Organic Chemistry · Organic Compounds: Alkanes and Their Stereochemistry

Properties of Alkanes

8 min read

Numerical values (boiling points, densities, molar masses) are standard reference values; verify against current sources before relying on them in assessments.

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
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In 30 seconds

Alkanes are nonpolar molecules: carbon and hydrogen have similar electronegativities (about 2.55 and 2.20 on the Pauling scale), so C–H bonds are nearly nonpolar and the molecules carry no permanent dipole. That fact drives almost every physical property: alkanes are insoluble in water, soluble in nonpolar solvents, less dense than water, and held together in the liquid and solid states only by weak . Because those forces depend on molecular surface area, boiling points rise with chain length but fall with branching: n-pentane boils at about 36 °C while its isomer 2,2-dimethylpropane boils at about 9.5 °C. Chemically, alkanes are the least reactive organic family — no π bonds, no polar bonds, no lone pairs — so their reactions are limited mainly to (with O₂, giving CO₂ and water) and (with Cl₂ or Br₂, heat or UV). This topic connects molecular structure to observable properties and to the combustion stoichiometry behind alkanes' role as the world's primary fuel.

Why this matters

Everyday materials: Natural gas (methane), propane grills, gasoline (C₅–C₁₂ alkanes), kerosene, candle wax, and asphalt are all alkane mixtures; boiling ranges decide which petroleum fraction is which.
Oil spills and environment: Alkanes float on water (density < 1 g/mL) and resist dissolving — why petroleum spills spread as slicks and are hard to clean up.
Safety: Alkane vapors are flammable; limited-oxygen combustion produces toxic carbon monoxide. General laboratory principle: keep alkanes away from ignition sources and work with ventilation.
Combustion stoichiometry: Fuel efficiency, CO₂ emissions, and oxygen requirements come from balanced combustion equations — a recurring real-world calculation and exam staple.

The college version

Core Concepts

Nonpolarity and intermolecular forces

Alkanes contain only C–C and C–H σ bonds, both nearly nonpolar, so they have no permanent dipole and no hydrogen-bond donors or acceptors. The only intermolecular attractions are London dispersion forces — transient induced dipoles that grow with electron count and molecular surface area. These weak forces explain the low boiling points relative to polar compounds of similar mass (water boils at 100 °C at 18 g/mol; butane at about −0.5 °C at 58 g/mol), insolubility in water, and mutual solubility with other nonpolar substances ("").

Boiling and melting point trends

increases with chain length: methane −161.5 °C, ethane −88.6 °C, propane −42.1 °C, butane −0.5 °C, pentane 36.1 °C, hexane 68.7 °C — roughly +20–30 °C per CH₂, because longer chains have more electrons and surface area. Branching lowers the boiling point: a compact molecule like 2,2-dimethylpropane has less surface area for intermolecular contact than its straight-chain isomer n-pentane. Melting points follow a less regular pattern — solid packing matters — so do not assume they mirror boiling points.

Solubility and density

Water is polar and hydrogen-bonding; alkanes are neither, so they are essentially insoluble in water (a few mg/L at best for small alkanes). They dissolve readily in nonpolar solvents (diethyl ether, hexane, carbon tetrachloride) — "like dissolves like." Liquid alkanes are less dense than water (roughly 0.6–0.8 g/mL vs. 1.00 g/mL), so they float — oil slicks and the water/hexane two-layer demo in lab.

Chemical reactivity: combustion and halogenation

Alkanes have no reactive sites (no π bonds, no polar bonds), so at room temperature they resist acids, bases, and oxidizing agents — "inert" in that limited sense. The two important reactions:

Combustion: complete combustion of an alkane gives CO₂ and H₂O: C_nH_2n+2 + 3n+12O₂ → nCO₂ + (n+1)H₂O. In limited oxygen, produces carbon monoxide (CO), a colorless, odorless, toxic gas — the reason furnaces and engines must be vented.
Free-radical halogenation: with Cl₂ or Br₂ under heat or UV light, an H is replaced: CH₄ + Cl₂ ^UV⟶ CH₃Cl + HCl. The three-stage mechanism — initiation (Cl₂ splits into two chlorine radicals), propagation (a chlorine radical abstracts an H; the methyl radical reacts with Cl₂), termination (two radicals combine) — usually gives mixtures for larger alkanes, so it is not a clean synthesis.

Common Confusions

Do Not Confuse	With	Difference
Boiling-point trend with branching	Melting-point trend with branching	Boiling points fall with branching (surface area); melting points depend on solid packing — no simple rule
"Alkanes are inert"	"Alkanes never react"	Inert toward acids/bases/oxidizers at room temperature, but they burn and halogenate
Complete combustion	Incomplete combustion	Limited O₂ yields CO (colorless, toxic) or soot, not CO₂ — a furnace/engine hazard
London dispersion forces	Dipole–dipole or hydrogen bonding	Alkanes have no permanent dipoles; only dispersion forces operate
Solubility in water	Miscibility with nonpolar liquids	Alkanes are insoluble in water but fully miscible with hexane, ether, etc.
Density of alkane liquids	Density of alkanes in general	Liquid alkanes (~0.6–0.8 g/mL) float on water; gases like methane have negligible density

Eli explains

The same idea, in plain words

Explain it like I’m 10

Alkanes are like oil: they don't mix with water, they float on top, and they catch fire easily. Think of each molecule as a smooth, slippery ball — big balls (long chains) stick to each other better than small ones, so they need more heat to boil apart; a round, branched ball sticks worse than a long skinny one, so it boils sooner. When an alkane burns with enough air, every carbon becomes carbon dioxide and every hydrogen becomes water — that's the fire, and it's why we can predict how much CO₂ a fuel makes.

Worked example

Example 1: Predicting relative boiling points

Which boils lower: n-pentane (C₅H₁₂) or 2,2-dimethylpropane (C₅H₁₂)? Explain using intermolecular forces.

Both are C₅H₁₂, so compare shape. n-Pentane is a long, flexible chain; 2,2-dimethylpropane is nearly spherical (a central carbon with four methyls). Dispersion forces depend on surface area available for contact, and the compact isomer has far less.

Answer: 2,2-dimethylpropane boils lower (about 9.5 °C vs. 36.1 °C). This is why refining separates isomers by boiling point — “same formula” never means “same properties.”

Example 2: Combustion stoichiometry — oxygen and CO₂ from octane

How many grams of O₂ are required for complete combustion of 114 g of octane (C₈H₁₈, molar mass 114.23 g/mol)?

Balanced equation first:

$2C 8 H 18 + 25O 2 \to 16CO 2 + 18H 2 O$

Step 1 — moles of octane:

$n = 114 g \times 1 mol 114.23 g = 0.998 mol \approx 1.00 mol$

Step 2 — moles of O₂ (ratio 25 mol O₂ : 2 mol C₈H₁₈):

$n O 2 = 1.00 mol C 8 H 18 \times 25 mol O 2 2 mol C 8 H 18 = 12.5 mol$

Step 3 — mass of O₂ (molar mass 32.00 g/mol):

$m = 12.5 mol \times 32.00 g mol = 400 g$

Answer: about 400 g of O₂ per 114 g of octane. The same skeleton works for any fuel — balance first, then convert mass → moles → moles → mass.

Example 3: Density — why alkanes float on water

A 1.00 L sample of hexane has a density of 0.66 g/mL. Compare its mass with 1.00 L of water (1.00 g/mL):

Formula first: m = d × V. Substitute for hexane:

$m = 0.66 g mL \times 1000 mL = 660 g$

Water: 1.00 gmL × 1000 mL = 1000 g.

Answer: the hexane layer (660 g) is less dense than the water layer (1000 g), so hexane floats — the physical basis of oil-slick behavior and water/hexane extractions in lab.

Key takeaways

Alkanes are nonpolar (C and H have similar electronegativity): no dipole, no hydrogen bonding.
Intermolecular forces = London dispersion only; strength rises with electron count and surface area. Boiling point rises with chain length (~+20–30 °C per CH₂) and falls with branching.
Solubility: insoluble in water, soluble in nonpolar solvents; liquid alkanes float on water (density ≈ 0.6–0.8 g/mL).
Phase at room temperature: C₁–C₄ gases, C₅–C₁₇ liquids, C₁₈+ waxy solids.
Complete combustion: C_nH_2n+2 + 3n+12O₂ → nCO₂ + (n+1)H₂O; incomplete combustion makes toxic CO.
Halogenation: free-radical mechanism (initiation, propagation, termination), needs heat or UV; gives mixtures.
Safety: alkane vapors are flammable; use ventilation and keep away from ignition sources.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Why are alkanes insoluble in water but soluble in hexane?
Show answer
Water is polar and hydrogen-bonding; alkanes are nonpolar. Like dissolves like — alkanes dissolve in nonpolar hexane, not in polar water.
Which has the higher boiling point: n-hexane or 2,2-dimethylbutane? Explain.
Show answer
n-Hexane — a straight chain has more surface area for dispersion forces than the branched 2,2-dimethylbutane, so its boiling point is higher.
Write the balanced equation for complete combustion of propane, C₃H₈.
Show answer
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
How many moles of O₂ are needed to burn 2.0 mol of octane completely?
Show answer
25 mol — from 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O, 2 mol octane needs 25 mol O₂.
Why is carbon monoxide a concern when alkanes burn?
Show answer
Limited oxygen gives incomplete combustion and CO, a colorless, odorless, toxic gas — hence ventilation around furnaces, engines, and heaters.
Arrange in order of increasing boiling point: methane, pentane, ethane, decane.
Show answer
Methane (−161.5 °C) < ethane (−88.6 °C) < pentane (36.1 °C) < decane (174 °C) — boiling point rises with chain length.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

London dispersion forces: Weak attractions from transient induced dipoles
Nonpolar molecule: No permanent separation of charge
Boiling point: Temperature where vapor pressure equals atmospheric pressure
Combustion: Reaction with O₂ producing CO₂ and H₂O (complete)
Incomplete combustion: Burning with limited O₂ producing CO (or soot)
Free-radical halogenation: H replaced by halogen via radical chain mechanism
Like dissolves like: Polar solvents dissolve polar solutes; nonpolar dissolve nonpolar

Sources & references

openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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In 30 seconds

Why this matters

The college version

Core Concepts

Nonpolarity and intermolecular forces

Boiling and melting point trends

Solubility and density

Chemical reactivity: combustion and halogenation

Common Confusions

The same idea, in plain words

Worked example

Example 1: Predicting relative boiling points

Example 2: Combustion stoichiometry — oxygen and CO₂ from octane

Example 3: Density — why alkanes float on water

Key takeaways

Check yourself

Keep learning

Key vocabulary

Related lessons

Sources & references