Carboxylic Acids

Carboxylic Acids

Carboxylic acids are organic compounds that have a carboxyl group (-COOH) as a functional group. This functional group account for three of the four bonds carbon can make and thus must appear at the end of the carbon chain. The carboxyl group is therefore a terminal function group since it must discontinue that portion of the carbon chain.

List of Carboxylic Acids

# Carbons  Name
1 methanoic acid
2 ethanoic acid
3 propanoic acid
4 butanoic acid
5 pentanoic acid
6 hexanoic acid
7 heptanoic acid
8 octanoic acid
9 nonanoic acid
10 decanoic acid

Dissociation

No organic acids are considered strong, therefore the carboxylic acids are weak acids. That is, they dissociate less than 100% in water. They are prime candidates for examination using Brønsted–Lowry acid/base theory. Consequently, the Ka values for the carboxylic acids are of particular importance. Consider some of the values given below:

methanoic acid 1.70 × 10-4
ethanoic acid 1.74 × 10-5
propanoic acid 1.32 × 10-5
butanoic acid 1.51 × 10-5
pentanoic acid 1.51 × 10-5

Interestingly, after an initial drop of an order of magnitude over the first few acids, the Ka remain fairly constant through decanoic acid. While the carboxyl group is the most soluble functional group (in water) due to its polarity, the remainder of the carbon chain is not soluble in water at all. This explains the lack of total dissociation among the carboxylic acids in water. Usually, increased carbon chain length results in decreased solubility as less and less of the molecule becomes soluble as the molecule grows in size. However, it has been speculated that the wrapping of the carbon chain around itself is the reason for no decrease in dissociation, and hence a decrease in the value for Ka.

Oxidation of Alcohols

Oxidation of alcohols describes the process in which primary and secondary alcohols can be made into aldehydes and ketones respectively. The aldehyde produced from the primary alcohol can be further oxidized to a carboxylic acid. Only primary and secondary alcohols are oxidized. Tertiary alcohols cannot be oxidized. An acidified dichromate solution, usually potassium dichromate or sodium dicrhomate is utilized, works as an oxidizer for the alcohol. If a primary alcohol is to be oxidized, the aldehyde that is produced is merely a step on the path to full oxidation to the carboxylic acid. Environmental conditions and concentration of the acidified dichromate must be strictly maintained if the aldehyde is the desired product.

Consider the general reaction for the oxidation of primary alcohols shown below. First the hydroxyl is converted to a carbonyl, and then to a carboxyl group.

Below is shown the general reaction for the oxidation of secondary alcohols. Note that after the hydroxyl group is converted to a carbonyl, the reaction stops.