Chemistry 2e · Organic Chemistry
Hydrocarbons
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Hydrocarbons are compounds of only carbon and hydrogen — the simplest members of organic chemistry and the foundation of this chapter. They divide into four families by bonding: alkanes (all single bonds, saturated, CnH2n+2), alkenes (one C=C, unsaturated, CnH2n), alkynes (one C≡C, CnH2n-2), and Aromatic Ring system with delocalized π electrons (benzene) Full entry → hydrocarbons (benzene rings with delocalized electrons). Because carbon forms four bonds and chains endlessly, even a few carbons give many distinct compounds — isomers — and each family has its own geometry, physical properties, naming rules, and characteristic reactions. Hydrocarbons are also the world's primary energy currency: natural gas is mostly methane, gasoline is a C₅–C₁₂ alkane mixture, and polyethylene is a polymer of ethene. This topic covers structures, IUPAC nomenclature, physical properties, and the reactions — combustion, halogenation, addition, Polymerization Many small monomers join into one long chain (ethene → polyethylene) Full entry → — that define each family.
Why this matters
- Energy: Hydrocarbon combustion heats homes and powers vehicles — and produces CO₂, the central driver of climate change. Incomplete combustion produces toxic carbon monoxide.
- Materials: Polyethylene, polypropylene, and PVC come from small hydrocarbon monomers; lubricants, solvents, asphalt, and paraffin wax are hydrocarbon mixtures.
- Feedstock: Hydrocarbons are the starting point for nearly all organic synthesis — the drugs, dyes, and polymers built in later topics.
- Safety: Hydrocarbon vapors are flammable, and benzene is a recognized carcinogen — general principle: work with ventilation and minimize exposure.
- Exams: Naming, classifying by formula, predicting combustion products, and identifying addition vs substitution are staple questions.
The college version
Core Concepts
Alkanes: the saturated backbone
In an alkane every carbon is sp³-hybridized and tetrahedral (~109.5°), bonded by σ bonds with free rotation about C–C. The general formula:
CnH2n+2
First ten: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane, hexane, heptane, octane, nonane, decane — prefix + "-ane." Beyond butane the same formula gives structural isomers: butane has 2, pentane 3, hexane 5, octane 18. Branching matters physically: 2,2-dimethylpropane (bp 9.5 °C) boils far below n-pentane (36 °C) because a compact molecule has less surface area for London dispersion forces.
Alkenes and alkynes: the unsaturated hydrocarbons
An alkene has at least one C=C: the carbons are sp², planar, ~120°, and the π bond Side-by-side p-orbital overlap; weaker than σ, exposed above/below the bond axis Full entry → locks the molecule against rotation. Simplest is ethene, CH₂=CH₂; with one double bond:
CnH2n
An alkyne has a C≡C: sp-hybridized, linear (180°). Simplest is ethyne (acetylene), HC≡CH:
CnH2n-2
The exposed π electrons make alkenes and alkynes far more reactive than alkanes: they undergo addition reactions in which the π bond breaks and two new σ bonds form. Restricted rotation also creates geometric (cis/trans) isomerism: in 2-butene the methyl groups sit on the same side (cis, bp 3.7 °C) or opposite sides (trans, bp 0.9 °C) — different compounds.
Aromatic hydrocarbons: benzene
Benzene, C₆H₆, is a planar six-membered ring whose six p orbitals overlap into a continuous ring of electron density. All six C–C bonds are identical (~1.39 Å, between single 1.54 Å and double 1.34 Å), and the molecule is far more stable than a hypothetical "cyclohexatriene." This aromatic stabilization (delocalization/resonance) changes the chemistry: benzene prefers substitution (replace an H, keep the ring) over addition (which would destroy delocalization). Typical reactions: halogenation with a catalyst, nitration, sulfonation. Methylbenzene (toluene) follows in later topics. Safety note: benzene is a recognized carcinogen — general laboratory principle: use fume hoods and minimize exposure.
IUPAC nomenclature: the naming engine
For alkanes: (1) find the longest continuous chain — the parent; (2) number from the end nearest the first substituent; (3) name substituents as prefixes (methyl, ethyl; fluoro, chloro…); (4) list substituents alphabetically, using di-/tri- for multiples (ignored for alphabetizing) with locant numbers. Examples: 2-methylpropane; 2,2-dimethylbutane; 3-ethyl-2-methylhexane; 2-chloropropane. For alkenes/alkynes the parent chain must contain the multiple bond, numbered to give it the lowest locant: 1-hexene, 2-butyne, 2-methyl-2-butene.
Physical properties and key reactions
Physical: All hydrocarbons are nonpolar — insoluble in water, soluble in nonpolar solvents, less dense than water. Boiling points rise with chain length (more electrons → stronger London forces) and fall with branching. At room temperature: C₁–C₄ gases, C₅–C₁₇ liquids, C₁₈+ waxy solids.
Combustion: complete → CO₂ + H₂O; limited O₂ → carbon monoxide (colorless, odorless, toxic) or soot. Balancing example:
2C8H18 + 25O2 → 16CO2 + 18H2O
Free-radical halogenation (alkanes; needs UV/heat): CH4 + Cl2 UV⟶ CH3Cl + HCl. Mechanism: initiation (Cl₂ → 2 Cl•), propagation (Cl• abstracts H; CH₃• + Cl₂), termination (radical + radical). Larger alkanes give mixtures — not a clean synthesis.
Addition reactions (alkenes/alkynes): hydrogenation (C2H4 + H2 Ni/Pt/Pd⟶ C2H6); halogenation (Br₂ adds — bromine water decolorizes, the classic unsaturation test); hydrohalogenation (HX; Markovnikov's rule In HX addition, H attaches to the carbon already holding more H Full entry →: H goes to the carbon already holding more H); hydration (H₂O + acid catalyst → alcohol).
Polymerization and Cracking Heat + catalyst split large alkanes into smaller alkanes and alkenes Full entry →: ethene polymerizes to polyethylene (n CH2=CH2 → -(CH2–CH2)n-); cracking splits large alkanes into smaller alkanes plus alkenes (C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆), and reforming branches chains — both boost gasoline yield from petroleum.
How It Works / Step-by-Step Process
Naming an alkane:
- Find the longest continuous chain → parent name (meth-…dec-).
- Number from the end giving the first substituent the lowest locant.
- List substituents alphabetically, each with its locant (di-, tri- for multiples).
- Assemble: locants + prefixes + parent + "-ane."
Predicting an alkene addition product:
- Identify the reagent (H₂, X₂, HX, or H₂O/H⁺).
- Break the π bond; add one atom to each alkene carbon.
- For HX or H₂O, apply Markovnikov: H to the carbon with more H.
- Check cis/trans possibilities when the carbons carry different groups.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Alkane addition | Alkene addition | Alkanes have no π bond: radical substitution (UV/heat); alkenes add across the double bond |
| Benzene's "alternating double bonds" | Equal delocalized bonds | All six C–C bonds are identical (~1.39 Å); resonance stabilizes the ring, so benzene substitutes |
| Markovnikov: "H to the carbon with fewer H" | H adds to the carbon with more H | In CH₃–CH=CH₂ + HBr, Br goes to the middle carbon; H goes to the CH₃ side |
| Complete combustion | Incomplete combustion | Limited O₂ gives CO (colorless, toxic) instead of CO₂ — why furnaces and cars need ventilation |
| Cis and trans are the same compound | They are geometric isomers | Different shapes → different boiling points (cis-2-butene 3.7 °C vs trans 0.9 °C) |
| "All hydrocarbons are liquids" | C₁–C₄ gases; C₁₈+ solids | Phase depends on chain length: natural gas is methane, gasoline a liquid mixture |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Hydrocarbons are like LEGO towers of carbon bricks with hydrogen studs on the sides. If every carbon holds hands with single bonds, you get flexible, stable alkanes — the stuff of candle wax and gas. If two carbons hold hands twice (a double bond), the tower is stiffer and eager to grab new pieces — that's how we make plastic from ethylene. Benzene is a magic ring where electrons run around in a circle, making it super calm: instead of reacting violently, it politely swaps one stud for a new piece. When any of these towers burn with enough oxygen, they become carbon dioxide and water — which is how cars move and how CO₂ gets into the air.
Worked example
Example 1: CO₂ from burning octane
How many grams of CO₂ are produced by complete combustion of 114 g of octane (C₈H₁₈, molar mass 114.23 g/mol)?
Balanced equation first:
2C8H18 + 25O2 → 16CO2 + 18H2O
Step 1 — moles of octane:
nC8H18 = 114 g × 1 mol114.23 g = 0.998 mol ≈ 1.00 mol
Step 2 — moles of CO₂ (mole ratio 16 CO₂ : 2 C₈H₁₈ = 8 : 1):
nCO2 = 1.00 mol × 8 mol CO21 mol C8H18 = 8.00 mol
Step 3 — mass of CO₂ (M = 44.01 g/mol):
mCO2 = 8.00 mol × 44.01 gmol = 352 g
Answer: 352 g of CO₂ from just 114 g of fuel — every gram of hydrocarbon carbon becomes ~3.1 g of CO₂ once oxygen is added. That ratio is why transportation emissions are so heavy.
Example 2: Identifying an alkane from its molar mass
A gaseous alkane has a molar mass of 72.15 g/mol. Determine its molecular formula and number of structural isomers.
Formula first: for an alkane, M = 12.011n + 1.008(2n+2), where n is the number of carbons. Set equal to 72.15:
12.011n + 2.016n + 2.016 = 72.15
14.027n = 70.13 ⇒ n = 5.00
Answer: C₅H₁₂ (pentane), which has 3 structural isomers: n-pentane, 2-methylbutane, and 2,2-dimethylpropane. Their boiling points (36 °C, 28 °C, 9.5 °C) fall as branching increases — a handy check on isomer assignments.
Example 3: Hydrogenation stoichiometry
How many moles of H₂ fully hydrogenate 1.0 mol of 1-hexene (C₆H₁₂) to hexane (C₆H₁₄)?
Equation first:
C6H12 + H2 catalyst⟶ C6H14
Answer: 1.0 mol of H₂ (1:1). An alkyne would need 2.0 mol per mole — first to the alkene, then to the alkane.
Key takeaways
- Families and formulas: alkane CnH2n+2 (sp³, saturated); alkene CnH2n (sp², one C=C); alkyne CnH2n-2 (sp, one C≡C); aromatic (benzene ring, delocalized π).
- Geometry: ~109.5° (sp³), ~120° (sp²), 180° (sp); free rotation in alkanes, locked in alkenes → cis/trans isomers.
- Physical: nonpolar, insoluble in water, float on water; bp rises with chain length, falls with branching.
- Combustion: complete → CO₂ + H₂O; incomplete → CO (toxic) — furnace and car-exhaust hazard.
- Alkanes substitute (radical, UV); alkenes/alkynes add; benzene substitutes (preserves aromaticity).
- Markovnikov's rule: in HX addition, H bonds to the carbon with more H atoms.
- Br₂ water decolorizes in the presence of C=C (unsaturation test).
- Petroleum refining: fractional distillation (by bp) + cracking + reforming.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Classify each formula: C₆H₁₄, C₆H₁₂ (one double bond), C₆H₆.
Show answer
C₆H₁₄ = alkane (fits CₙH₂ₙ₊₂); C₆H₁₂ = alkene (CₙH₂ₙ); C₆H₆ = aromatic (benzene).
Name the alkane CH₃–CH(CH₃)–CH₂–CH₃.
Show answer
2-methylbutane — longest chain is 4 carbons (butane), methyl on carbon 2.
Propane burns completely. Write the balanced equation and the mole ratio of CO₂ per mole of propane.
Show answer
C3H8 + 5O2 → 3CO2 + 4H2O; 3 mol CO₂ per mol propane.
Which reacts with bromine water in the dark: hexane or 1-hexene? Why?
Show answer
1-Hexene — the C=C π bond adds Br₂ and decolorizes the bromine; hexane needs UV light for radical substitution and won't react in the dark.
Why does 2,2-dimethylpropane boil lower than n-pentane even though both are C₅H₁₂?
Show answer
Branching reduces surface area for London dispersion forces, so intermolecular attraction (and boiling point) falls despite equal molar mass.
In CH₃–CH=CH₂ + HCl, which product forms — CH₃–CHCl–CH₃ or CH₃–CH₂–CH₂Cl? State the rule.
Show answer
CH₃–CHCl–CH₃ (2-chloropropane) — Markovnikov's rule: H adds to the carbon already holding more H (the CH₃ end), so Cl lands on the middle carbon.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Saturated hydrocarbon
- Alkane with only single C–C bonds; no more H can be added
- Unsaturated hydrocarbon
- Contains C=C or C≡C bonds that can accept more atoms
- Structural isomer
- Same formula, different connectivity (n-butane vs 2-methylpropane)
- π bond
- Side-by-side p-orbital overlap; weaker than σ, exposed above/below the bond axis
- Aromatic
- Ring system with delocalized π electrons (benzene)
- Markovnikov's rule
- In HX addition, H attaches to the carbon already holding more H
- Addition reaction
- π bond breaks; two atoms add across the double/triple bond
- Polymerization
- Many small monomers join into one long chain (ethene → polyethylene)
- Cracking
- Heat + catalyst split large alkanes into smaller alkanes and alkenes
Sources & references
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