Phenol

Question 1

Chemical properties of phenols

Answer 1

1. Acidic
2. Does not undergo nucleophilic sub.
3. Stability depends on the groups present in a phenol

Question 2

Why are phenols acidic?

Answer 2

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

Question 3

Why phenols do not undergo nu:sub

Answer 3

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

Question 4

The stability of phenols depends on the groups they are attached to

Answer 4

1. Electron withdrawing groups (NO2, Cl)
   - Electron withdrawing groups disperse the negative charge on O hence further stabilising the phenoxide ion
2. Electron donating groups
   - Electron donating groups intensify the negative charge on O hence destabilising the phenoxide ion

Question 5

Reactions

Answer 5

1. Redox
   - Na(s) to from phenoxide Na+ and 1/2H2
2. Acid base
   - NaOH(aq) from phenoxide Na+ and H2O
3. Condensation (nu:sub)
   - Na(s), RCOCl, heat to form ester
   Na generates phenoxide ion which is more stronger nucleophile than phenol
4. 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

Question 6

Test for phenol

Answer 6

neutral FeCl3(aq)
- violet colouration observed