Acid-Base Behavior and Chemical Structure

How does the chemical structure of a substance determine whether it will behave as an acid, base, or neither?

For a molecule to donate a proton, the bond between the molecule and the hydrogen in question must be polarized such that the shared electrons in the bond are attracted away from the leaving hydrogen, and towards the rest of the molecule:

• H has an electronegativity of 2.1 and carbon 2.5. This bond is slightly polar, but does not promote withdrawal of the bonding electrons towards carbon. Thus, compounds containing exclusively C-H bonds do not behave as acids
• Na has an electronegativity of 0.9, in comparison to hydrogen's 2.1. Thus, in metal hydride compounds (e.g. NaH) the shared electrons are actually withdrawn towards the hydrogen. In this case, the hydrogen may be present as the hydride ion (H^-) and act as a proton acceptor

In addition to appropriate polarity, the overall strength of the H-X bond is important in determining whether the molecule can donate a proton

• The H-F bond is the most polar bond that H can engage in. Although the polarity is such that the shared electrons are withdrawn towards the F atom, H-F is actually considered to be a weak acid. This is because the H-F bond is somewhat difficult to break (although when it does break, the hydrogen leaves the bonding electrons behind).

Another consideration is the nature of the conjugate base that will form (X^-).

• If this compound is stable (i.e. chemically unreactive), then H-X will behave as an acid.
• If X^- is entirely unstable (for example, will readily react with H₂O to produce HX and OH^-), then H-X will not behave as an acid

Are there periodic trends for hydrogen compounds of elements (i.e. hydrides)? The answer: somewhat.

• Towards the left, hydride compounds will generally be basic
• Towards the right, hydride compounds will generally be acidic

• In this case, in a bond with hydrogen, the metal tends to give electrons to the departing hydrogen, forming a hydride ion, H^-. The hydride ion acts as a base, i.e. it will accept a proton to form H₂(g).

• In this case, in a bond with hydrogen, the non-metal tends to keep the shared electrons, and the departing hydrogen leaves as H⁺. This is the definition of an acid

What about moving down the periodic table?

• A weaker H-X bond means that the compound is more likely to donate a proton. Thus, a hydride compound is generally more acidic as we move down the periodic table

Acids in which OH groups (and potentially other O atoms) are bonded to a central atom are called oxyacids:

Sulfuric acid (H₂SO₄) - a strong acid!

• Being an acid, H₂SO₄ donates a proton during chemical reactions

OH groups are also found in bases. However, in bases, the entire OH group is released (as OH^- ion) and H⁺ is not released

Consider the effects of the other atoms in the molecule. In particular, the atom that the OH group is bonded to:

• If Y has low electronegativity, the Y-O bond will most likely be ionic in nature and separate with O retaining the shared electrons. This would be the case if Y was a metal (giving a basic metal hydroxide)
• If Y has higher electronegativity, the Y-O bond would be more covalent in character. In this case, it is unlikely that the Y-O bond will break, and more likely that the O-H bond will break, yielding a proton. This would be the case for Y = nonmetals, and the compound would most likely represent an oxyacid.

• The high electronegativity of Y is felt by the H atom also. The attraction of Y for electrons helps to "withdraw" the electrons in the O-H bond towards the Oxygen atom. The O-H bond becomes more polar with increasing electronegativity of the neighboring Y atom.
• In order for the resulting anion, Y-O^-, to recombine with a proton, there must be a freely available non-bonding pair of electrons on the oxygen. As the electronegativity of Y increases, it tends to withdraw the non-bonding electrons of the oxygen (making them less available for a proton). Thus, the greater the electronegativity of Y, the more stable the conjugate base (oxyanion)
• Likewise, the more electronegative atoms (e.g. O) bonded to the central Y atom, the more they help to withdraw electrons from the Y-O bond in question. The strength of an acid will increase as additional electronegative groups are bonded to the central atom.

Carboxylic acids are organic compounds that have the general formula R-COOH, where R is either a hydrogen or a chain of carbon-based groups:

• The -COOH portion of the structure is known as the carboxyl group
• Acids that contain carboxyl groups are known as carboxylic acids

Compare the following compounds:

• The additional oxygen atom attached to the central carbon, due to its electronegativity, helps to withdraw electrons from the other C-O bond. In particular, it makes the O-H bond more polar.
• The conjugate base of a carboxylic acid can exhibit resonance. This helps to stabilize (i.e. make less chemically reactive) the conjugate base by spreading the negative charge over two oxygen atoms:

• The strength of carboxylic acids increase as the number of electronegative atoms directly, or indirectly, bonded to the central carbon increases. For example, trifluoroacetic acid has a K_a = 5.0 x 10^-1