AROMATIC HYDROXY COMPOUNDS
PHENOL
MF: C₆H₆O
functional group: phenolic group, C₆H₅OH
structural formula:
Physical properties
-at rtp, pure phenol = hygroscopic while solid w/ antiseptic smell (might be a little purple due to air oxidation)
(hygroscopic solid: solid absorbs water vapour in air > moist,
hydroscopic solid: absorbs so much water vapour > dissolve > colourless sol, eg FeCl₃)
-corrosive
-partially soluble in small amt of water > cloudy emulsion (tiny immiscible oily pheol drops in water)
cloudy emulsion > colourless solution by: (1) dilution- add more H₂O (due to more H-bonds w/ water), (2) heating- only temporary, on cooling > emulsion reforms

Chemical properties of phenols
-very weak acid (damped blue litmus: purple)
[cloudy aq. phenol + NaOH >(neutralization)> colourless sol: C₆H₅OH(aq) + NaOH(aq) > C₆H₅O^-Na⁺(aq) + H₂O(l)]
[C₆H₅O^-: phenoxide ion]
[if dil H₂SO₄ / stream of CO₂ added to sodium phenoxide sol, cloudy emulsion reforms:
C₆H₅O^-Na⁺(aq) + H⁺(aq) > C₆H₅OH(aq) + Na⁺(aq)]
(phenol: much weaker than carbonic acid, H₂CO₃ & doesn't react w/ Na₂CO₃ to give CO₂- distinguishes phenol from organic acids)

O atom's 2 π e^-s interact w/ 6 π electrons of benzene ring > delocalisation of electrons stabilises phenoxide ion & weakens O-H bond (> phenoxide ion + H⁺) (ethanol: not acidic as ethoxide ion not stablised by delocalised e^-s)
acidity of subs: based in pH / acid dissociation constant, ka (ka; not affected by changes in pH values) (greater ka: stronger acid) [pka = -lg ka (smaller pka: stronger acid)]

Reaction w/ Na
reagent: Na metal
condition: room temp
2C₆H₅OH(l) + 2 Na(s) > 2C₆H₅O^-Na⁺ +H₂
observation: -Na dissolves, -H₂ gas

Alkanoylation
reagent: alkanoyl chloride, R-C(=O)Cl
condition: room temp

observation: -steamy white fumes of HCl, -sweet, fruity ester evolved
to form white solid phenylbenzoate: use alkaline sol of phenol
(C₆H₅OH + NaOH > C₆H₅O^-Na⁺ + H₂O
C₆H₅O^-Na⁺ + C₆H₅-C(=O)Cl > C₆H₅-O-C(=O)-C₆H₅)

Reaction w/ aq Br
reagent: aq. Br
condition: room temp

observation: immediate white ppt of 2,4,6-tribromophenol (excess phenol: red Br decolourised)

Nitration
reagent: conc HNO₃
condition: room temp

reagent: conc HNO₃
condition: ~20°C


Test for phenol
-excess phenol + aq. Br > red Br decolourised + white ppt formed
-phenol + aq FeCl₃ > violet colouration
(C₆H₅OH + PCl₅ > C₆H₅Cl+ POCl₃ + HCl: v.low yield ~ no reaction)
phenol is not oxidised

Aromatic hydroxy compounds part 1

Back to 'A' level notes index

Back to notes index