Organic chemistry
Alkanes, alkenes, alkynes, arenes
Classify hydrocarbons by carbon framework and bonding, relate their formulas and structures to characteristic reactions, and compare the stability of aromatic rings.
IntuitionIntuition: a carbon skeleton with different bond patterns
Hydrocarbons contain only carbon and hydrogen. Single bonds let a chain rotate relatively freely; double and triple bonds constrain geometry and make addition reactions possible. A ring with delocalized electrons behaves differently from an ordinary alkene, so its reactions often preserve the ring.
SchoolSchool: classify and write formulas
Definition: Families and general formulas
Acyclic alkanes are saturated and follow CₙH₂ₙ₊₂; acyclic alkenes with one C=C and alkynes with one C≡C follow CₙH₂ₙ and CₙH₂ₙ₋₂, respectively. These formulas do not apply unchanged to compounds with additional rings or multiple bonds. Arenes include benzene and related aromatic systems.
| Family | Representative pattern | Typical chemistry |
|---|---|---|
| Alkane | C–C single bonds; CₙH₂ₙ₊₂ (acyclic) | Combustion; radical substitution |
| Alkene | One C=C; CₙH₂ₙ (acyclic) | Addition; polymerization |
| Alkyne | One C≡C; CₙH₂ₙ₋₂ (acyclic) | Stepwise addition |
| Arene | Aromatic π system | Substitution often preserves aromaticity |
Example: Formula of a five-carbon alkene
Solution
Use CₙH₂ₙ with n = 5: C₅H₁₀. This assumes no additional ring or multiple bond.
IUPAC names encode both the parent chain and the location of unsaturation: pent-1-ene and pent-2-ene are constitutional isomers. Restricted rotation about C=C can also produce E/Z stereoisomers when each alkene carbon bears two different substituents.
UndergraduateUniversity: structure explains reactivity
In an alkene, the σ bond joins the carbons while a π bond occupies electron density above and below the molecular plane. Electrophiles interact with this accessible π electron density; the exact outcome depends on substrate, reagent, solvent and conditions rather than on a memorized slogan alone.
Definition: Degree of unsaturation
For a neutral formula containing C, H, N and halogens X, the double-bond equivalent is DBE = (2C + 2 + N − H − X)/2. Each ring or π bond contributes one unit; a triple bond contributes two. Oxygen and sulfur do not enter this formula.
Example: Interpreting C₆H₆
Solution
DBE = (2×6 + 2 − 6)/2 = 4. Benzene has one ring and three π-bond equivalents; its actual bonding is delocalized rather than three isolated double bonds.
AdvancedAdvanced: aromaticity and selectivity
A planar, cyclic, conjugated system with (4n + 2) π electrons is aromatic in the Hückel model. Benzene has six π electrons (n = 1), stabilizing the delocalized ring. Electrophilic aromatic substitution replaces a ring hydrogen while restoring conjugation; addition would generally sacrifice aromatic stabilization.