Preparation of alcohols

From alkenes: acid-catalysed hydration and its mechanism

1. 1★
   CH₃CH=CH₂ + H₂O → CH₃CH(OH)CH₃

   Dilute acid (H⁺) as catalyst. Markovnikov's rule: the -OH ends up on the carbon with fewer H atoms
2. 2
   Step 1, protonation: CH₃CH=CH₂ + H₃O⁺ → CH₃C⁺HCH₃ + H₂O

   The π electrons of the alkene attack H₃O⁺ (electrophilic attack); the more stable 2° carbocation forms
3. 3
   Step 2, attack by water: CH₃C⁺HCH₃ + H₂O → CH₃CH(O⁺H₂)CH₃

   Water acts as a nucleophile and joins the positive carbon
4. 4
   Step 3, deprotonation: CH₃CH(O⁺H₂)CH₃ + H₂O → CH₃CH(OH)CH₃ + H₃O⁺

   H₃O⁺ is given back, so the acid is a catalyst

★ key step

From alkenes: hydroboration-oxidation

1. 1
   6CH₃CH=CH₂ + B₂H₆ → 2(CH₃CH₂CH₂)₃B

   Diborane adds across C=C. Boron attaches to the terminal carbon (the one with more H atoms), giving a trialkylborane
2. 2★
   (CH₃CH₂CH₂)₃B + 3H₂O₂ → 3CH₃CH₂CH₂OH + B(OH)₃

   H₂O₂ in aqueous NaOH oxidises it. Propan-1-ol forms in excellent yield

★ key step

Same alkene, two different alcohols

Choose the method by where you want the -OH group.

From carbonyl compounds, acids and esters (reduction)

1. 1
   RCHO + H₂ → RCH₂OH

   Finely divided Pd, Pt or Ni catalyst, or NaBH₄ or LiAlH₄. An aldehyde gives a 1° alcohol
2. 2
   RCOR' + H₂ → RCH(OH)R'

   Same reagents. A ketone gives a 2° alcohol
3. 3★
   RCOOH + 4[H] → RCH₂OH + H₂O

   [H] stands for hydrogen from the reducing agent: (i) LiAlH₄, then (ii) H₂O. LiAlH₄ is strong but costly, so it is used only for special chemicals
4. 4
   RCOOR' + 2H₂ → RCH₂OH + R'OH

   Industry turns acids into esters and reduces them with H₂ and a catalyst

★ key step

From Grignard reagents

A Grignard reagent, R-MgX, has a carbon that behaves like a carbanion (a negative carbon). It adds to the carbon of the C=O group.

1. 1
   Step 1, nucleophilic addition: >C=O + R-MgX → >C(R)-OMgX

   The R group of the Grignard reagent attacks the δ⁺ carbonyl carbon, giving an adduct
2. 2★
   Step 2, hydrolysis: >C(R)-OMgX + H₂O → >C(R)-OH + Mg(OH)X
3. 3
   HCHO + CH₃MgBr → CH₃CH₂OMgBr; CH₃CH₂OMgBr + H₂O → CH₃CH₂OH + Mg(OH)Br

   Methanal gives a 1° alcohol (ethanol)
4. 4
   CH₃CHO + CH₃MgBr → (CH₃)₂CHOMgBr; (CH₃)₂CHOMgBr + H₂O → (CH₃)₂CHOH + Mg(OH)Br

   Other aldehydes give a 2° alcohol (propan-2-ol)
5. 5
   CH₃COCH₃ + CH₃MgBr → (CH₃)₃COMgBr; (CH₃)₃COMgBr + H₂O → (CH₃)₃COH + Mg(OH)Br

   Ketones give a 3° alcohol (2-methylpropan-2-ol)

★ key step