Organic Chemistry · Organic Compounds: Alkanes and Their Stereochemistry
Alkanes and Alkane Isomers
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In 30 seconds
Alkanes are the simplest organic molecules: hydrocarbons containing only carbon–carbon and carbon–hydrogen single bonds. Because every carbon has four bonds and every bond is a σ bond, each carbon is sp³-hybridized and tetrahedral, with bond angles near 109.5°. Alkanes are described as Saturated Holds the maximum possible hydrogens for its carbon count Full entry → — each carbon holds the maximum possible number of hydrogens, so no more hydrogen atoms can be added without breaking the skeleton. All alkanes fit the general formula:
CnH2n+2
The family forms a Homologous series Family where members differ by repeated CH2 units Full entry →: methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12), and so on, each member differing from the next by a CH2 unit. A key consequence of the tetravalent carbon is isomerism: molecules with the same molecular formula can connect their atoms differently, producing distinct compounds called constitutional (structural) isomers. Butane, C4H10, exists as two isomers; pentane has three; hexane has five. This topic explains what defines an Alkane Hydrocarbon with only C–C and C–H single bonds Full entry →, how to read condensed structures, and how to recognize and count constitutional isomers.
Why this matters
- Fuels and materials: Natural gas is mostly methane; gasoline, diesel, and jet fuel are alkane mixtures; candle wax, paraffin, and many lubricants are long-chain alkanes. Understanding their composition explains why they burn and how refining separates them.
- Naming foundation: Alkanes are the backbone of the entire IUPAC naming system. Every later functional-group name (alkene, alcohol, amine…) builds on alkane parent names.
- Isomerism concepts: Constitutional isomerism is the first step toward the richer isomerism you will meet later — cis/trans, conformational, and enantiomeric. Getting "same formula, different structure" clear now prevents confusion later.
- Property prediction: Isomers are different compounds with different boiling points, densities, and reactivities, even though they share a formula. Recognizing isomers explains real observations, such as why isobutane boils lower than n-butane.
- Exams: General-formula problems ("an alkane has 7 carbons — what is its formula?"), isomer counting, and isomer identification are standard test questions.
The college version
Core Concepts
What makes a molecule an alkane
An alkane contains only C–C and C–H single bonds. Every carbon is sp³-hybridized and tetrahedral (~109.5°), and every bond is a σ bond with free rotation. There are no double bonds, triple bonds, rings, or heteroatoms (atoms other than carbon and hydrogen). "Saturated" means the molecule holds the maximum possible hydrogen count for its number of carbons — which is exactly what the formula CnH2n+2 enforces.
The homologous series and the general formula
Members of a homologous series share a general formula and differ by repeated CH2 units. The first ten unbranched alkanes are methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane. Given the carbon count n, the hydrogen count is 2n + 2; conversely, the formula CnH2n+2 identifies a saturated acyclic alkane. A formula with fewer hydrogens than 2n+2 must contain rings or multiple bonds — a useful diagnostic you will use again in Chapter 7 (degree of unsaturation).
Reading condensed structures
Full structures like CH3-CH2-CH2-CH3 are tedious, so chemists write condensed structures: CH3CH2CH2CH3 for butane, or (CH3)2CHCH3 for its branched isomer 2-methylpropane. In a Condensed structure Shorthand formula showing each carbon with its attached atoms Full entry →, each carbon is written with the atoms attached to it, and parentheses indicate a branch. You must be able to expand a condensed structure into a line-bond or full structure — and, crucially, to recognize when two condensed structures represent the same molecule versus different isomers.
Constitutional isomers
Constitutional isomers (also called structural isomers) share a molecular formula but differ in the connectivity of their atoms. For C4H10:
- n-Butane: CH3CH2CH2CH3 — a straight chain of four carbons.
- 2-Methylpropane (isobutane): CH3CH(CH3)CH3 — a three-carbon chain with a methyl branch on the middle carbon.
Both are C4H10, but they are different compounds: n-butane boils at about −0.5 °C, isobutane at about −11.7 °C. Isomer counts grow quickly: pentane has 3, hexane 5, heptane 9, and octane 18 constitutional isomers. There is no simple formula for the count — you must systematically vary the skeleton, moving branches and shortening the main chain one carbon at a time, and check for duplicates.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Constitutional isomer | Same molecule drawn differently | Isomers differ in connectivity; redrawing or renumbering the same skeleton does not create a new isomer |
| Alkane formula CnH2n+2 | Any hydrocarbon formula | CnH2n has one ring or double bond; CnH2n-2 has two (or a triple bond) — only single-bond acyclic molecules are alkanes |
| "Straight chain" | "Longest chain" | A branched alkane's longest continuous chain is often not the horizontal row of atoms you first see |
| Saturated | Unreactive | Saturated refers only to hydrogen content; alkanes do react (combustion, halogenation) |
| Isomer count pattern | Linear growth | Counts grow steeply (butane 2 → octane 18); never extrapolate a straight-line trend |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Alkanes are like LEGO chains made of carbon bricks, with hydrogen studs snapped on top. Every carbon brick holds exactly four things — other carbons or hydrogens — and it likes to spread them out like a three-legged stool with a hat on top. Because carbon can connect in different orders, four carbons and ten hydrogens can build two different toys: a straight train (n-butane) or a T-shaped one (isobutane). Same LEGO pieces, different toys — that's what an isomer is.
Worked example
Example 1: Finding the formula of an alkane from its carbon count
An acyclic alkane contains 7 carbon atoms. Determine its molecular formula.
Write the general formula first:
CnH2n+2
Substitute n = 7:
H = 2(7) + 2 = 16
Answer: The formula is C7H16 — heptane. As a check, a molecule with 7 carbons and fewer than 16 hydrogens would not be a saturated acyclic alkane.
Example 2: Molar mass of an alkane from the formula
Calculate the molar mass of propane, C3H8, using C = 12.011 g/mol and H = 1.008 g/mol.
Formula first:
M = 3(12.011 g/mol) + 8(1.008 g/mol)
Compute:
M = 36.033 g/mol + 8.064 g/mol = 44.097 g/mol ≈ 44.1 g/mol
Answer: Propane's molar mass is about 44.1 g/mol. The same procedure works for any alkane once its formula is known — and molar mass is often the clue used to identify an unknown alkane.
Example 3: Are these structures isomers or the same compound?
Compare CH3CH2CH(CH3)CH3 and CH3CH(CH3)CH2CH3.
Expand each. The first is a four-carbon chain with a methyl on carbon 2: CH3-CH2-CH(CH3)-CH3. The second is also a four-carbon chain with a methyl on carbon 2 when numbered from the other end: CH3-CH(CH3)-CH2-CH3 is the same connectivity read right-to-left.
Answer: They are the same molecule (2-methylbutane), not isomers — isomerism requires different connectivity, not just a different way of writing the same connectivity. This is a classic trap on exams: flip the paper mentally before declaring two structures different.
Key takeaways
- Alkanes: only C–C and C–H single bonds; sp³ carbons; tetrahedral (~109.5°); σ bonds only.
- General formula: CnH2n+2 for acyclic saturated hydrocarbons; hydrogen count = 2n+2.
- Saturated = maximum possible hydrogens; fewer hydrogens than 2n+2 implies rings or multiple bonds.
- Constitutional isomers: same molecular formula, different atom connectivity — different compounds with different properties.
- Isomer counts to memorize: butane 2, pentane 3, hexane 5 (octane 18).
- Homologous series: each member differs from the next by CH2 (14.03 g/mol).
- Condensed structures are shorthand; always expand them before comparing two structures for isomerism.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What is the molecular formula of an acyclic alkane with 9 carbons?
Show answer
C9H20 — hydrogens = 2(9) + 2 = 20, so nonane.
How many constitutional isomers does C4H10 have? Name them.
Show answer
Two: n-butane (CH3CH2CH2CH3) and 2-methylpropane (CH3CH(CH3)CH3).
Are CH3CH2CH2CH3 and CH3CH(CH3)CH3 isomers? Explain.
Show answer
Yes — both are C4H10 but the atoms are connected differently (straight chain vs. branched), so they are constitutional isomers with different boiling points.
Why is every carbon in an alkane described as sp³-hybridized?
Show answer
Each carbon forms four σ bonds to four atoms with no lone pairs or π bonds, which requires sp³ hybridization and tetrahedral geometry (~109.5°).
A hydrocarbon has formula C6H10. Can it be an alkane? Why or why not?
Show answer
No — an alkane with 6 carbons must have 2(6) + 2 = 14 hydrogens. C6H10 has four fewer, so it contains rings or multiple bonds (it is unsaturated).
What is the molar mass of hexane, C6H14?
Show answer
M = 6(12.011) + 14(1.008) = 72.066 + 14.112 = 86.18 g/mol.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Alkane
- Hydrocarbon with only C–C and C–H single bonds
- Saturated
- Holds the maximum possible hydrogens for its carbon count
- Homologous series
- Family where members differ by repeated CH2 units
- Constitutional isomer
- Same molecular formula, different atom connectivity
- Condensed structure
- Shorthand formula showing each carbon with its attached atoms
- sp³ hybrid
- Carbon orbital set with four σ bonds, tetrahedral geometry
Sources & references
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