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Useful Concepts | ![]() |
Typically students are not comfortable
when asked to identify the
most acidic protons or the most basic site in a molecule.
Acidity and basicity are the based on the same chemical reaction, just
looking at it from opposite sides.
Acidity
Here are some general guidelines of principles to look for the help
you address the issue of acidity:
First, consider the general equation of a simple acid reaction:
The more stable the conjugate base, A-,
is then the more the equilibrium favours the product side.....
The more the equilibrium favours products, the more H+
there is....
The more H+ there is then the stronger H-A
is as an acid....
So looking for factors that stabilise the conjugate base, A-,
gives us a "tool" for assessing acidity.
Key Factors that affect the stability of the
conjugate base, A-,
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Electronegativity. When comparing atoms within the same row of the periodic table, the more electronegative the anionic atom in the conjugate base, the better it is at accepting the negative charge. |
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Size. When comparing atoms within the same group of the periodic table, the larger the atom the weaker the H-X bond and the easier it is to accommodate negative charge (lower charge density) |
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Resonance. In the carboxylate ion, RCO2- the negative charge is delocalised across 2 electronegative atoms which makes it more stable than being localised on a specific atom as in the alkoxide, RO-. |
Basicity
A convinient way to look at basicity is based on electron pair
availability....
the more available the electrons, the more readily they can be donated
to form a new bond to the proton and, and therefore the stronger base.
Key Factors that affect electron pair
availability in a base, B
|
Electronegativity. When comparing atoms within the same row of the periodic table, the more electronegative the atom donating the electrons is, the less willing it is to share those electrons with a proton, so the weaker the base. |
|
Size. When comparing atoms within the same group of the periodic table, the larger the atom the weaker the H-X bond and the lower the electron density making it a weaker base. |
|
Resonance. In the carboxylate ion, RCO2- the negative charge is delocalised across 2 electronegative atoms which makes it the electrons less available than when they localised on a specific atom as in the alkoxide, RO-. |
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© Dr. Ian Hunt, Department of Chemistry, University of Calgary | ![]() |
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