Carboxylic Acids

Introduction

The carboxyl functional group that characterizes the carboxylic acids is unusual in that it is composed of two functional groups hydroxyl  and carbonyl . As may be seen in the formula on the structure below, the carboxyl group is made up of a hydroxyl group bonded to a carbonyl group. It is often written in condensed form as –CO₂H or –COOH. Other combinations of functional groups were described previously, and significant changes in chemical behavior as a result of group interactions were described (e.g. phenol & aniline). In this case, the change in chemical and physical properties resulting from the interaction of the hydroxyl and carbonyl group are so profound that the combination is customarily treated as a distinct and different functional group.

 

1. Nomenclature of Carboxylic Acids

Substituent suffix = -oic acid  e.g. ethanoic acid

Substituent prefix = carboxy

• The root name is based on the longest chain including the carboxylic acid group.

• Since the carboxylic acid group is at the end of the chain, it must be C1.

• The carboxylic acid suffix is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -oic acid = -anoic acid etc.

 

As with aldehydes, the carboxyl group must be located at the end of a carbon chain. In the IUPAC system of nomenclature the carboxyl carbon is designated #1, and other substituents are located and named accordingly. The characteristic IUPAC suffix for a carboxyl group is "oic acid", and care must be taken not to confuse this systematic nomenclature with the similar common system. These two nomenclatures are illustrated in the following table, along with their melting and boiling points

               Name and some physical properties Table:

Formula	Common Name	Source	IUPAC Name	Melting Point	Boiling Point
HCO2H	formic acid	ants (L. formica)	methanoic acid	8.4 ºC	101 ºC
CH3CO2H	acetic acid	vinegar (L. acetum)	ethanoic acid	16.6 ºC	118 ºC
CH3CH2CO2H	propionic acid	milk (Gk. protus prion)	propanoic acid	-20.8 ºC	141 ºC
CH3(CH2)2CO2H	butyric acid	butter (L. butyrum)	butanoic acid	-5.5 ºC	164 ºC
CH3(CH2)3CO2H	valeric acid	valerian root	pentanoic acid	-34.5 ºC	186 ºC
CH3(CH2)4CO2H	caproic acid	goats (L. caper)	hexanoic acid	-4.0 ºC	205 ºC
CH3(CH2)5CO2H	enanthic acid	vines (Gk. oenanthe)	heptanoic acid	-7.5 ºC	223 ºC
CH3(CH2)6CO2H	caprylic acid	goats (L. caper)	octanoic acid	16.3 ºC	239 ºC
CH3(CH2)7CO2H	pelargonic acid	pelargonium (an herb)	nonanoic acid	12.0 ºC	253 ºC
CH3(CH2)8CO2H	capric acid	goats (L. caper)	decanoic acid	31.0 ºC	219 ºC

 

Substituted carboxylic acids are named either by the IUPAC system or by common names. If you are uncertain about the IUPAC rules for nomenclature you should review them now. Some common names, the amino acid threonine for example, do not have any systematic origin and must simply be memorized. In other cases, common names make use of the Greek letter notation for carbon atoms near the carboxyl group. Some examples of both nomenclatures are provided below.

 

Simple dicarboxylic acids having the general formula HO₂C–(CH₂)_n–CO₂H (where n = 0 to 5) are known by the common names: Oxalic (n=0), Malonic (n=1), Succinic (n=2), Glutaric (n=3), Adipic (n=4) and Pimelic (n=5) Acids.

2. Carboxylic Acid Natural Products

Carboxylic acids are widespread in nature, often combined with other functional groups. Simple alkyl carboxylic acids, composed of four to ten carbon atoms, are liquids or low melting solids having very unpleasant odours. The fatty acids are important components of the biomolecules known as lipids, especially fats and oils. As shown in the following table, these long-chain carboxylic acids are usually referred to by their common names, which in most cases reflect their sources.
Interestingly, the molecules of most natural fatty acids have an even number of carbon atoms. Analogous compounds composed of odd numbers of carbon atoms are perfectly stable and have been made synthetically. Since nature makes these long-chain acids by linking together acetate units, it is not surprising that the total carbon atoms composing the natural products are multiples of two. The double bonds in the unsaturated compounds listed on the right are all cis (or Z).

FATTY ACIDS
Saturated Formula Common Name Melting Point CH3(CH2)10CO2H lauric acid 45 ºC CH3(CH2)12CO2H myristic acid 55 ºC CH3(CH2)14CO2H palmitic acid 63 ºC CH3(CH2)16CO2H stearic acid 69 ºC CH3(CH2)18CO2H arachidic acid 76 ºC	Unsaturated Formula Common Name Melting Point CH3(CH2)5CH=CH(CH2)7CO2H palmitoleic acid 0 ºC CH3(CH2)7CH=CH(CH2)7CO2H oleic acid 13 ºC CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H linoleic acid -5 ºC CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7CO2H linolenic acid -11 ºC CH3(CH2)4(CH=CHCH2)4(CH2)2CO2H arachidonic acid -49 ºC

 

The following formulas are examples of other naturally occurring carboxylic acids. The molecular structures range from simple to complex, often incorporate a variety of other functional groups, and many are chiral.

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3. Related Carbonyl Derivatives

Other functional group combinations with the carbonyl group can be prepared from carboxylic acids, and are usually treated as related derivatives. Five common classes of these carboxylic acid derivatives are listed in the following table. Although nitriles do not have a carbonyl group, they are included here because the functional carbon atoms all have the same oxidation state. The top row (yellow shaded) shows the general formula for each class, and the bottom row (light blue) gives a specific example of each. As in the case of amines, amides are classified as 1º, 2º or 3º, depending on the number of alkyl groups bonded to the nitrogen.

 

Functional groups of this kind are found in many kinds of natural products. Some examples are shown below with the functional group colored red. Most of the functions are amides or esters, cantharidin being a rare example of a natural anhydride. Cyclic esters are called lactones, and cyclic amides are referred to as lactams. Penicillin G has two amide functions, one of which is a β-lactam. The Greek letter locates the nitrogen relative to the carbonyl group of the amide.

 

Properties of Carboxylic Acids

1. Physical Properties of Carboxylic Acids

• The polar nature of both the O-H and C=O bonds (due to the electonegativity difference of  the atoms) results in the formation of strong hydrogen bonds with other carboxylic acid molecules or other H-bonding systems (e.g. water). The implications are:
  • higher melting and boiling points compared to analogous alcohols
  • high solubility in aqueous media
  • hydrogen bonded dimers in gas phase and dimers or aggregates in pure liquid

The table at the beginning of this page gave the melting and boiling points for a homologous group of carboxylic acids having from one to ten carbon atoms. The boiling points increased with size in a regular manner, but the melting points did not. Unbranched acids made up of an even number of carbon atoms have melting points higher than the odd numbered homologs having one more or one less carbon. This reflects differences in intermolecular attractive forces in the crystalline state. In the table of fatty acids we see that the presence of a cis-double bond significantly lowers the melting point of a compound. Thus, palmitoleic acid melts over 60º lower than palmitic acid, and similar decreases occur for the C₁₈ and C₂₀ compounds. Again, changes in crystal packing and intermolecular forces are responsible.

The factors that influence the relative boiling points and water solubilities of various types of compounds were discussed earlier. In general, dipolar attractive forces between molecules act to increase the boiling point of a given compound, with hydrogen bonds being an extreme example. Hydrogen bonding is also a major factor in the water solubility of covalent compounds . The following table lists a few examples of these properties for some similar sized polar compounds (the non-polar hydrocarbon hexane is provided for comparison).

Physical Properties of Some Organic Compounds
Formula	IUPAC Name	Molecular Weight	Boiling Point	Water Solubility
CH3(CH2)2CO2H	butanoic acid	88	164 ºC	very soluble
CH3(CH2)4OH	1-pentanol	88	138 ºC	slightly soluble
CH3(CH2)3CHO	pentanal	86	103 ºC	slightly soluble
CH3CO2C2H5	ethyl ethanoate	88	77 ºC	moderately soluble
CH3CH2CO2CH3	methyl propanoate	88	80 ºC	slightly soluble
CH3(CH2)2CONH2	butanamide	87	216 ºC	soluble
CH3CON(CH3)2	N,N-dimethylethanamide	87	165 ºC	very soluble
CH3(CH2)4NH2	1-aminobutane	87	103 ºC	very soluble
CH3(CH2)3CN	pentanenitrile	83	140 ºC	slightly soluble
CH3(CH2)4CH3	hexane	86	69 ºC	insoluble

The first five entries all have oxygen functional groups, and the relatively high boiling points of the first two is clearly due to hydrogen bonding. Carboxylic acids have exceptionally high boiling points, due in large part to dimeric associations involving two hydrogen bonds.  The high boiling points of the amides and nitriles are due in large part to strong dipole attractions, supplemented in some cases by hydrogen bonding.

Structure:

• The CO₂H unit is planar and consistant with sp² hybridisation and a resonance interaction of the lone pairs of the hydroxyl oxygen with the π system of the carbonyl.

Chemical Reactivity

Organic chemistry encompasses a very large number of compounds ( many millions ), and our previous discussion and illustrations have focused on their structural characteristics. Now that we can recognize these actors ( compounds ), we turn to the roles they are inclined to play in the scientific drama staged by the multitude of chemical reactions that define organic chemistry.
We begin by defining some basic terms that will be used frequently as this subject is elaborated.

Chemical Reaction: A transformation resulting in a change of composition, constitution and/or configuration of a compound ( referred to as the reactant or substrate ).
Reactant or Substrate: The organic compound undergoing change in a chemical reaction. Other compounds may also be involved, and common reactive partners ( reagents ) may be identified. The reactant is often ( but not always ) the larger and more complex molecule in the reacting system. Most ( or all ) of the reactant molecule is normally incorporated as part of the product molecule.
Reagent: A common partner of the reactant in many chemical reactions. It may be organic or inorganic; small or large; gas, liquid or solid. The portion of a reagent that ends up being incorporated in the product may range from all to very little or none.
Product(s) The final form taken by the major reactant(s) of a reaction.
Reaction Conditions The environmental conditions, such as temperature, pressure, catalysts & solvent, under which a reaction progresses optimally. Catalysts are substances that accelerate the rate ( velocity ) of a chemical reaction without themselves being consumed or appearing as part of the reaction product. Catalysts do not change equilibria positions

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2. Acidity of Carboxylic Acids

The pK_a 's of some typical carboxylic acids are listed in the following table. When we compare these values with those of comparable alcohols, such as ethanol (pK_a = 16) and 2-methyl-2-propanol (pK_a = 19), it is clear that carboxylic acids are stronger acids by over ten powers of ten! Furthermore, electronegative substituents near the carboxyl group act to increase the acidity.

Compound	pKa		Compound	pKa
HCO2H	3.75		CH3CH2CH2CO2H	4.82
CH3CO2H	4.74		ClCH2CH2CH2CO2H	4.53
FCH2CO2H	2.65		CH3CHClCH2CO2H	4.05
ClCH2CO2H	2.85		CH3CH2CHClCO2H	2.89
BrCH2CO2H	2.90		C6H5CO2H	4.20
ICH2CO2H	3.10		p-O2NC6H4CO2H	3.45
Cl3CCO2H	0.77		p-CH3OC6H4CO2H	4.45

 

Why should the presence of a carbonyl group adjacent to a hydroxyl group have such a profound effect on the acidity of the hydroxyl proton? To answer this question we must return to the nature of acid-base equilibria and the definition of pK_a , illustrated by the general equations given below. These relationships were described in an previous section of this text.

We know that an equilibrium favors the thermodynamically more stable side, and that the magnitude of the equilibrium constant reflects the energy difference between the components of each side. In an acid base equilibrium the equilibrium always favors the weaker acid and base (these are the more stable components). Water is the standard base used for pK_a measurements; consequently, anything that stabilizes the conjugate base (A:^(–)) of an acid will necessarily make that acid (H–A) stronger and shift the equilibrium to the right. Both the carboxyl group and the carboxylate anion are stabilized by resonance, but the stabilization of the anion is much greater than that of the neutral function, as shown in the following diagram. In the carboxylate anion the two contributing structures have equal weight in the hybrid, and the C–O bonds are of equal length (between a double and a single bond). This stabilization leads to a markedly increased acidity.

 

 

Vinylagous Acids Compounds in which an enolic hydroxyl group is conjugated with a carbonyl group also show enhanced acidity.

 

 

 

The resonance effect described here is undoubtedly the major contributor to the exceptional acidity of carboxylic acids. However, inductive effects also play a role. For example, alcohols have pK_a's of 16 or greater but their acidity is increased by electron withdrawing substituents on the alkyl group. The following diagram illustrates this factor for several simple inorganic and organic compounds (row #1), and shows how inductive electron withdrawal may also increase the acidity of carboxylic acids (rows #2 & 3). The acidic hydrogen is colored red in all examples.

Water is less acidic than hydrogen peroxide because hydrogen is less electronegative than oxygen, and the covalent bond joining these atoms is polarized in the manner shown. Alcohols are slightly less acidic than water, due to the poor electronegativity of carbon, but chloral hydrate, Cl₃CCH(OH)₂, and 2,2,2,-trifluoroethanol are significantly more acidic than water, due to inductive electron withdrawal by the electronegative halogens (and the second oxygen in chloral hydrate). In the case of carboxylic acids, if the electrophilic character of the carbonyl carbon is decreased the acidity of the carboxylic acid will also decrease. Similarly, an increase in its electrophilicity will increase the acidity of the acid. Acetic acid is ten times weaker an acid than formic acid (first two entries in the second row), confirming the electron donating character of an alkyl group relative to hydrogen, as noted earlier in a discussion of carbocation stability. Electronegative substituents increase acidity by inductive electron withdrawal. As expected, the higher the electronegativity of the substituent the greater the increase in acidity (F > Cl > Br > I), and the closer the substituent is to the carboxyl group the greater is its effect (isomers in the 3rd row). Substituents also influence the acidity of benzoic acid derivatives, but resonance effects compete with inductive effects. The methoxy group is electron donating and the nitro group is electron withdrawing (last three entries in the table of pK_a values).

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Substituent Effects on the Acidity of Carboxylic Acids

he carboxylic acids are a large and structurally diverse class of compounds. Since most are at least partially soluble in water and have pK_a's in the 2 to 5 region, the influence of functional substituents and structural features on aqueous acidity have been studied extensively. Formic acid, HCO₂H, is the simplest member of this class, and will serve as a useful reference point, pK_a=3.75. Although the greater acidity of formic acid compared with methanol has been attributed to resonance stabilization of the formate anion, the different solvation demands of the respective conjugate anions result in an entropy difference that also favors the formate base. Both factors are depicted in the following illustration.. Resonance delocalization of the negative charge in the formate anion produces a large enthalpic stabilization shown by the magenta arrow. In water solution both methanol and formic acid are incorporated into the dynamic hydrogen bonded structure of liquid water. On ionization, each of these solutes produces a hydrated proton (hydronium ion) and a negatively charged conjugate base. The hydronium ion is common to both cases and can be ignored. The negative charge in the methoxide anion is concentrated on a single oxygen atom and demands strong solvation by water molecules, indicated by the aqua-colored dots. This solvation forces significant structural organization on many water molecules at the cost of decreased entropy. The formate anion also carries a single negative charge, but it is distributed over two oxygen atoms, so the charge density at either site is halved, compared with methoxide. This lower charge density demands much less solvation by water, resulting in a smaller entropy cost.

The importance of solvation and the accompanying entropy changes to any discussion of acidity may be seen by comparing the pK_a's of methanol and formic acid in water and DMSO, a solvent that poorly solvates anions. In water the pK_a of methanol is 15.5, nearly 12 powers of ten less acidic than formic acid (3.75). In DMSO the pK_a's of methanol and formic acid are roughly 29 and 13 respectively, representing a very large decrease in Brønsted acid strength for both compounds (more than ten powers of ten). Furthermore, the difference in acid strength between methanol and formic acid in DMSO is magnified about ten thousand times, even though the enthalpic resonance stabilization presumably remains constant. When comparing the acidities of different acids, care must be taken to use pK_a's measured in the same solvent. In this discussion all the pK_a's were taken in or extrapolated to water at 25 ºC. Measurements in mixed aqueous solvents, using water-soluble organic co-solvents such as ethanol, acetonitrile, dioxane, DMSO and acetone, generally give significantly larger pK_a's.

In all other carboxylic acids an organic substituent replaces the hydrogen of formic acid, and it is instructive to analyze the change in acid strength caused by this change. To begin with, we must recognize that the carbonyl moiety of the carboxyl group is electrophilic and withdraws electrons from substituents. The deactivating nature of the carboxyl group on electrophilic substitution of benzoic acid is one example of this property. Resonance structures, such as A, B & C in the following diagram, are often drawn to describe this electrophilic character. The inductive effect of substituent Z in this diagram may enhance or diminish this character, depending on its overall electronegativity. Inductive electron withdrawal will increase the electrophilic character and the acidity of the carboxyl group, as shown in the green shaded box on the right. Resonance electron donation, either by p-π or π-π interaction, would act to stabilize the carboxylic acid, reducing its electrophilicity and acidity. These two effects often act in opposition, and in the case of carbonic acid ( H₂CO₃ ) electron donation overcomes inductive withdrawal, resulting in a pK_a¹=6.63.

Saturated aliphatic acids are generally ten times weaker than formic acid, which may seem surprising since carbon has a higher Pauling electronegativity than hydrogen (2.55 versus 2.20). However, we must recognize that a carbon atom is larger and more polarizable than hydrogen, allowing it to shift electrons toward the more electronegative carbonyl carbon of the carboxyl group. Also, hydrogen and alkyl substituents on the α-carbon assist in this inductive electron shift, as shown in the green box on the left. This analysis is supported by the activating influence of alkyl substituents in electrophilic aromatic substitution, the Markovnikov rule, and the greater reactivity of aldehydes with nucleophiles compared with equivalent methyl ketones.
The four carboxylic acids in the first row of the following table illustrate the electron donating quality of alkyl groups. As the number of carbon atoms in the group increases from one to five, the inductive electron donation also increases. The compounds in the next three rows of the table demonstrate that electronegative substituents on an alkyl group can shift its inductive effect from donating to withdrawing (relative to hydrogen). Thus, all the haloacetic acids are more acidic than formic acid, with fluoroacetic acid being the most acidic. Additional halogen substituents have an additive influence, and moving the substituent from the α to a β-carbon reduces its influence on the acidity. Note that a hydroxyl substituent has a much weaker effect than any of the halogens, despite the higher electronegativity of oxygen (3.44 compared with 3.16 for chlorine).

 

pK_a Values for Some Aliphatic Carboxylic Acids ( 25 ºC in H₂O )

Compound	pKa		Compound	pKa		Compound	pKa		Compound	pKa
CH3CO2H	4.76		CH3CH2CO2H	4.87		CH3(CH2)2CO2H	4.91		(CH3)3CCO2H	5.05
FCH2CO2H	2.59		ClCH2CO2H	2.85		BrCH2CO2H	2.89		ICH2CO2H	3.13
NCCH2CO2H	2.50		HOCH2CO2H	3.82		Cl2CHCO2H	1.25		Cl3CCO2H	0.77
NCCH2CH2CO2H	3.98		ClCH2CH2CO2H	3.95		BrCH2CH2CO2H	4.00		ICH2CH2CO2H	4.06

 

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Reactivity:

The image shows the electrostatic potential for acetic acid (ethanoic acid).  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  There is low electron density (blue) on H atom of the -CO2H group alcohol, i.e. H+ character.  The H atom of the RCO2H is acidic (pKa ~ 5). The most important reactions of carboxylic acids converts them into carboxylic acid derivatives such as acyl halides, esters and amides via nucleophilic acyl substitution reactions.

The image shows the electrostatic potential for the acetate ion (ethanoate ion)  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  There is high electron density (red) on both O atoms of the -CO2- group alcohol, i.e. resonance and basic or nucleophilic behaviour

Preparation of Carboxylic Acids

The carbon atom of a carboxyl group has a high oxidation state. It is not surprising, therefore, that many of the chemical reactions used for their preparation are oxidations. Such reactions have been discussed in previous sections of this text, and the following diagram summarizes most of these. To review the previous discussion of any of these reaction classes simply click on the number (1 to 4) or descriptive heading for the group.

 

Two other useful procedures for preparing carboxylic acids involve hydrolysis of nitriles and carboxylation of organometallic intermediates. As shown in the following diagram, both methods begin with an organic halogen compound and the carboxyl group eventually replaces the halogen. Both methods require two steps, but are complementary in that the nitrile intermediate in the first procedure is generated by a S_N2 reaction, in which cyanide anion is a nucleophilic precursor of the carboxyl group. The hydrolysis may be either acid or base-catalyzed, but the latter give a carboxylate salt as the initial product.
In the second procedure the electrophilic halide is first transformed into a strongly nucleophilic metal derivative, and this adds to carbon dioxide (an electrophile). The initial product is a salt of the carboxylic acid, which must then be released by treatment with strong aqueous acid.

 

An existing carboxylic acid may be elongated by one methylene group, using a homologation procedure called the Arndt-Eistert reaction.

 

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Reactions of Carboxylic Acids

• In principle, all carboxylic acids derivatives can be made from the parent carboxylic acid see above.
• In practice, there may be better methods, e.g. amides are more readily prepared from the more reactive acyl chlorides.
• However, appreciating the relationship between these groups is important and useful.

1. Salt Formation

Because of their enhanced acidity, carboxylic acids react with bases to form ionic salts, as shown in the following equations. In the case of alkali metal hydroxides and simple amines (or ammonia) the resulting salts have pronounced ionic character and are usually soluble in water. Heavy metals such as silver, mercury and lead form salts having more covalent character (3rd example), and the water solubility is reduced, especially for acids composed of four or more carbon atoms.

RCO2H	+	NaHCO3		RCO2(–) Na(+)   +   CO2   +   H2O
RCO2H	+	(CH3)3N:		RCO2(–) (CH3)3NH(+)
RCO2H	+	AgOH		RCO2δ(-) Agδ(+)   +   H2O

Carboxylic acids and salts having alkyl chains longer than six carbons exhibit unusual behavior in water due to the presence of both hydrophilic (CO₂) and hydrophobic (alkyl) regions in the same molecule. Such molecules are termed amphiphilic (Gk. amphi = both) or amphipathic. Depending on the nature of the hydrophilic portion these compounds may form monolayers on the water surface or sphere-like clusters, called micelles, in solution.

2. Substitution of the Hydroxyl Hydrogen

This reaction class could be termed electrophilic substitution at oxygen, and is defined as follows (E is an electrophile). Some examples of this substitution are provided in equations (1) through (4).

RCO2–H   +   E(+)

RCO2–E  +  H(+)

If E is a strong electrophile, as in the first equation, it will attack the nucleophilic oxygen of the carboxylic acid directly, giving a positively charged intermediate which then loses a proton. If E is a weak electrophile, such as an alkyl halide, it is necessary to convert the carboxylic acid to the more nucleophilic carboxylate anion to facilitate the substitution. This is the procedure used in reactions 2 and 3. Equation 4 illustrates the use of the reagent diazomethane (CH₂N₂) for the preparation of methyl esters. This toxic and explosive gas is always used as an ether solution (bright yellow in color). The reaction is easily followed by the evolution of nitrogen gas and the disappearance of the reagent's color. This reaction is believed to proceed by the rapid bonding of a strong electrophile to a carboxylate anion.
The nature of S_N2 reactions, as in equations 2 & 3, has been described elsewhere. The mechanisms of reactions 1 & 4 will be displayed by clicking the "Toggle Mechanism" button below the diagram.

 

Alkynes may also serve as electrophiles in substitution reactions of this kind, as illustrated by the synthesis of vinyl acetate from acetylene. Intramolecular carboxyl group additions to alkenes generate cyclic esters known as lactones. Five-membered (gamma) and six-membered (delta) lactones are most commonly formed. Electrophilic species such as acids or halogens are necessary initiators of lactonizations. Even the weak electrophile iodine initiates iodolactonization of γ,δ- and δ,ε-unsaturated acids.

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3. Substitution of the Hydroxyl Group

Reactions in which the hydroxyl group of a carboxylic acid is replaced by another nucleophilic group are important for preparing functional derivatives of carboxylic acids. The alcohols provide a useful reference chemistry against which this class of transformations may be evaluated. In general, the hydroxyl group proved to be a poor leaving group, and virtually all alcohol reactions in which it was lost involved a prior conversion of –OH to a better leaving group. This has proven to be true for the carboxylic acids as well.
Four examples of these hydroxyl substitution reactions are presented by the following equations. In each example, the new bond to the carbonyl group is colored magenta and the nucleophilic atom that has replaced the hydroxyl oxygen is colored green. The hydroxyl moiety is often lost as water, but in reaction #1 the hydrogen is lost as HCl and the oxygen as SO₂. This reaction parallels a similar transformation of alcohols to alkyl chlorides, although its mechanism is different. Other reagents that produce a similar conversion to acyl halides are PCl₅ and SOBr₂.
The amide and anhydride formations shown in equations #2 & 3 require strong heating, and milder procedures that accomplish these transformations will be described in the next chapter.

Reaction #4 is called esterification, since it is commonly used to convert carboxylic acids to their ester derivatives. Esters may be prepared in many different ways; indeed, equations #1 and #4 in the previous diagram illustrate the formation of tert-butyl and methyl esters respectively. The acid-catalyzed formation of ethyl acetate from acetic acid and ethanol shown here is reversible, with an equilibrium constant near 2. The reaction can be forced to completion by removing the water as it is formed. This type of esterification is often referred to as Fischer esterification. As expected, the reverse reaction, acid-catalyzed ester hydrolysis, can be carried out by adding excess water.
A thoughtful examination of this reaction (#4) leads one to question why it is classified as a hydroxyl substitution rather than a hydrogen substitution. The following equations, in which the hydroxyl oxygen atom of the carboxylic acid is colored red and that of the alcohol is colored blue, illustrate this distinction (note that the starting compounds are in the center).

H2O  +   CH3CO-OCH2CH3

H-substitution

CH3CO-OH  +   CH3CH2-OH

HO-substitution

CH3CO-OCH2CH3  +   H2O

In order to classify this reaction correctly and establish a plausible mechanism, the oxygen atom of the alcohol was isotopically labled as ¹⁸O (colored blue in our equation). Since this oxygen is found in the ester product and not the water, the hydroxyl group of the acid must have been replaced in the substitution. A mechanism for this general esterification reaction in table below . Addition-elimination mechanisms of this kind proceed by way of tetrahedral intermediates (such as A and B in the mechanism diagram) and are common in acyl substitution reactions. Acid catalysis is necessary to increase the electrophilic character of the carboxyl carbon atom, so it will bond more rapidly to the nucleophilic oxygen of the alcohol. Base catalysis is not useful because base converts the acid to its carboxylate anion conjugate base, a species in which the electrophilic character of the carbon is reduced.
Since a tetrahedral intermediate occupies more space than a planar carbonyl group, we would expect the rate of this reaction to be retarded when bulky reactants are used. To test this prediction the esterification of acetic acid was compared with that of 2,2-dimethylpropanoic acid, (CH₃)₃CO₂H. Here the relatively small methyl group of acetic acid is replaced by a larger tert-butyl group, and the bulkier acid reacted fifty times slower than acetic acid. Increasing the bulk of the alcohol reactant results in a similar rate reduction.

SUMMARY MECHANISM FOR REACTION FOR ACID CATALYSED ESTERIFICATION

Step 1: An acid/base reaction. Protonation of the carbonyl makes it more electrophilic.
Step 2: The alcohol O functions as the nucleophile attacking the electrophilic C in the C=O, with the electrons moving towards the oxonium ion, creating the tetrahedral intermediate.
Step 3: An acid/base reaction. Deprotonate the alcoholic oxygen.
Step 4: An acid/base reaction. Need to make an -OH leave, it doesn't matter which one, so convert it into a good leaving group by protonation.
Step 5: Use the electrons of an adjacent oxygen to help "push out" the leaving group, a neutral water molecule.
Step 6: An acid/base reaction. Deprotonation of the oxonium ion reveals the carbonyl in the ester product.

 

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Reductions & Oxidations of Carboxylic Acids

1. Reduction

The carbon atom of a carboxyl group is in a relatively high oxidation state. Reduction to a 1º-alcohol takes place rapidly on treatment with the powerful metal hydride reagent, lithium aluminum hydride, as shown by the following equation. One third of the hydride is lost as hydrogen gas, and the initial product consists of metal salts which must be hydrolyzed to generate the alcohol. These reductions take place by the addition of hydride to the carbonyl carbon, in the same manner noted earlier for aldehydes and ketones. The resulting salt of a carbonyl hydrate then breaks down to an aldehyde that undergoes further reduction.

 

4 RCO2H   +   3 LiAlH4

ether

4 H2  +   4 RCH2OM   +   metal oxides

H2O

4 RCH2OH   +   metal hydroxides

Diborane, B₂H₆, reduces the carboxyl group in a similar fashion. Sodium borohydride, NaBH₄, does not reduce carboxylic acids; however, hydrogen gas is liberated and salts of the acid are formed. Partial reduction of carboxylic acids directly to aldehydes is not possible, but such conversions have been achieved in two steps by way of certain carboxyl derivatives. These will be described later.

Reaction usually in Et₂O or THF followed by H₃O⁺work-ups

 

Reaction type:  Nucleophilic Acyl Substiution then Nucleophilic Addition

Summary

• Carboxylic acids are less reactive to reduction by hydride than aldehydes, ketones or esters.
• Carboxylic acids are reduced to primary alcohols.
• As a result of their low reactivity, carboxylic acids can only be reduced by LiAlH₄ and NOT by the less reactive  NaBH₄

 

a-Halogenation (Hell-Volhard-Zelinsky reaction)

Reaction type: Substitution

Summary

• Reagents most commonly : Br₂ and either PCl3, PBr3 or red phosphorous in catalytic amounts.
• Carboxylic acids can be halogenated at the C adjacent to the carboxyl group.
• This reaction depends on the enol type character of carbonyl compounds.
• The product of the reaction, an a-bromocarboxylic acid can be converted via substitution reactions to a-hydroxy- or a-amino carboxylic acids.

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Decarboxylation

Reaction type: Elimination

Summary

• Loss of carbon dioxide is called decarboxylation.
• Simple carboxylic acids rarely undergo decarboxylation.
• Carboxylic acids with a carbonyl group at the 3- (or b-) position readily undergo thermal decarboxylation, e.g. derivatives of malonic acid.

 

 

2. Oxidation

Because it is already in a high oxidation state, further oxidation removes the carboxyl carbon as carbon dioxide. Depending on the reaction conditions, the oxidation state of the remaining organic structure may be higher, lower or unchanged. The following reactions are all examples of decarboxylation (loss of CO₂). In the first, bromine replaces the carboxyl group, so both the carboxyl carbon atom and the remaining organic moiety are oxidized. Silver salts have also been used to initiate this transformation, which is known as the Hunsdiecker reaction. The second reaction is an interesting bis-decarboxylation, in which the atoms of the organic residue retain their original oxidation states. Lead tetraacetate will also oxidize mono-carboxylic acids in a manner similar to reaction #1. Finally, the third example illustrates the general decarboxylation of β-keto acids, which leaves the organic residue in a reduced state (note that the CO₂ carbon has increased its oxidation state.). 

The meta- dihalobenzene formed in reaction 4 could not be made by direct halogenation reactions, since chlorine and bromine are ortho/para-directing substituents. Also, various iodide derivatives may be prepared directly from the corresponding carboxylic acids. A heavy metal carboxylate salt is transformed into an acyl hypohalide by the action of a halogen. The weak oxygen-halogen bond in this intermediate cleaves homolytically when heated or exposed to light, and the resulting carboxy radical decarboxylates to an alkyl or aryl radical. A chain reaction then repeats these events. Since acyl hypohalites are a source of electrophilic halogen, this reaction takes a different course when double bonds and reactive benzene derivatives are present. In this respect remember the addition of hypohalous reagents to double bonds and the facile bromination of anisole.

 

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Spectroscopic Analysis

 

• IR - The -O-H and C=O should be obvious

Absorbance (cm-1)	Interpretation
2500 - 3500 (very broad)	OH stretch
1700	C=O stretch
1200-1250	C-O stretch

• ¹H NMR - The -CO₂H proton is very deshielded

 

Resonance (ppm)	Interpretation
10 -12 (exchangeable)	-COOH proton
2 - 3	H-C-COOH

• ¹³C NMR
  CO₂H carbon  160 - 185 ppm (deshielded due to O, but not as much as aldehydes and ketones, 190-215 ppm)
   
• UV-VIS
  Simple carboxylic acids absorb at 210 nm, but this is too low to be particularly useful.
   
• Mass Spectrometry
  Peak for the molecular ion, M⁺, is usually prominent.
  Fragments due to loss of OH (M - 17)⁺ and then loss of CO (M - 45)⁺

                                                                                                                                                     Alcohols

                                                                                                                       Aldehydes & Ketones

                                                                                                                                                      Carboxylic Acid Derivatives  

 

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© M.EL-Fellah ,Chemistry Department, Garyounis University