Carboxylic acids, esters, and other acid derivativesChapter 16

Structure of carboxylic acids and their derivativesThe functional group present in a carboxylic acid is a combination of a carbonyl group and a hydroxyl group; however, the resulting carboxyl group ( -COOH) possesses properties that are unlike those present in aldehydes/ketones and alcohols.

Structure of carboxylic acids and their derivativesCarboxylic acids have the following general formula:

Some simple carboxylic acids:

Since carbon can have only four bonds, there are no cyclic carboxylic acids (i.e. the carboxyl group cannot form part of a carbon ring)

Structure of carboxylic acids and their derivativesThe following molecules have a similar structure to carboxylic acids, and will be encountered in this chapter and the next.

IUPAC nomenclature for carboxylic acidsFor monocarboxylic acids (one COOH group):Select the longest, continuous carbon chain that involves the carboxyl group. This is the parent chain and the COOH carbon is designated as C-1.Name the parent chain by dropping the e from the corresponding alkane name and changing to oic acidIndicate the identity and location of substituents on the parent chain at the front of the carboxylic acids nameBenzoic acid

IUPAC nomenclature for carboxylic acidsDicarboxylic acids:For these compounds, both ends of a chain will end with a COOH group. The parent chain is the one that involves both COOH groups.The parent chain is named as an alkane and the term dioic acid is added afterwards to indicate the diacid structure.

Common names for carboxylic acids

Common names for dicarboxylic acids

Common names for carboxylic acidsFor common-name carboxylic acids and diacids, substituents are often numbered using a Greek system:

So the following molecule could be called a-Methylpropionic acid (or, using the IUPAC system, 2-Methylpropanoic acid)

Polyfunctional carboxylic acidsCarboxylic acids that contain other functional groups besides the COOH group are called polyfunctional carboxylic acids. Some examples are shown below:

Polyfunctional carboxylic acidsUnsaturated acids possess a unit of unsaturation (double/triple C-C bond) in the structure. Three of the most common unsaturated acids are shown:Unsaturated acidsa metabolic acid

Polyfunctional carboxylic acidsFour of the simplest hydroxy acids:Hydroxy acids

Polyfunctional carboxylic acidsFor keto acids, a carbonyl group is present as part of a carbon chain that involves a carboxyl group:

For IUPAC naming of this structure, the O-atom of the carbonyl group is treated as an oxo substituent and the molecule is called 2-Oxopropanoic acidKeto acidsOxo-group

Metabolic carboxylic acidsPolyfunctional acids are intermediates in metabolic reactions that occur as food is processed to obtain energy.Eight key intermediates in these processes are derived from only three carboxylic acids (propanoic, butanoic, and pentanoic acids, see below)

Physical properties of carboxylic acidsCarboxylic acids are the most polar functional group we have seen so far. The presence of the carbonyl group next to the OH causes the O-H bond to be even more polar.

Physical properties of carboxylic acidsBecause of the very polar COOH group, carboxylic acids exhibit strong intermolecular attractions.As expected, carboxylic acids of a given number of carbon atoms have higher boiling points than alcohols.Carboxylic acids also tend to dimerize, producing molecules that are twice as heavy which have enhanced London forces (and thus still higher boiling points).

Physical properties of carboxylic acidsIn terms of water-solubility, because of H-bonding, carboxylic acids dissolve well in water (up to 4-carbon chains).Beyond 4 carbons, water-solubility drops off rapidly.

Preparation of carboxylic acidsWe saw in Ch-15 that carboxylic acids can be prepared from aldehydes (which can be prepared from primary alcohols):

Aromatic carboxylic acids can be made by oxidizing alkyl-substituted aromatic molecules:

Acidity of carboxylic acidsWhen carboxylic acids are placed in water, they undergo de-protonation as discussed in Ch-10:Remember from Ch-10:HA + H2O D A- + H3O+

Acidity of carboxylic acids

Acidity of carboxylic acids

Carboxylic acid saltsWhen carboxylic acids are reacted with strong bases, they are converted to salts as follows:

Carboxylic acid saltsSalts of carboxylic acids are much more water-soluble than the acids themselves. Also, they can be converted back to the acid form by reacting them with a strong acid:

Uses of carboxylic acid saltsBecause of their enhanced solubility in water compared to the acid form, many drugs and medicines that possess acid groups are marketed as carboxylic acid salts (sodium or potassium salts).frogurt

Structure of estersEsters are carboxylic acid derivatives having an alkoxy group instead of a hydroxyl group.

Preparation of estersEsters are prepared by condensation reactions involving carboxylic acids and alcohols. Such reactions are called esterification reactions:

This an ester consists of an acid portion and an alcohol portion:Reaction is encouraged by thepresence of excess alcohol(Le Chateliers Principle)

Preparation of estersCyclic esters (lactones) are created from hydroxy acids (bear both a hydroxyl group and a carboxyl group) in an intramolecular esterification reaction:

Preparation of estersIndicate the structures of the carboxylic acid and alcohol that are needed to make each of the following esters:estercarboxylic acid + alcoholH+H+H+H+H+H2O +H2O +H2O +H2O +H2O +

Nomenclature for estersThinking of an ester in terms of an alcohol portion and a carboxylic acid portion is important for naming esters using the IUPAC system:The name for the alcohol portion comes first: name the alkyl part of the alcohol (e.g., for the ester shown below, the first part of the esters name is methyl (alcohol part comes from methanol). Present the alkyl name separate from the remainder of the ester name.The carboxylic acid portion is named as if it were deprotonated, changing the -ic acid part of that name to -ateMethyl propanoateThis part would be called propanoate

Nomenclature for estersSome other examples:Ethyl butanoateEthyl 2-methylbutanoateEthyl a-methylbutyrate2-Butyl butanoateSec-Butyl butanoate2-Methylpropyl butanoateIsobutyl butanoateabg

Selected common estersFlavor/fragrance agents

Selected common estersPheromones:

Medications:

Selected common estersSynthesis of Aspirin

Synthesis of oil of wintergreen:

Isomerism in carboxylic acids and estersRecall that constitutional isomers are molecules that share the same formula but differ in their atom-to-atom connectivities. Three kinds of constitutional isomers (in the order we encountered them):positional isomersskeletal isomersfunctional group isomersCarboxylic acids and esters that have a given number of carbon atoms form another example of functional group isomers:

Isomerism in carboxylic acids and estersFor both carboxylic acids and esters, skeletal isomers are possible:(carboxylic acids)(esters)

Isomerism in carboxylic acids and estersPositional isomers are possible for esters, but not carboxylic acids.

Physical properties of estersBecause they dont possess OH groups, esters cannot form H-bonds with other ester molecules. As a result, esters have lower boiling points than carboxylic acids and alcohols that have approximately the same molar mass.Water molecules can H-bond to esters, at the oxygen atoms. This makes low molecule weight esters water-soluble.

Chemical reactions of estersEster hydrolysis: the hydrolysis of an ester is accomplished by reacting water with the ester in the presence of an acid catalyst (this is the reverse reaction of esterification).

An example:

Chemical reactions of estersEster saponification: another hydrolysis reaction, but this time, under basic conditions. Rather than a carboxylic acid, the acid salt is produced here.

Example:

Sulfur analogs of estersIn Ch-14 and 15, we saw sulfur analogs of alcohols, ethers, aldehydes, and ketones. Esters also have known sulfur analogs, thioesters:

Thioesters are made by condensation reactions involving carboxylic acids and thiols.

Sulfur analogs of estersThioesters, like esters, have relatively low boiling points (compared to alcohols and carboxylic acids) and may be found in foods as flavorings.

Acetyl coenzyme A, a thioester, is important in metabolic cycles that provide our bodies with energy.Methyl thiobutanoate

PolyestersIn Ch-13, we saw how unsaturated molecules like ethylene can undergo addition polymerization.Condensation polymerization creates polymers through the loss of water molecules:A polyesterDifunctional reactants

PolyestersPolyesters are very important materials for textiles (clothing, upholstery, etc.) and plastics.About 50% of PET production goes towards textiles and the other 50% towards plastics/materials for applications:soda bottlesaudio/video tapesmedicinal applications like heart valves and mesh for arterial replacements

PolyestersBiodegradable polymers are also made by this route. Polyesters of lactic acid and glycolic acid are used to make the lactomer shown below, which undergoes hydrolysis after a few weeks to produce lactic acid and glycolic acid.Used as materials for sutures, this polymer enables dissolvable stiches which produce materials that are already present in the body.Lactic acidGlycolic acid

Acid chlorides and acid anhydridesAcid chlorides and anhydrides have structures that are quite similar to carboxylic acids and esters.Both are able to be hydrolized to produce carbolxylic acids.

Acid chlorides are produced by reaction of a carboxylic acid with an inorganic chloride (PCl3, PCl5, or SOCl2).

Acid chlorides and acid anhydridesAcid chlorides are named as follows:first as a carboxylic acidRemove the -ic acid and add -oyl chlorideHydrolysis reaction:Esterification:

Acid chlorides and acid anhydridesAcid anhydrides are made by condensation reactions between carboxylic acids (or between acid chlorides and carboxylic acids):

Naming is straightforward: indicate the names of the acids used to make the anhydride (without the acid part included)

Acid chlorides and acid anhydridesCarboxylic acids (esters, acid chlorides) possess an acyl group (notice how this is different that the carboxyl group of the acid)

Acyl transfer reactions occur during protein synthesis. The reaction involves the formation of a new bond between the acyl group and another molecular fragment.One example of an acyl transfer occurs during an esterification:Acyl transfer reactionsAcyl group

Esters and anhydrides of inorganic acidsWe have seen inorganic acids like H2SO4, H3PO4, and HNO3 in the first semester of this course. These can also react with alcohols to make inorganic esters:

Esters and anhydrides of inorganic acidsPhosphate esters are the most important class of inorganic esters.Because phosphoric acid has three OH groups, it can form mono-, di- and tri-esters:

Esters and anhydrides of inorganic acidsPhosphoric acid anhydrides are also made via condensation reactions:

These systems are important components of cellular processes for biochemical energy production.