structural effects on acidity
negative charges are stabilized by allowing the charge to
delocalize over a bigger space or over more atoms. Typically,
a molecule that holds the negative charge over several atoms
will be more stable than a molecule that localizes the full
charge on a single atom. The
main features that determine the stability of a negative charge
are the following:
electronegativity of the atom on which the negative charge
size of the atom on which the negative charge is located
of the negative charge through resonance
of the negative charge by adjacent electronegative atoms
look at each of these factors individually.
of negatively charged atom
put, negative charges prefer to rest on more electronegative
elements (see here for more on
electronegativity) than on more electropositive elements (like
carbon). That's why water is more acidic than ammonia (NH3),
because oxygen is more electronegative than nitrogen. Similarly,
both water and ammonia are more acidic than methane (CH4)
since carbon is more electropositive than nitrogen or oxygen.
of negatively-charged atom
size of the atom on which the negative charge rests also effects
the stability. As a general rule, negative charges prefer
to rest on larger atoms, as the charge can spread over a much
larger region of space (making it more stable) than when the
charge is localized in a much smaller space on a smaller atom.
Generally, this preference for placing the charge on a larger
atom trumps electronegativity considerations. For example,
HI is more acidic than HF, even though based on the last argument
about electronegativity you might suspect that F-
would be more stable than I- because fluorine is
more electronegative than iodine (and thus HF would be a stronger
acid than HI). This turns out not to be the case. Iodine is
so much bigger than fluorine that the charge is more stable
on this larger atom. Thus, the trends for acidity of the hydrohalic
acids are as follows: HI is strongest, followed by HBr, HCl,
and finally HF.
of the most important effects on the acidity of a molecule
is whether the conjugate base anion can be stabilized by resonance.
Phenol, for example (shown below), is about 1,000,000 times
stronger an acid than cyclohexanol because the conjugate base
of phenol is much more stable than the conjugate base of cyclohexanol.
This added stability of the phenol conjugate base arises because
this anion can delocalize the negative charge throughout the
ring through resonance, effectively stabilizing it. The conjugate
base of cyclohexanol has no resonance structures to stabilize
the charge and so is less stable.
on phenol are stabilized by their delocalization throughout
the ring (shown below); cyclohexane has no electron
delocalization, and thus is a much weaker acid.
presence of electronegative groups near a hydrogen also makes
it more acidic. You can see why this is so by using the same
analysis as before. That is, somehow the presence of these
electronegative atoms must stabilize the conjugate base anion.
For example, substituting the electronegative atom chlorine
for hydrogen in acetic acid makes the molecule about 100 times
more acidic (recall that the pKa scale is defined logarithmically).
This increase in acidity is due to the electronegative chlorine
atom pulling some of the electron density away from the oxygens,
making the oxygens not having to bear all of the negative
charge by themselves.