Organic Chemistry I

For elements in the same group, the larger the size of the atom, the stronger the acid is; the acidity
increases from top to bottom along the group.
3.4.2. Resonance Effect
The resonance effect accounts for the acidity difference between ethanol and acetic acid. For both ethanol and acetic
acid, the hydrogen is bonded with the oxygen atom, so there is no element effect that matters. However, the p
K
a
values
(and the acidity) of ethanol and acetic acid are very different. What makes a carboxylic acid so much more acidic than an
alcohol? As stated before, we begin by considering the stability of the conjugate bases, remembering that a more stable
(weaker) conjugate base corresponds to a stronger acid.
For acetate, the conjugate base of acetic acid, two resonance contributors can be drawn and therefore the negative
charge can be delocalized (shared) over two oxygen atoms. However, no other resonance contributor is available in the
ethoxide ion, the conjugate base of ethanol, so the negative charge is localized on the oxygen atom. As we have learned
in
section 1.3
,
the species that has more resonance contributors gains stability
, therefore acetate is more stable than
ethoxide, and is weaker as the base, so acetic acid is a stronger acid than ethanol.
The charge delocalization by resonance has a very powerful effect on the reactivity of organic molecules, enough to
account for the big difference of over 10 p
K
a
units between ethanol and acetic acid. Because p
K
a
= –log
K
a
, that means
that there is a factor of about 10
10
between the K
a
values for the two molecules!
100 | 3.4 Structural Effects on Acidity and Basicity

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Examples
The p
K
a
of the OH group in alcohol is about 15, however OH in phenol (OH group connected on a benzene
ring) has a pK
a
of about 10, which is much stronger in acidity than other alcohols. Explain the difference.
Solution
:
The difference can be explained by the resonance effect. There is no resonance effect on the conjugate base
of ethanol, as mentioned before. However, the conjugate base of phenol is stabilized by the resonance effect
with four more resonance contributors, and the negative is delocalized on the benzene ring, so the conjugate
base of phenol is much more stable and is a weaker base. Therefore phenol is much more acidic than other
alcohols.
Exercises 3.2
•
Practice drawing the resonance structures of the conjugate base of phenol by yourself!
•
It is because of the special acidity of phenol (and other aromatic alcohols), that NaOH can be used to
deprotonate phenol effectively, but not to normal alcohols, like ethanol. Show the reaction equations of
3.4 Structural Effects on Acidity and Basicity | 101

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these reactions and explain the difference by applying the p
K
a
values.
Answers to Practice Questions Chapter 3