Organic chemistry tricks that make hydrocarbons stick

Alkane = All single bonds (saturated). Alkene = has a double bond. Alkyne = has a triple bond. Memory: All Linear, Alkane Double = Doesn't apply, It's sIngle only.

Alkanes follow CnH(2n+2). Methane=CH4, Ethane=C2H6, Propane=C3H8. Each carbon adds 2 H's plus the 2 end H's.

At room temperature: C1–C4 alkanes are gases (methane, ethane, propane, butane). C5–C17 are liquids. C18+ are solids (wax, tar).

E (from German 'entgegen' = opposite): higher priority groups on opposite sides. Z (from German 'zusammen' = together): higher priority groups on same side. Use Cahn-Ingold-Prelog priority rules.

Degrees of unsaturation (DoU) = (2C + 2 + N - H - X) / 2. Each ring or double bond = 1 DoU. Triple bond = 2 DoU. Benzene ring = 4 DoU.

When HX (or H₂O) adds to an alkene, H adds to the carbon that already has MORE hydrogens. The result: more substituted carbocation forms (more stable). Memory: 'The rich get richer' — carbon with more H gets the extra H. Modern explanation: more substituted carbocation (3° > 2° > 1°) is more stable. Anti-Markovnikov: radical addition (ROOR + HBr, H goes to more substituted carbon, Br to less).

Tertiary carbocations (3°) are most stable — three alkyl groups donate electrons by hyperconjugation and induction. Stability order: 3° > 2° > 1° > methyl. Allylic and benzylic carbocations are especially stable (resonance delocalization). Vinyl and aryl carbocations are very unstable. Rearrangements occur: hydride or methyl shift from adjacent C can form a more stable carbocation (Wagner-Meerwein).

Alkanes react with halogens under UV light (or heat) via radical chain mechanism. Three stages: Initiation: X₂ → 2X• (UV breaks X-X bond). Propagation: X• + R-H → HX + R•; R• + X₂ → RX + X•. Termination: any two radicals combine. Selectivity: F₂ (most reactive, least selective, explosive) > Cl₂ (selective) > Br₂ (most selective — most discriminating) > I₂ (too slow, endothermic). Bromine prefers most substituted C (more stable radical).

BH₃/THF adds across alkene syn (both B and H add to same face). B goes to less hindered carbon (anti-Markovnikov). Oxidation with H₂O₂/NaOH replaces B with OH (retention of configuration). Overall: anti-Markovnikov, syn addition of OH. Net result: OH on less substituted carbon. Compare to acid-catalyzed hydration (Markovnikov, OH on more substituted). Key reagents: (1) BH₃·THF, (2) H₂O₂, NaOH.

Ozonolysis cleaves the C=C double bond. Reductive workup (Zn/AcOH or Me₂S): aldehydes and ketones. Oxidative workup (H₂O₂): aldehydes oxidized to carboxylic acids; ketones unchanged. Used to determine alkene structure (work backwards from carbonyl fragments). Disubstituted alkene → 2 ketones. Monosubstituted → 1 aldehyde + 1 ketone. Terminal alkene → formaldehyde + ketone/aldehyde.

Two ways to add two OH groups across a double bond: Syn dihydroxylation: OsO₄ (catalytic with N-oxide co-oxidant) → syn-diol (both OH on same face). Also: KMnO₄ (cold, dilute) → syn-diol. Anti dihydroxylation: (1) mCPBA → epoxide, (2) H₂O/acid or base → anti-diol (ring opens with inversion). Sharpless asymmetric dihydroxylation uses chiral ligands with OsO₄ → enantioselective syn diol.

Terminal alkynes (R-C≡C-H) have unusually acidic C-H bonds (pKa ~25) because the sp-hybridized carbon holds electrons closer to the nucleus (more s-character = more electronegative). Deprotonated by NaNH₂ (sodium amide) or n-BuLi to give acetylide anion (R-C≡C⁻). Acetylides are strong bases AND good nucleophiles — react with alkyl halides (SN2) and carbonyl compounds. Key carbon-carbon bond forming reaction.

Cyclohexane adopts chair conformation (most stable). Each carbon: one axial bond (up or down, alternating) and one equatorial bond (nearly horizontal). Ring flip interconverts axial and equatorial. Large substituents prefer equatorial (less steric strain — 1,3-diaxial interactions avoided). A-value = energy cost of putting group axial vs equatorial. tert-Butyl group locks ring (too large to go axial). Trans-diequatorial is most stable disubstituted isomer.