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Acids and Bases

Many students start organic chemistry thinking they know all about acids and bases, but then quickly discover that they can't really use the principles involved.
For example,
many students are typically not comfortable when they are asked to identify the most acidic protons or the most basic site in a molecule.
Yet this is critical since an acid will typically react at the most basic site first and a base will remove the most acidic proton first.

Remember that acidity and basicity are the based on the same chemical reaction, just looking at it from opposite sides, so they are opposites.

When evaluating acidity / basicity, look at the atom bearing the proton / electron pair first. Then you may also need to consider resonance, inductive (remote electronegativity effects), the orbitals involved and the charge on that atom.

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:

equation defining Bronsted acidity

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-,



HF > H2O > NH3 > CH
Electronegativity but only 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.


HI > HBr > HCl > HF
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)


RCO2H > ROH
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 



CH3-  > NH2- > HO- > F-
Electronegativity but only 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.


F- > Cl > Br > I
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.


 RO-  >  RCO2
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-.


© Dr. Ian Hunt, Department of Chemistry, University of Calgary