Chemistry: Atoms First 2e · Organic Chemistry
Hydrocarbons
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In 30 seconds
Hydrocarbons contain only carbon and hydrogen — the simplest members of the organic family. Because carbon forms four bonds and links to itself in chains and rings, two elements make an enormous variety of molecules: the methane in natural gas, the octane in gasoline, the ethylene of plastics, and the benzene rings of many drugs. This topic organizes them into four families — alkanes, alkenes, alkynes, aromatic hydrocarbons — and shows how bonding, naming, and properties follow from structure. The central idea: the type of carbon–carbon bond controls the general formula, the reactions, and whether a molecule is Saturated Contains only single bonds Full entry → or Unsaturated Contains double or triple bonds Full entry →.
Why this matters
Hydrocarbons are the backbone of the energy economy and chemical industry. Natural gas is mostly methane; gasoline, diesel, and jet fuel are Alkane Hydrocarbon with only single bonds Full entry → mixtures; ethylene and propylene feed the plastics in bottles, pipes, and packaging; aromatic hydrocarbons appear in dyes, drugs, and polymers. Safety literacy depends on this too: alkanes are flammable fuels, alkenes polymerize readily, and benzene is a recognized carcinogen. Finally, hydrocarbons are the vocabulary base for every functional group in this chapter — alcohols, ethers, carbonyl compounds, amines, and amides are hydrocarbons with atoms replaced or added.
The college version
Core Concepts
Carbon: the four-bond rule
Carbon has four valence electrons and forms four covalent bonds — the single most important fact in organic chemistry. A carbon bonded by four single bonds is saturated; one in a double or triple bond is unsaturated. Because C–H and C–C bonds are nearly nonpolar (electronegativities: C 2.55, H 2.20), hydrocarbons are nonpolar: insoluble in water, soluble in nonpolar solvents.
Alkanes: saturated chains and rings
Alkanes contain only C–C and C–H single bonds. A chain alkane with n carbons fits \(\mathrm{CnH{2n+2}}\): methane \(\mathrm{CH_4}\), ethane \(\mathrm{C_2H_6}\), propane \(\mathrm{C_3H_8}\). IUPAC naming: (1) find the longest chain and name it with the parent stem (meth-, eth-, prop-, but-, pent-, …) plus -ane; (2) number the chain so substituents get the lowest locants; (3) name branches with -yl (methyl, ethyl), listed alphabetically with position numbers. Cycloalkanes use the prefix cyclo- (cyclohexane \(\mathrm{C6H{12}}\)); each ring costs two hydrogens, so ring alkanes fit \(\mathrm{CnH{2n}}\).
Alkenes and alkynes: the unsaturated hydrocarbons
Alkenes contain at least one C=C bond; alkynes at least one C≡C bond. One double bond gives \(\mathrm{CnH{2n}}\); one triple bond gives \(\mathrm{CnH{2n-2}}\). Suffixes are -ene and -yne, with the multiple bond's locant included (1-butene vs. 2-butene). Because rotation around a double bond is blocked, alkenes show geometric (cis/trans) isomerism: in cis-2-butene the methyl groups sit on the same side; in trans-2-butene, opposite sides. These are different compounds with different boiling points. The exposed π electrons make alkenes react readily — the signature reaction is addition across the double bond.
Aromatic hydrocarbons: benzene's special stability
Benzene, \(\mathrm{C_6H_6}\), looks highly unsaturated yet does not react like an Alkene Hydrocarbon with at least one C=C bond Full entry →. Its six π electrons are delocalized around the ring, making all six C–C bonds identical and the ring unusually stable. Aromatics therefore undergo substitution reactions that preserve the ring rather than addition reactions that destroy it. Derivatives are named with parent benzene (methylbenzene, commonly toluene). Because benzene is a recognized carcinogen, laboratory work with it requires a fume hood — a general principle for volatile aromatic solvents.
Physical properties: what structure predicts
All hydrocarbons are nonpolar, held together by weak dispersion (London) forces. Consequences: (1) insoluble in water; (2) boiling points rise with molar mass — methane boils at −162 °C, pentane at 36 °C; (3) branched isomers boil lower than straight chains; (4) methane through butane are gases at room temperature, mid-size alkanes are liquids, larger ones waxy solids. All burn in oxygen to \(\mathrm{CO_2}\) and \(\mathrm{H_2O}\) — the basis of their use as fuels.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Alkane formula \(\mathrm{CnH{2n+2}}\) | Alkene formula \(\mathrm{CnH{2n}}\) | Each double bond removes two H atoms |
| Saturated (hydrocarbon) | Saturated (solution) | Hydrocarbon meaning = only single bonds; unrelated to solutions |
| cis vs. trans | Structural isomers | cis/trans differ only in double-bond arrangement; same connectivity |
| Aromatic compounds | Alkenes | Aromatics look unsaturated but are far more stable; prefer substitution |
| Benzene \(\mathrm{C_6H_6}\) | Cyclohexane \(\mathrm{C6H{12}}\) | Benzene has delocalized π electrons; cyclohexane is a saturated ring |
| Longest-chain naming | Choosing a chain that includes the branch | Always pick the longest continuous chain, even if a branch sits on a "side" |
| Substituent locant | Multiple-bond locant | Double bonds generally take priority when locants compete |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Hydrocarbons are molecules made of just two kinds of LEGO bricks: carbon and hydrogen. Carbon bricks have four connection points and snap into chains or rings; hydrogen bricks have one and fill the leftover spots. If all carbon–carbon connections are single, the molecule is an alkane (like grill-tank gas); if two carbons share a double connection, it's an alkene (plastic-maker); benzene is a ring with half-double connections, making it extra sturdy. The shape gives the name; the name predicts behavior.
Worked example
Example 1: Finding the degree of unsaturation
The degree of unsaturation (index of hydrogen deficiency, IHD) counts rings and π bonds using
\[ \mathrm{IHD} = \frac{2C + 2 - H}{2} \]
for hydrocarbons (halogens count as H; each N adds one to the numerator). Find the IHD of (a) octane \(\mathrm{C8H{18}}\), (b) 1-butene \(\mathrm{C_4H_8}\), (c) benzene \(\mathrm{C_6H_6}\).
Substitution (a):
\[ \mathrm{IHD} = \frac{2(8) + 2 - 18}{2} = \frac{16 + 2 - 18}{2} = \frac{0}{2} = 0 \]
Octane has no rings or π bonds — a saturated alkane.
Substitution (b):
\[ \mathrm{IHD} = \frac{2(4) + 2 - 8}{2} = \frac{8 + 2 - 8}{2} = \frac{2}{2} = 1 \]
One degree of unsaturation matches the single double bond.
Substitution (c):
\[ \mathrm{IHD} = \frac{2(6) + 2 - 6}{2} = \frac{12 + 2 - 6}{2} = \frac{8}{2} = 4 \]
Benzene's four degrees of unsaturation = three double bonds + one ring. The formula works even though the electrons are delocalized.
Example 2: How much CO₂ does burning octane produce?
Gasoline is often modeled as octane, \(\mathrm{C8H{18}}\), which combusts:
\[ \mathrm{2\,C8H{18}(l) + 25\,O_2(g) \rightarrow 16\,CO_2(g) + 18\,H_2O(g)} \]
How many grams of \(\mathrm{CO_2}\) form when 100.0 g of octane burns? Molar masses: octane 114.22, \(\mathrm{CO_2}\) 44.01 g/mol.
Step 1 — Grams to moles:
\[ 100.0\ \text{g C}8\mathrm{H}{18} \times \frac{1\ \text{mol C}8\mathrm{H}{18}}{114.22\ \text{g}} = 0.8755\ \text{mol C}8\mathrm{H}{18} \]
Step 2 — Mole ratio (16 mol CO₂ per 2 mol octane):
\[ 0.8755\ \text{mol C}8\mathrm{H}{18} \times \frac{16\ \text{mol CO}_2}{2\ \text{mol C}8\mathrm{H}{18}} = 7.004\ \text{mol CO}_2 \]
Step 3 — Moles to grams:
\[ 7.004\ \text{mol CO}_2 \times \frac{44.01\ \text{g CO}_2}{1\ \text{mol CO}_2} = 308.2\ \text{g CO}_2 \]
Check: each octane makes 8 CO₂, so 0.8755 mol × 8 = 7.004 mol; units cancel g → mol → mol → g. Burning 100 g of gasoline releases ~308 g of CO₂.
Key takeaways
- Hydrocarbons contain only C and H; carbon always forms 4 bonds.
- General formulas: alkane \(\mathrm{CnH{2n+2}}\), alkene \(\mathrm{CnH{2n}}\), alkyne \(\mathrm{CnH{2n-2}}\), cycloalkane \(\mathrm{CnH{2n}}\).
- Saturated = only single bonds; unsaturated = double/triple bonds present.
- IUPAC: longest chain → lowest locants → alphabetical substituents with -yl; suffix -ane/-ene/-yne. Alkenes show cis/trans isomerism because the double bond prevents rotation.
- Benzene is aromatic: delocalized π electrons, extra stability, substitution rather than addition.
- Hydrocarbons are nonpolar and water-insoluble; boiling point rises with molar mass and falls with branching.
- Complete combustion of any hydrocarbon gives \(\mathrm{CO_2}\) and \(\mathrm{H_2O}\).
- Degree of unsaturation: each ring or π bond = two hydrogens "missing" relative to \(\mathrm{CnH{2n+2}}\).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
General formula of an alkane with 7 carbons? Of an alkene with 7?
Show answer
Alkane: \(\mathrm{C7H{16}}\) (2(7) + 2). Alkene (one double bond): \(\mathrm{C7H{14}}\) (2(7)).
Name \(\mathrm{CH_3CH_2CH_2CH_3}\) by IUPAC rules.
Show answer
Butane — four-carbon chain (but-), all single bonds (-ane).
Why do cis- and trans-2-butene exist, while butane has no such pair?
Show answer
The double bond prevents rotation, locking the methyl groups into cis or trans arrangements. Butane's single bonds rotate freely, so no such pair exists.
A Hydrocarbon Compound containing only C and H Full entry → has the formula \(\mathrm{C_5H_8}\). How many degrees of unsaturation does it have?
Show answer
\(\mathrm{IHD} = (2(5) + 2 - 8)/2 = 2\). Examples: a diene (two double bonds), an alkyne (one triple bond), or a ring with one double bond.
Why does pentane boil far above methane?
Show answer
Dispersion forces strengthen as molecules grow. Pentane's larger molar mass and longer chain give much stronger attractions, so more energy is needed to vaporize it.
Write the balanced equation for propane's complete combustion \(\mathrm{C_3H_8}\).
Show answer
\(\mathrm{C_3H_8(g) + 5\,O_2(g) \rightarrow 3\,CO_2(g) + 4\,H_2O(g)}\).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Hydrocarbon
- Compound containing only C and H
- Alkane
- Hydrocarbon with only single bonds
- Alkene
- Hydrocarbon with at least one C=C bond
- Alkyne
- Hydrocarbon with at least one C≡C bond
- Aromatic hydrocarbon
- Ring with delocalized π electrons, e.g., benzene
- Saturated
- Contains only single bonds
- Unsaturated
- Contains double or triple bonds
- Geometric isomer
- cis/trans pair around a double bond
- Substituent
- Atom/group attached to the main chain
- Dispersion force
- Weak attraction between nonpolar molecules
Sources & references
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