Preparing Alcohols: Hydration, Hydroboration, Grignard

Alcohols are organic compounds in which a hydroxyl group (–OH) is attached directly to a carbon atom, for example ethanol (CH3CH2OH). They are some of the most useful intermediates in chemistry because one alcohol molecule can be converted into aldehydes, ketones, ethers, esters and alkyl halides.

You rarely buy every alcohol you need — you build it. Three classic routes are acid-catalyzed hydration, hydroboration–oxidation, and the Grignard reaction. Each gives a different substitution pattern, deciding whether you get a primary, secondary or tertiary alcohol.

Key Concepts

• Markovnikov's rule: in acid-catalyzed hydration, the –OH group adds to the more substituted carbon of the double bond (the carbon that can form the more stable carbocation).
• Anti-Markovnikov addition: in hydroboration–oxidation, the –OH group ends up on the less substituted carbon — the opposite pattern.
• Grignard reagent: an organomagnesium compound, R–MgX (X = Cl, Br, I), prepared from an alkyl halide and dry magnesium in dry ether. It behaves like R− and attacks carbonyl carbons.
• Syn addition: both new groups add to the same face of the double bond, as in hydroboration.

How It Works

1. Acid-catalyzed hydration. An alkene is treated with dilute sulphuric acid (or passed with steam over phosphoric acid at about 573 K and high pressure industrially). A proton adds to the double bond to give the more stable carbocation, water attacks it, and loss of a proton gives the alcohol. Propene gives propan-2-ol. Because a carbocation is involved, rearrangements are possible.

2. Hydroboration–oxidation. The alkene first adds diborane (B2H6), placing boron on the less crowded carbon, then oxidation with alkaline hydrogen peroxide (H2O2/NaOH) replaces boron with –OH with retention of position. Net result: anti-Markovnikov, syn addition of water, with no rearrangement. Propene gives propan-1-ol.

3. Grignard reaction. The Grignard reagent adds to the carbonyl carbon: formaldehyde (HCHO) gives a primary alcohol, any other aldehyde gives a secondary alcohol, and a ketone gives a tertiary alcohol, after aqueous acid work-up. Grignard reagents are destroyed by water, alcohols, amines and carboxylic acids, so everything must be perfectly dry.

Worked Example

Prepare propan-2-ol from propene and calculate the theoretical yield from 42 g of propene.

1. Molar mass of propene (C3H6) = 3 × 12 + 6 × 1 = 42 g/mol, so 42 g = 1.00 mol.
2. Acid-catalyzed hydration adds one molecule of water across the double bond following Markovnikov's rule, so the –OH lands on carbon 2, giving propan-2-ol (CH3CH(OH)CH3).
3. Molar mass of propan-2-ol (C3H8O) = 36 + 8 + 16 = 60 g/mol.
4. The reaction is 1:1, so 1.00 mol of propene gives 1.00 mol of product = 60 g of propan-2-ol.

Common Misconceptions

• "Hydroboration puts –OH on the more substituted carbon." Wrong — it is anti-Markovnikov: boron attaches to the less hindered carbon and –OH replaces it there.
• "Grignard reagents tolerate –OH or –NH2 groups." Wrong. Any acidic proton destroys the reagent instantly; such groups must be protected first.
• "Acid hydration never rearranges." Wrong. The carbocation intermediate can undergo hydride or alkyl shifts, changing the product skeleton.

Real-World Applications

• Ethanol fuel: industrial ethanol for fuel blending is made by hydration of ethene from petroleum, or by fermentation of sugar.
• Isopropanol: propan-2-ol made by propene hydration is the rubbing alcohol used as an antiseptic and cleaning solvent.
• Pharmaceuticals: Grignard reactions build the carbon skeletons of many drugs, since they form new C–C bonds while installing the alcohol group.

In short, acid hydration gives Markovnikov alcohols through carbocations, hydroboration–oxidation gives the anti-Markovnikov alcohol cleanly, and Grignard reagents convert formaldehyde, aldehydes and ketones into primary, secondary and tertiary alcohols. Pick the route that puts the –OH exactly where you want it.