Carboxylic Acids (reformat)

Carboxylic Acids. As is true with the word “alcohol”, the term “acid” is widely used in common parlance; it can be used to describe materials that are corrosive, or those with a sour or pungent taste. When we discuss the essential ideas of acid/base chemistry in Chapter 5, we’ll discuss acids in greater detail. There are actually many different types of acids but they all do one thing that gives rise to the acidic properties just mentioned: they react with water to form the hydronium ion, H₃O⁺ (this is essentially a water molecule with an extra hydrogen atom and a +1 charge [16]; we’ll discuss ions in detail soon and the details of the reaction need not concern us at the moment). For now we want to introduce a certain type of acid, carboxylic acids, for two reasons: 1) it illustrates another important functional group, and 2) it further illustrates how the structure of molecules impacts their chemical and physical properties. 

Carboxylic acids are compounds that have an -OH group immediately adjacent to a carbon-oxygen double bond, called a carbonyl group (Figure 1-24). Unlike alcohols, which don’t chemically react with water to a significant extent, the presence of the carbonyl group enhances the reaction with water by orders of magnitude. So, when looking at molecular structures to determine what functional groups are present, don’t make the mistake of concluding that something with a hydroxyl group is necessarily an alcohol: if it is adjacent to a C=O, it is a carboxylic acid.

Figure 1-24. The generic structures of carboxylic acids (left) and alcohols (right); recall that R can be any alkyl or other organic substituent. 

There are many examples of carboxylic acids in Nature, including the linoleic acid found in Burt’s Bees sunscreen. Digestion of plant and animal fats and oils gives rise to several commonly encountered carboxylic acids. These typically have long carbon chains, but there are several relatively familiar carboxylic acids that have short carbon chains. By far the most common of these is acetic acid (pronounced uh-CEE-tick), the compound responsible for the taste and aroma of vinegar. This acid and two other simple examples are shown in Table 1-6 along with some of their properties. 

Table 1-6: Examples of Simple Carboxylic Acids (RCOOH)
R =	-H	-CH3	-CH2CH3

As with alcohols, the presence of the carboxylic acid functional group (sometimes abbreviated as -COOH) bestows properties to these compounds that are quite different from simple alkanes. (Speculate about why that might be.) The boiling points of these compounds are even higher than those for alcohols with the same number of carbon atoms. Carboxylic acids also show a trend of decreasing solubility in what as the carbon chain increases in length, for precisely the same reason that the same effect was seen in alcohols. The three acids shown in Table 1-6 are miscible with water at room temperature, but if the series is extended a bit, that is no longer true. For example, only about 50 grams of pentanoic acid (C₄H₉COOH, also known as valeric acid), can dissolve in 1 liter of water at room temperature. 

IUPAC has developed a fairly simple systematic naming system for carboxylic acids. For all but the simplest of carboxylic acids, the root of the name is the same as that for the alkane with the same number of carbon atoms, but the -ane suffix for the alkane is replaced by -anoic acid. For example, propane is the alkane having three carbon atoms, and propanoic acid is the carboxylic acid with three carbon atoms. Pentanoic acid acid is the name for the carboxylic acid with a five-carbon backbones, hexanoic acid has six, etc. The only exceptions to this are the two smallest carboxylic acids. According to the IUPAC naming system, one would expect these two acids to be named methanoic and ethanoic acids. Instead, because these two compounds have such deeply entrenched common names, those have been retained by IUPAC. The preferred name for methanoic acid is actually formic acid, named after the Latin word for ants (formic acid is a chief component in certain ant venoms), and that for ethanoic acid is acetic acid, named for the Latin word for vinegar. Other common names are still widely used too, but are not the IUPAC-preferred terms. One final colorful example: butanoic acid, which smells of vomit, has the common name butyric acid (from the Latin for butter, in which butyric acid is formed when it turns rancid); it is also a minor component in parmesan cheese, where it definitely makes its presence known!