Carboxylic acids are unsaturated organic compounds with a hydroxyl group bonded to a carbonyl. The carboxylic acid functional group is usually abbreviated as COOH or CO2H. Examples of molecules with COOH groups include vinegar, acetic acid, fatty acids, and amino acids.
The “R” in the structure above represents any generic alkyl chain. The important thing is that the blue carbon has three bonds to oxygen: one to a hydroxyl group and two to the carbonyl oxygen.
Carboxylic acids are generally named in much the same way alcohols are. First, you find out how many carbons are in the longest chain, and then you add the modifier –oic acid instead of –ol. Since carboxylic acids, by structural necessity, are at the ends of molecules location is assumed to be 1. Let’s take a look at the molecule below:
To name this molecule, we started counting at the carbonyl carbon. There are 5 carbons in the longest chain, so our root name is pentane. Since this is a carboxylic acid, we modify the root to be pentanoic acid. All that’s left is to indicate our substituent, so we end up with 4-methylpentanoic acid as the name.
Are carboxylic acids polar or nonpolar. Polar, of course! Carbon 1 has two electronegative atoms creating a dipole, which makes carboxylic acids generally soluble in water—until the alkyl chains or "tails" get past five carbons in length. Once that threshold is crossed, they become less and less soluble as the number of carbons increases.
When trying to predict boiling points, what we're really looking at are the intermolecular forces at play. Using 1-pentanol and pentanoic acid again, which of the following would have the higher boiling point?
Both molecules have the same number of carbons and they both exhibit hydrogen bonding, but pentanoic acid is more polar! Carbon 1 has two electronegative atoms creating a dipole, which increases boiling point. The boiling point of pentanoic acid is 186ºC, while the boiling point of 1-pentanol is 138ºC.
As the name might suggest, a carboxylic acid can be deprotonated by a base like NaOH. The deprotonated form of a carboxylic acid is called a carboxylate anion. Carboxylic acids have a relatively high acidity—they generally have a pKa around 5—because carboxylates are resonance-stabilized.
Instead of just undergoing deprotonation, that hydroxyl group can also undergo substitution through the following reactions.
When in solution with acid and an alcohol, the carboxylic acid can actually undergo nucleophilic acyl substitution (NAS) to form an ester.
Using SOCl2 (thionyl chloride) we can transform a carboxylic acid into an acid halide, which are much more prone to NAS than are carboxylic acids. The carbonyl carbon acts as a Lewis acid in NAS reactions, and the very electronegative chlorine increases the electrophilicity of that carbon while also being a great leaving group.
Carboxylic acids can be dehydrated to form anhydrides, which are basically two carbonyls linked together through a single oxygen atom.
Lactones and lactams are cyclic esters and cyclic amides, respectively. Lactones can be formed via NAS when a carboxylic acid and an alcohol are on the same molecule; lactams require a carboxyl group and an amine on the same molecule. Five- and six-membered rings can form spontaneously.
Carboxylic acids with carbonyls at the beta position, called beta-keto acids, can undergo a process called decarboxylation when heated. In this process, the carboxyl group is lost as CO2.