Reactions of phenols, and tests for alcohols and phenols

Nitration

The -OH group **activates** the ring and is **ortho-para directing**: its lone pair raises the electron density at the ortho and para positions by resonance.

1. 1
   C₆H₅OH + HNO₃ → 2-nitrophenol or 4-nitrophenol, + H₂O

   Dilute HNO₃ at 298 K
2. 2★
   Separate the isomers by steam distillation. o-Nitrophenol is steam volatile because of intramolecular hydrogen bonding (within one molecule). p-Nitrophenol is less volatile because intermolecular hydrogen bonds link its molecules.
3. 3
   C₆H₅OH + 3HNO₃ → C₆H₂(NO₂)₃OH + 3H₂O

   Conc HNO₃ gives 2,4,6-trinitrophenol (picric acid), but in poor yield
4. 4
   Better route to picric acid: phenol + conc H₂SO₄ → phenol-2,4-disulphonic acid; then conc HNO₃ → 2,4,6-trinitrophenol.

★ key step

Halogenation

1. 1
   C₆H₅OH + Br₂ → 4-bromophenol (major) or 2-bromophenol (minor), + HBr

   Bromine in a solvent of low polarity (CHCl₃ or CS₂), at low temperature (273 K)
2. 2★
   C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

   Bromine water: the orange colour fades and a white precipitate of 2,4,6-tribromophenol forms
3. 3
   No Lewis acid is needed. The strongly activating -OH group makes Br₂ polarise on its own, unlike benzene, which needs FeBr₃.

★ key step

Kolbe's reaction

1. 1
   C₆H₅OH + NaOH → C₆H₅ONa + H₂O

   The phenoxide ion is even more reactive than phenol towards electrophiles
2. 2★
   C₆H₅ONa + CO₂ → HOC₆H₄COONa (sodium salicylate)

   CO₂, a weak electrophile, under pressure on heating (typical values, not given in your textbook: about 400 K and 4 to 7 atm). It enters the ortho position
3. 3
   HOC₆H₄COONa + HCl → HOC₆H₄COOH + NaCl

   Acidify: salicylic acid (2-hydroxybenzoic acid) is the main product

★ key step

Reimer-Tiemann reaction

A -CHO (formyl) group is brought into the ortho position of phenol.

1. 1
   Warm phenol with CHCl₃ and aqueous NaOH (about 340 K is typical; your textbook gives no temperature). A -CHCl₂ group enters the ortho position of the phenoxide ion, giving an intermediate (a substituted benzal chloride).
2. 2★
   The alkali hydrolyses -CHCl₂ to -CHO, giving the sodium salt of salicylaldehyde. Overall: C₆H₅OH + CHCl₃ + 4NaOH → NaOC₆H₄CHO + 3NaCl + 3H₂O
3. 3
   NaOC₆H₄CHO + HCl → HOC₆H₄CHO + NaCl

   Acidify: salicylaldehyde (2-hydroxybenzaldehyde)

★ key step

Two more reactions of phenol

1. 1
   C₆H₅OH + Zn → C₆H₆ + ZnO

   Heat with zinc dust: phenol is reduced to benzene. This is the only C-O bond cleavage that phenols show
2. 2
   C₆H₅OH + 2[O] → C₆H₄O₂ (benzoquinone) + H₂O

   Chromic acid (Na₂Cr₂O₇ with H₂SO₄) oxidises phenol to benzoquinone, a conjugated diketone. In air, phenols slowly darken as coloured quinones form

Activity: tests for alcoholic and phenolic -OH groups

1. 1
   Sodium metal test (alcohols and phenols): add a small piece of dry sodium to 1 mL of the dry liquid. Brisk effervescence of hydrogen shows an -OH group.

   2R-OH + 2Na → 2R-ONa + H₂
2. 2
   Ester test (alcohols): warm 1 mL of the liquid with 1 mL of ethanoic acid and 1 to 2 drops of conc H₂SO₄, then pour into water. A fruity smell shows an alcohol.
3. 3
   Ceric ammonium nitrate test (alcohols): add a few drops of the reagent and shake. A red colour shows an alcoholic -OH; it fades on standing or with excess reagent.
4. 4
   Iodoform test: warm with iodine and NaOH solution. A yellow precipitate of CHI₃ shows ethanol, or a 2° alcohol with a CH₃CH(OH)- group. Methanol does not give it.

   CH₃CH₂OH + 4I₂ + 6NaOH → CHI₃ + HCOONa + 5NaI + 5H₂O
5. 5
   Lucas test tells 1°, 2° and 3° alcohols apart (see the reactions of alcohols).
6. 6★
   Litmus and ferric chloride tests (phenols): phenol turns blue litmus faintly red. With neutral FeCl₃ solution it gives a violet colour.

   6C₆H₅OH + FeCl₃ → [Fe(OC₆H₅)₆]³⁻ + 3H⁺ + 3HCl
7. 7
   Bromine water test (phenols): the orange colour fades and a white precipitate forms.
8. 8
   Phthalein dye test (phenols): heat a little of the compound with phthalic anhydride and 1 to 2 drops of conc H₂SO₄ for a minute; cool and pour into dilute NaOH. Phenol gives a dark pink colour (phenolphthalein); other phenols give other colours. Record each test in three columns: experiment, observation, inference.

★ key step