Preparation of Alcohols & Phenols
Synthesize the hydroxyl group. Master selective reductions with $LiAlH_4$ and $NaBH_4$, the definitive Grignard rules, and the industrial Cumene process.
Module Focus: The Versatile -OH Group
Alcohols ($R-OH$) and Phenols ($Ar-OH$) are formed when a hydrogen atom in an aliphatic or aromatic hydrocarbon is replaced by a hydroxyl group. Synthesizing these compounds requires precision. You must know exactly which reagent reduces a ketone but leaves an ester untouched, and exactly which carbonyl compound reacts with a Grignard reagent to yield a primary, secondary, or tertiary alcohol.
1. Preparation of Alcohols from Alkenes
The hydration of alkenes is a standard industrial and laboratory method. Reviewing the hydration triad from the Hydrocarbons chapter is crucial here.
| Method | Regioselectivity | Key Trap |
|---|---|---|
| Acid-Catalyzed Hydration ($H_2O / H^+$) |
Markovnikov | Rearrangements Occur! (Carbocation shifts) |
| Hydroboration-Oxidation (HBO) ($B_2H_6$, then $H_2O_2/OH^-$) |
Anti-Markovnikov | Syn-Addition, NO Rearrangements |
| Oxymercuration-Demercuration ($Hg(OAc)_2/H_2O$, then $NaBH_4$) |
Markovnikov | Anti-Addition, NO Rearrangements |
2. Reduction of Carbonyl Compounds
Aldehydes reduce to Primary ($1^\circ$) alcohols. Ketones reduce to Secondary ($2^\circ$) alcohols. The critical NEET skill is choosing the correct reducing agent.
- A very powerful, non-selective reducing agent.
- Reduces aldehydes, ketones, carboxylic acids, and esters directly to alcohols.
- Highly reactive with moisture; must be used in a dry ether solvent.
- A milder, highly selective reducing agent.
- Reduces aldehydes and ketones exclusively.
- Does NOT reduce Carboxylic acids or Esters!
- Can be safely used in aqueous or alcoholic solvents.
3. From Grignard Reagents ($RMgX$)
Grignard reagents react with carbonyl compounds via Nucleophilic Addition to form an adduct, which upon acidic hydrolysis yields an alcohol. This is the most heavily tested method because the choice of carbonyl dictates the degree of the alcohol.
4. Preparation of Phenols
Chlorobenzene is fused with $NaOH$ under extreme conditions due to the inertness of the aryl halide bond.
Aniline is diazotized ($NaNO_2 + HCl, 0-5^\circ\text{C}$). The resulting diazonium salt is simply warmed with water to yield phenol.
Most of the world's phenol is produced this way from Cumene (Isopropylbenzene). It is highly tested because it yields a valuable commercial by-product.
- Oxidation: Cumene is oxidized in air to form Cumene hydroperoxide.
- Hydrolysis: It is treated with dilute acid, triggering a rearrangement that cleaves the molecule.
- Products: Phenol + Acetone (Propanone).
5. Physical Properties
The $-OH$ group dominates the physical properties of alcohols and phenols due to its ability to form strong intermolecular hydrogen bonds.
- BPs of alcohols/phenols are significantly higher than corresponding hydrocarbons, ethers, and haloalkanes of similar mass due to H-bonding.
- Branching effect: Among isomeric alcohols, BP decreases as branching increases. Branching makes the molecule spherical, decreasing surface area and van der Waals forces.
- Lower alcohols are highly miscible in water because they form H-bonds with $H_2O$.
- Solubility rapidly decreases as the size of the hydrophobic alkyl/aryl group (the hydrocarbon tail) increases.
NEET Grand Test: Alcohols Prep
15 High-Yield Questions testing selective reduction, Grignard additions, and the Cumene pathway.
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