Preparation
Alcohols are one of the most important functional groups in organic chemistry, characterised by the presence of a hydroxyl group (–OH) bonded to a saturated carbon atom. Their preparation from simpler starting materials is a foundational skill in synthesis, and three methods dominate at this level: acid-catalysed hydration of alkenes, hydroboration–oxidation of alkenes, and Grignard reagent synthesis. Each method gives alcohols with different regiochemistry or carbon skeletons, making them complementary tools in the chemist's toolkit. The choice of preparation method depends on what alcohol is needed and what starting materials are available. Acid-catalysed hydration follows Markovnikov's rule, placing the –OH on the more substituted carbon. Hydroboration–oxidation is anti-Markovnikov and proceeds with syn addition, providing the opposite regiochemical outcome. Grignard synthesis goes further by forming entirely new carbon–carbon bonds, enabling the construction of primary, secondary, or tertiary alcohols from carbonyl compounds such as aldehydes, ketones, or formaldehyde. Understanding these three methods also illustrates broader principles of organic reaction mechanisms: carbocation stability, electrophilic addition, concerted pericyclic steps, and nucleophilic addition to carbonyl groups. Mastering alcohol preparation builds the conceptual scaffold needed for more advanced synthesis problems encountered in A-level and university chemistry.
- Acid-Catalysed Hydration of Alkenes
- Hydroboration–Oxidation
- Grignard Reagent Synthesis
Acid-Catalysed Hydration of Alkenes
An alkene is treated with dilute strong acid (commonly H₂SO₄ or H₃PO₄) and water, typically under heat and pressure. Step 1: The π electrons of the double bond attack a proton (H⁺) from the acid. The proton adds to the carbon that bears more hydrogen atoms (the less substituted carbon), generating the more stable secondary or tertiary carbocation at the more substituted carbon — this is Markovnikov's rule. Step 2: A water molecule acts as a nucleophile, donating a lone pair to the carbocation to form an oxonium ion (R₃O⁺). Step 3: A second water molecule deprotonates the oxonium ion, releasing H⁺ (regenerating the acid catalyst) and yielding the alcohol. For propene, the product is propan-2-ol, not propan-1-ol.
Hydroboration–Oxidation
This two-step sequence, developed by Herbert C. Brown, delivers anti-Markovnikov alcohols with syn (same-face) addition. Step 1 — Hydroboration: Borane (BH₃, often as the THF complex BH₃·THF) adds across the double bond in a single concerted four-centre transition state. The boron atom, being electron-deficient, bonds to the less hindered (less substituted) carbon, while hydrogen delivers simultaneously to the more substituted carbon. The result is an organoborane (R–BH₂, then R₂BH, then R₃B as three alkene molecules react). Step 2 — Oxidation: The organoborane is treated with hydrogen peroxide (H₂O₂) in basic solution (NaOH/H₂O). Peroxide replaces the boron with an –OH group with retention of configuration. The net result: –OH ends up on the less substituted carbon, opposite to acid hydration. For propene, the product is propan-1-ol.
Grignard Reagent Synthesis
Grignard reagents (R–MgX, where X = Cl, Br, or I) are prepared by reacting an alkyl or aryl halide with magnesium turnings in dry diethyl ether or THF. The carbon bonded to magnesium carries a partial negative charge (δ−) and acts as a powerful nucleophile and carbanion equivalent. When a Grignard reagent is added to a carbonyl compound, the carbanion attacks the electrophilic carbonyl carbon, forming a new C–C bond and an alkoxide salt. Aqueous acid workup (NH₄Cl or dilute HCl) protonates the alkoxide to give the alcohol. The type of alcohol produced depends on the carbonyl partner: formaldehyde (HCHO) gives a primary alcohol, any other aldehyde (RCHO) gives a secondary alcohol, and a ketone (RCOR') gives a tertiary alcohol. The Grignard reagent is highly sensitive to moisture and air and must be prepared and used under strictly anhydrous, inert conditions.
The Key Equation
- \ce{CH3CH=CH2} — Propene (—): The alkene starting material; the π bond is broken during the reaction.
- \ce{H3PO4} — Phosphoric acid (—): The acid catalyst that provides H⁺ to initiate addition; it is regenerated at the end.
- \ce{CH3CH(OH)CH3} — Propan-2-ol (—): The Markovnikov product; the –OH group is on the secondary (more substituted) carbon.
Worked Example
Problem. Starting from but-1-ene (CH₂=CHCH₂CH₃), predict the major product of (a) acid-catalysed hydration and (b) hydroboration–oxidation. Explain the regiochemical outcome for each.
Answer. Acid hydration gives butan-2-ol (Markovnikov); hydroboration–oxidation gives butan-1-ol (anti-Markovnikov).
Origins · 1 minHerbert C. Brown and how this was discovered
Herbert C. Brown (born Herbert Brovarnik in London) pioneered the chemistry of organoboron compounds and discovered hydroboration in 1956 while at Purdue University. Recognising that borane adds across double bonds in a regioselective, syn fashion, he developed the hydroboration–oxidation sequence as a versatile anti-Markovnikov route to alcohols. This discovery transformed synthetic organic chemistry by giving chemists precise control over which carbon receives the –OH group. Brown was awarded the Nobel Prize in Chemistry in 1979, jointly with Georg Wittig. Brown's interest in boron chemistry began almost by accident: as a graduate student, he used diborane because it was the cheapest available reducing agent, and he became fascinated by its unexpected reactivity.
Related Concepts
- Electrophilic Addition to Alkenes — Acid-catalysed hydration and hydroboration–oxidation both proceed via electrophilic addition to the C=C double bond; understanding this mechanism is essential to explain regiochemistry.
- Properties and Reactions of Alcohols — Once prepared, alcohols undergo oxidation, esterification, and elimination; knowing how they are made contextualises their reactivity.
- Grignard Reagents and Organometallics — Grignard synthesis is one of the three preparation methods; a deeper treatment of Grignard chemistry explains nucleophilic addition to carbonyls and the scope of alcohols obtainable.
- Markovnikov's Rule and Regioselectivity — Predicting whether acid hydration or hydroboration–oxidation gives the desired alcohol requires a firm grasp of Markovnikov's rule and carbocation stability.