Molecular Structure and Acid-Base Behavior

Learning Goals

You will develop an understanding of the relationship between the bonding properties of molecules and their acid-base strength.

Synopsis

For an acid to produce a hydrogen atom in solution, first the bond with which the hydrogen atom is held must be broken and then the freed hydrogen atom must lose an electron. The remaining molecular group must then gain an electron. So can we order the acid strength of some acids by looking at the sum of the strength with which that hydrogen is bound and the electron affinity of the remaining group (we ignore the ionization potential of the hydrogen atom for that factor is the same for all acids)? We can try. Let's try with the HA series where A is an atom from the halogen series.

HF HCl HBr HI

H-A bond strength, kJ/mol 569 431 368 297

EA of A, kJ/mol 328 349 325 295

Sum, kJ/mol 897 780 693 592

The acid strength increases from left to right and the bond strength decreases from left to right. The sum also decreases in going from the left to the right. So the weaker the H-A bond the stronger the acid and the less the sum, the stronger the acid. So we have made a reasonable correlation of bonding concepts with acid strength. But you might remember one of your review questions was why was HF not included as a strong acid (it has Ka = 7.2 x 10^-4). One explanation is that the bond strength is so high that dissociation requires too much energy to readily occur in water solution. What more is different among these molecules? The electronegativity of the halogens is decreasing considerably as we go from left to right. The electronegativity of F is so large that HF has very strong hydrogen bonds holding the HF monomers together as polymers of HF. This intermolecular attraction could also keep the HF molecules from dissociating as easily as the rest of the HA series dissociates.

The electronegativity differences between atoms can also help us understand the relative strengths of some other acids.

molecules

As you see from the above, the more oxygen atoms that are attached to the central atom the more acidic is the compound. The oxygen atom is also very electronegative so it can attract electron density from neighboring atoms to itself (the inductive effect). As the oxygen atom removes electron density from the neighboring atoms, the bonds to those neighboring atoms are weakened. Because the acidic hydrogen is attached to one of those neighboring groups, the hydrogen is more easily dissociated from the molecule.

You can also see why the acidic hydrogen in compounds like acetic acid is the one bonded to the oxygen atom rather than one of the ones attached to the carbon atom. The resulting anion can accommodate a negative charge on the oxygen much more readily than a negative charge on the carbon.

Review Questions

Work Exercise 17.18 on page 724.
Draw the Lewis structure for oxalic acid so that you can see the bonding of the acidic hydrogens.
Draw the Lewis structure for formic acid and show the acidic hydrogen.

	HF	HCl	HBr	HI
H-A bond strength, kJ/mol	569	431	368	297
EA of A, kJ/mol	328	349	325	295
Sum, kJ/mol	897	780	693	592