Phenol Flashcards
Chemical properties of phenols
- Acidic
- Does not undergo nucleophilic sub.
- Stability depends on the groups present in a phenol
Why are phenols acidic?
Phenoxide ions (the conjugate base of phenols) are resonant stabilised. The p-p orbital overlap between unhybridised p orbital of O and pi electron cloud of benzene results in the delocalisation of lone pair of electrons on O into the benzene ring. This disperses the negative charge on O hence stabilising the phenoxide ion.
Thus the forward dissociation reaction of phenols to form phenoxide ions and H+ hence phenols are acidic
Why phenols do not undergo nu:sub
The lone pair of electrons on O delocalises into the benzene ring due to p-p orbital overlap. Hence the lone pair on O are less available to nu: attack.
Moreover since the C-O bond is strengthened and has partial double bond character which is harder to overcome
The stability of phenols depends on the groups they are attached to
- Electron withdrawing groups (NO2, Cl)
- Electron withdrawing groups disperse the negative charge on O hence further stabilising the phenoxide ion - Electron donating groups
- Electron donating groups intensify the negative charge on O hence destabilising the phenoxide ion
Reactions
- Redox
- Na(s) to from phenoxide Na+ and 1/2H2 - Acid base
- NaOH(aq) from phenoxide Na+ and H2O - Condensation (nu:sub)
- Na(s), RCOCl, heat to form ester
Na generates phenoxide ion which is more stronger nucleophile than phenol - Electrophilic subs
- Br2(aq)
a) trisub 2,4,6, bromo phenol
white ppt/orange Br2 decolourises
- Br2 (aq) in CCl4(aq)
b) monosub 2/4 bromo phenol
orange-red decolourises
c) HNO3(aq)
monosub nitrophenol
d) conc HNO3
trisub nitrophenol, white ppt
Test for phenol
neutral FeCl3(aq)
- violet colouration observed