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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

Introduction to Carboxylic Acids

• we have looked at a number of nucleophilic addition reactions of aldehydes and ketones. These groups can also be reduced to alcohols and oxidized to carboxylic acids. These can be either chemically or enzymatically modulated.
• examples of a chemical oxidiation are the Tollen's and Fehling's tests that you did in the lab on carbonyls. In Tollen's test, the oxidizing agent is Ag(I) in the form of Ag₂O. It is reduced in the reaction to metallic silver. This is what forms the silver mirror in a Tollen's test.
• enzyme catalyzed oxidations are rather complicated and usually involve a number of intermediates. We will look at a number of them in biochemistry next semester.
• the product of both reactions is a carboxylic acid (or a carboxylic acid derivative)

2. Structure and Nomenclature of Carboxylic Acids:

• the name is derived from carbonyl and hydroxyl. As the name suggests, the carboxyl group is a carbonyl carbon with a hydroxyl group on it.
• the carbon atom of the group is sp² hybridized, so it is trigonal planar
• to name carboxylic acids, one identifies the longest carbon chain containing the carboxyl group. The "e" is dropped from the parent name and the ending "-oic acid" is added
• the numbering is unambiguous, the carboxyl carbon is carbon number 1. Substituents are named as usual
• if the chain contains a double bond, the "an" of the parent name is replaced with "en" and the position is given with a number
• please make use of the table that we looked at the other week to name compounds with many groups on them
• dicarboxylic acids have carboxyl groups on both ends. They get the suffix "-dioc acid"
• for example: ethanedioc acid (oxalic acid), butanedioc acid (succinic acid)
• if the carboxylic acid group is directly attached to a cycloalkane, the suffix "-carboxylic acid" is added. The numbering of the ring is such that the attachment site is number 1.
• for example: 2-cyclohexenecarboxylic acid.
• when the carboxyl group is on a benzene ring, this is called benzoic acid
• see Table 13.1 in Brown (p. 375) for a list of the common names for caarboxylic acids

3. Physical Properties:

• let's start to explore the physical properties of carboxylic acids by looking at the group in a little more detail
• the carbon atom of the groups is bonded to 2 oxygen atoms, one in a double bond, one singly. Thus, the carbon atom is strongly partially positive, and the oxygen's are partially negative.
• the hydrogen on the hydroxyl group is partially positive as well. Thus, carboxylic acids are able to form hydrogen bonds between molecules
• the boiling points are thus higher than for corresponding hydrocarbons. This leads to an interesting property of carboxylic acids, they tend to form dimers in the liquid and solid states (in the pure form)
• Table 13.2 on page 378 of Brown summarizes the main properties of a number of carboxylic acids
• The solubility in water is also increased by the hydrogen bonding ability of these groups. They have many possible hydrogen bonds with water. The hydroxyl hydrogen is an H-bond donor and both oxygens act as H-bond acceptors
• the solubility of carboxylic acids decreases with size, as for alcohols... This is because the molecules has different parts with different properties. The carboxyl group is polar or hydrophilic, whereas the hydrocarbon tail is nonpolar or hydrophobic. So, as the hydrocarbon tail increases in length the solubility decreases.
• this is particularly important for the long chain carboxylic acids, which are called fatty acids

4. Acidity of Carboxylic Acids:

• as the name suggest, carboxylic acids are acidic. That means that the hydrogen of the hydroxyl group is acidic, it is able to dissociate.
• in general, the K_a values are between 10^-4 and 10^-5. For example the K_a of acetic acid is 1.74 x 10^-5
• a value that has more obvious meaning is the pK_a, which is the -logK_a. Thus the pK_as of carboxylic acids are around 4 to 5. That of acetic acid is 4.76
• this means that at pH 4.76 acetic acid is half protonated and half deprotonated.
• to put these values in perspective, the pK_a os water is 14, alcohols have pK_a of 16-18. Thus, carboxylic acids are much stronger acids.
• the conjugate base of a carboxylic acid is called a carboxylate. For example for acetic acid (ethanoic acid), the conjugate base is called acetate (ethanoate)
• why is the carboxyl group so much stronger as an acid? We must look at the resonance structure of a carboxylate to understand this. The negative charge is shared evenly over the two oxygen atoms, neither has to bear a full positive charge. This is not so with the alcohol.
• why is the pK_a of phenol so low (9.95) compared to ethanol? Resonance stabilization of the alkoxide.
• in addition to resonance stabilization, the carbonyl group of the carboxylic acid is so polarized that the electrons of the O-H bond are shifted more towards the oxygen, making the hydrogen even more positive than usual
• you can extend this argument, by looking at electron withdrawing substitutents near the carboxyl group. For example the pK_a of acetic acid is greater than that for iodoacetic, which is greater than that for bromoacetic, which is greater than that for chloroacetic acid, which is greater than that for fluoroacetic.
• this affect is increased when more groups are present, and is sensitive to the distance from the carboxyl group.