3.2.1 Organic Acids

The acids that we talked about in General Chemistry usually refers to inorganic acids, such as HCl, H₂SO₄, HF etc. If the structure of the acid contains a “carbon” part, then it is an organic acid. Organic acids donate protons in the same way as inorganic acids, however the structure may be more complicated due to the nature of organic structures.

Carboxylic acid, with the general formula of R-COOH, is the most common organic acid that we are familiar with. Acetic acid (CH₃COOH), the ingredient of vinegar, is a simple example of a carboxylic acid. The K_a of acetic acid is 1.8×10^-5.

Another common organic acid is the organic derivative of sulfuric acid H₂SO₄.

The replacement of one OH group in H₂SO₄ with a carbon-containing R (alkyl) or Ar (aromatic) group leads to the organic acid named “sulfonic acid”, with the general formula of RSO₃H, or ArSO₃H. Sulfonic acid is a strong organic acid with a K_a in the range of 10⁶. The structure of a specific sulfonic acid example called p-toluenesulfonic acid is shown here:

Other than the acids mentioned here, technically any organic compound could be an acid, because organic compounds always have hydrogen atoms that could potentially be donated as H⁺. Only a few examples are shown here with the hydrogen atoms highlighted in blue:

Therefore, the scope of acids has been extended to be much broader in an organic chemistry context. We will have further discussions on the acidity of organic compounds in section 3.3, and we will see more acid-base reactions applied to organic compounds later in this chapter.

3.2.2 Organic Bases

While it is relatively straightforward to identify an organic acid since hydrogen atoms are always involved, sometimes it is not that easy to identify organic bases. According to the definition, a base is the species that is able to accept the proton. Organic bases may involve a variety of different structures, but they must all share the common feature of having electron pairs that are able to accept protons. The electron pairs could be lone pair electrons on a neutral or negative charged species, or π electron pairs. Organic bases could therefore involve the following types:

• Negatively charged organic bases: RO^–(alkyloxide), RNH^–(amide), R^–(alkide, the conjugate base of alkane). Since the negatively charged bases have a high electron density, they are usually stronger bases than the neutral ones.

• Neutral organic bases, for example amine, C=O group and C=C group
  • Amine: RNH₂, R₂NH, R₃N, ArNH₂ etc (section 2.3). As organic derivatives of NH₃, which is an inorganic weak base, amines are organic weak bases with lone pair electrons on N that are able to accept the proton.

• • Functional groups containing oxygen atoms: carbonyl group C=O, alcohol R-OH, ether R-O-R. The lone pair electrons on O in these groups are able to accept the proton, so functional groups like aldehyde, ketone, alcohol and ether are all organic bases. It may not that easy to accept this concept at the first, because these groups do not really look like bases. However, they are bases according to the definition because they are able to accept the proton with the lone pair on the oxygen atom.