Organic chemistry tricks that make alcohols & ethers stick

Lucas reagent = ZnCl2 + HCl. Tertiary alcohols react immediately (cloudy). Secondary alcohols react slowly (5 min). Primary alcohols — no reaction at room temp. Used to distinguish alcohol classes.

Primary alcohols: PCC/PDC → aldehyde (stops). KMnO4 or K2Cr2O7 → carboxylic acid. Secondary alcohols → ketone with any oxidant. Tertiary alcohols — cannot be oxidized (no H on OH carbon).

Ethers are resistant to most reagents — no reaction with bases, mild acids, oxidants, or reducing agents. Cleaved only by concentrated HI or HBr at high temperature via SN2 or SN1.

Best method to make ethers. Alkoxide (RO⁻) attacks alkyl halide via SN2. Use primary alkyl halide to avoid elimination. NaH converts alcohol to alkoxide first.

Alcohols have OH — can H-bond with each other → high boiling points. Ethers have no OH — cannot H-bond with each other → much lower boiling points. Both dissolve in water via H-bonding.

Alcohols (pKa ~16) are weak acids. Strong bases (NaH, NaNH₂, alkyllithiums) deprotonate them to alkoxides (RO⁻). Smaller alkyl groups = more acidic (methanol > ethanol > isopropanol) because larger groups destabilize the alkoxide through steric electron donation. Alkoxides are strong bases and good nucleophiles — used in Williamson synthesis and other reactions.

Alcohols dehydrate to alkenes under acidic conditions with heat. Tertiary alcohols dehydrate easily (E1, stable 3° carbocation). Secondary alcohols dehydrate under stronger conditions. Primary alcohols require harsh conditions (E2-like). Reagents: H₂SO₄ or H₃PO₄ + heat. Follows Zaitsev — most substituted alkene is major product. Reverse of acid-catalyzed hydration of alkenes.

Epoxides (oxiranes) are 3-membered rings containing oxygen. Ring strain makes them far more reactive than ethers. Synthesis: alkene + mCPBA (meta-chloroperoxybenzoic acid) → epoxide (stereospecific — cis alkene gives cis epoxide). Ring opening: acid conditions → Markovnikov attack (nucleophile attacks more substituted carbon). Base conditions → SN2 (nucleophile attacks less substituted carbon). Anti addition in both cases.

Grignard reagents (RMgX) open epoxides at the less hindered carbon (SN2). This is a powerful synthetic method: opens epoxide → extends chain by R group → gives primary alcohol after workup. Example: ethylene oxide + RMgX → RCH₂CH₂OH (primary alcohol, 2 carbons longer than R). Useful for extending carbon chains in synthesis.

Crown ethers are cyclic ethers with multiple oxygen atoms arranged in a ring. They selectively bind metal cations whose size matches the cavity: 12-crown-4 binds Li⁺, 15-crown-5 binds Na⁺, 18-crown-6 binds K⁺. Used as phase-transfer catalysts — dissolve ionic reagents in organic solvents. Named by ring size and number of oxygens (18-crown-6 = 18 atoms in ring, 6 oxygens).

Thiols (R-SH) are the sulfur analogs of alcohols. More acidic than alcohols (pKa ~10 vs ~16) because S-H bond weaker and sulfur polarizability stabilizes thiolate. Thiolates (RS⁻) are excellent nucleophiles — sulfur is larger and more polarizable than oxygen. Thiols oxidize to disulfides (R-S-S-R) — important in protein structure (cysteine bridges). Oxidation: 2 RSH → RSSR + 2H⁺ + 2e⁻.

When you need to react at one functional group without disturbing another: Protect the alcohol as a TMS ether (R-OH + TMSCl/Et₃N → R-OTMS). Remove with fluoride (TBAF) or acid. Protect carbonyl as an acetal (R₂C=O + HO(CH₂)₂OH/H⁺ → cyclic acetal). Stable to base/nucleophiles; remove with aqueous acid. Strategy: PROTECT → REACT → DEPROTECT.

Symmetrical ethers form when primary alcohols react with H₂SO₄ at 140°C (lower temp than dehydration to alkene at 180°C). One alcohol is protonated → good leaving group. Second alcohol acts as nucleophile. Only works for primary alcohols — secondary and tertiary undergo elimination instead. Temperature control is key: 140°C → ether, 180°C → alkene.