromatics: benzene, naphthalene (2 fused rings), anthracene (3 fused), benzo[a]pyrene, naphthacene.

• Key property: conjugated π-electron system leading to special stability (aromaticity).

Industrial & Everyday Uses of Hydrocarbons

• Alkanes

  • Gaseous (propane, butane): fuels (LPG).

  • Liquid (hexane, octane): solvents, motor & illuminating fuels.

  • Solid (paraffin wax): candles, waterproofing fabrics.

• Alkenes

  • Lower members as fuels.

  • Ethene (ethylene): artificial fruit ripening; precursor for polymers, solvents, anesthetics.

• Alkynes

  • Ethyne (acetylene): oxy-acetylene welding/cutting; precursor to acetic acid & vinyl monomers.

• Benzene & Aromatics

  • Manufacture of dyes, pharmaceuticals, lubricants.

  • Paint removers (thinners), rubber production (tires, shoes), after-shave lotions.

  • Solvents for degreasing metals; intermediates for synthesis of many chemicals.

Ethical, Environmental & Practical Considerations

• Combustion releases $CO_2$ → greenhouse gas; incomplete combustion may produce CO (toxic).

• Volatile organic compounds (VOCs) contribute to smog; handling requires proper ventilation.

• Benzene classified as carcinogenic; strict exposure limits in industry.

Connections & Context

• Understanding nomenclature underpins further study of functionalized organics (alcohols, aldehydes, acids).

• Physical-property trends illustrate intermolecular forces (London dispersion) — key concept in general chemistry.

• Hydrocarbon fuels central to energy sector; alternative/renewable sources aimed at reducing carbon footprint.