Given
two different acids, you will need to determine which is the most acidic. This
seems like a daunting problem, but there are in fact only a few intuitive
parameters with which to determine the answer.
The ability to determine which acid, acid A, or acid B, is
the most acidic is a good skill to
develop for taking tests, and also a skill that has practical importance in the
laboratory. To answer this question, you can follow the simple steps outlined
below.
1)
Find the most acidic hydrogen in molecule A, and
in molecule B separately by following these three criteria:
a.
Look at the atom to which each hydrogen is
bonded and find which is the most electronegative (remember that
electronegativity increases UP and to the RIGHT on the periodic table)
b.
What is the hybridization of the atom to which
the hydrogen is bonded? Just remember the trend that SP3 is least
acidic, and SP is most acidic
c.
Now imagine that the hydrogen you chose is
dissociated, what will the resulting conjugate base look like?
i.
Is there a highly electronegative atom in the
structure that could lead to the inductive affect? If there is, then the
inductive effect leads to a more stabilized conjugate base, therefore a stronger acid
ii.
How many resonance forms are available to the
conjugate base? The more resonance structures, the more acidic is the hydrogen
2)
Now that you have chosen the most acidic
hydrogen of each molecule, compare the two by using the same rules as above.
3)
Label one of each set of hydrogen atoms
4)
Draw the resultant conjugate of each version of
the acid
5)
Make a table for each of the four criteria and
each of the acidic hydrogens
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Criteria
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Hydrogen 1
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Hydrogen 2
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Hydrogen 3
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Electronegativity of connected atom
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Inductive Affect
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Hybridization of connected atom
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Resonance forms of conjugate
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