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26.3 Introduction to Phenols

26.3    Introduction to Phenols

 

What are phenols?

  • Phenols are benzene compounds which have an -OH group attached directly to it

 

  • In a phenol molecule, one of the lone pairs on the oxygen overlaps with the delocalised electron system to give a structure like this:
  • This increases the electron density of the delocalised electron system. It makes phenols much more reactive than benzene itself. Also, it increases the acidity of phenol as well
  • Physical properties of phenols:
    1. Phenol has a higher melting and boiling points than methylbenzene
      • This is because phenols can form hydrogen bonds between them in addition of van der Waals’ forces and permanent dipole-dipole forces
    2. Phenol itself is more soluble in water than other phenols
      • This is because a small phenol molecule can form more effective hydrogen bonds with water molecules
      • However, most phenols are generally insoluble in water

 

Acidity of  phenols

 

  • Compounds like alcohols and phenols which contain an -OH group attached to a hydrocarbon are very weak acids.

 

  • Alcohol’s strength as an acid is so weak that it is ignored under normal situations. However, phenols have recognisable acid strength

 

  • This is because phenol can donate a proton to form a phenoxide The presence of hydroxonium ions makes it acidic.

 

Comparing the strength of organic acids

  • The strength of organic acids depends on:
  1. The strength of the O-H bond which is to be broken.
  2. The stability of the anion formed.
  • The strengths of the acids are as The lower the value of pfta, the stronger the acid is.
  • Ethanoic acid is the strongest because of the stability of ethanoate ion formed. In an ethanoate ion, the negative charge is spread throughout the -COO group. This delocalisation of electron and negative charge stabilises it to a greater However, since oxygen atoms are the most electronegative, more negative charge will still be concentrated here.
  • Phenol is a weaker acid. This is because electron delocalisation of the phenoxide ion is not as great as in ethanoate Although the delocalised ring electrons are involved, the electrons are still heavily distorted towards the one electronegative oxygen atom rather than two in the ethanoate ion case.
    1. If an electron-withdrawing group is attached to phenol, its acidity increases. This is because the electron-withdrawing group can attract electrons away from the oxygen, stabilising the phenoxide ion formed
    2. If an electron-donating group is attached to phenol, its acidity decreases. This is because the electron-donating group increases the electron density in the benzene ring, intensifying the charge on oxygen atom.

  • Ethanol is the weakest acid among This is because the ethoxide ion formed has no spreading of its negative charge to other parts, all the negative charge is concentrated on the oxygen atom. This makes it very attractive to hydrogen ions and it can easily accept one.
  • On the other hand, water is a stronger acid than ethanol but weaker than phenol. This is because in ethoxide ion, the presence of an alkyl group intensiftes the negative charge on the oxygen atom. In a hydroxide ion, no such thing happens

 

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