Structure and Acidity II

Its not uncommon to be asked to explain or rationalize the difference in acidity of various compounds.  You should definitely understand the following acidities based on their structures.  Provided are a brief explanation describing why each compound from our pKa table has the acidity they do.

Some general notes to keep in mind are as follows;

  1. Acidity depends on the stability of the conjugate base.
  2. Strong acids have weak conjugate bases.
  3. Look for reasons why one conjugate base is more stable than another.
    • negative charge on more electronegative element (X>O>N>C)
      • Electronegativity of C atoms depends on amount of s character (sp C > sp2 C > sp3 C)
    • more resonance structures possible
    • negative charge on more polarizable (soft and fluffy) element (e.g. I- is soft, F- is hard)
pKa explanation

HI=-10
HBr=-6
HCl=-4
HF=3

 negative charge on very electronegative (F,Cl) or polarizable element(Br,I)

 

-9

 negative charge on somewhat electronegative element (O) and 3 resonance structure possible

 

4

 negative charge on somewhat electronegative element (O) and 2 resonance structure possible

 

10

 negative charge on somewhat electronegative element (O) and 4 resonance structures possible

 

15.7

 negative charge on somewhat electronegative element (O)

 

16

 negative charge on somewhat electronegative element (O)

 

19

 negative charge on non-electronegative element (C) and 2 resonance structures

 

26

negative charge on not so electronegative element (C).

C atom electronegativity as follows sp>sp2>sp3

 

33

 negative charge on not very electronegative element (N) and 2 resonance structures

 

45

negative charge on not so electronegative  sp2 C.

C atom electronegativity as follows sp>sp2>sp3

 

60

negative charge on a very non-electronegative sp3 C.

C atom electronegativity as follows sp>sp2>sp3