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Chapter 24: Phenols



Summary | Phenols | Preparation of Phenols | Reactions of Phenols | Reactions of Aryl Ethers | Spectroscopic Analysis | Self Assessment | Quiz |


Phenols

Chapter 24: Phenols

Nomenclature:
Functional group suffix = -common - phenol, systematic - benzenol (review)
Functional group prefix = hydroxy
Numbering of the ring begins at the hydroxyl-substituted carbon and proceeds in the direction of the next substituted carbon that possesses the lower number.

Ortho, meta or para ? Mono-substituted phenols are characterized using the prefix ortho (o-), meta (m-) or para (p-) depending on the placement of the substituent from the hydroxyl group or the hydroxyl group from a higher priority functional group, 1,2-, 1,3- or 1,4- respectively.
 

1,2-chlorophenol
or
o-chlorophenol
1,3-chlorophenol
or
m-chlorophenol
1,4-chlorophenol
or
p-chlorophenol

There are several other functional group suffixes, for various substituted phenols, that one needs to know, check here.

Physical Properties:

Structure:
 
  • The alcohol functional group consists of an O atom bonded to an sp2-hybridized aromatic C atom and a H atom via s bonds.
  • Both the C-O and the O-H bonds are polar due to the high electronegativity of the O atom.
  • Conjugation exists between an unshared electron pair on the oxygen and the aromatic ring.
  • This results in, compared to simple alcohols:
    • a shorter carbon-oxygen bond distance
    • a more basic hydroxyl oxygen
    • a more acidic hydroxyl proton (-OH)

phenol resonance

Acidity:

Substituent Effects on Acidity
Substituents, particularly those located ortho or para to the -OH group, can dramatically influence the acidity of the phenol due to resonance and / or inductive effects. Electron withdrawing groups enhance the acidity, electron donating substituents decrease the acidity.  The resonance stabilization of o-nitrophenol is shown below:
resonance stabilisation of ortho-nitrophenolate
Compound
  pKa   
Compound
  pKa 
Phenol
  10.0      
o-Methoxyphenol
  10.0  
p-Methoxyphenol
  10.2
o-Methylphenol
  10.3  
p-Methylphenol
  10.3
o-Chlorophenol
   8.6  
p-Chlorophenol
    9.4
o-Nitrophenol
   7.2  
p-Nitrophenol
    7.2
m-Nitrophenol
   8.4      

Questions:
Draw resonance structures to show the stabilization of p-nitrophenol.click for diagram
How about m-nitrophenol ?click for diagram

Study Tip:
Did you answer question about the m-nitrophenol correctly ? If you did, you probably have a good understanding of resonance and curly arrows. If not, you may want to review resonance and curly arrows again

Reactivity:
 

electrostatic potential of phenol
electrostatic potential of phenolate
The image to the left shows the electrostatic potential for phenol. 
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. 
  • The hydroxyl O atom is a region of high electron density (red) due to the lone pairs. 
  • The hydroxyl O atom can function as a nucleophile or Lewis base.
  • There is low electron density (blue) on H atom of the hydroxyl group, i.e. H+ character, therefore phenols are acidic (pKa ~ 10)
  • Due to conjugation with the ring, phenols are more acidic than alcohols (pKa ~ 16).
  • Removal of the proton generates a phenolate ion.
The image to the left shows the electrostatic potential for a phenolate ion.
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. 
  • Note the increased electron density on the oxygen compared to the phenol


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