Carboxylic Acids Chemistry Notes PDF

1. The carboxyl functional group that characterizes the carboxylic acids is unusual in that it is composed of two functional groups described earlier in this text. As may be seen in the formula on the right, the carboxyl group is made up of a hydroxyl group bonded to a carbonyl group. It is often...

1. The carboxyl functional group that characterizes the carboxylic acids is unusual in that it is composed of two functional groups described earlier in this text. As may be seen in the formula on the right, 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 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. --------------- ----------------- ------------------------- ---------------- --------------- --------------- 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]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/nomen1.htm#start) 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. Common names, such as these can be troublesome to remember, so mnemonic aids, which take the form of a catchy phrase, have been devised. For this group of compounds one such phrase is: \"**O**h **M**y **S**uch **G**ood **A**pple **P**ie\". ### 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 odors. 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. A mnemonic phrase for the C10 to C20 natural fatty acids capric, lauric, myristic, palmitic, stearic and arachidic is: \"**C**urly, **L**arry & **M**oe **P**erform **S**illy **A**ntics\" (note that the names of the three stooges are in alphabetical order).\ 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 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 | | | | | | +---------+---------+---------+ | | | | #### Sa | | | | | | | turated | | | | | | +---------+---------+---------+ | | | | Formula | Common | Melting | | | | | | Name | Point | | | | +---------+---------+---------+ | | | | CH3(CH2 | lauric | 45 ºC | | | | | )10CO2H | acid | | | | | +---------+---------+---------+ | | | | CH3(CH2 | myristi | 55 ºC | | | | | )12CO2H | c | | | | | | | acid | | | | | +---------+---------+---------+ | | | | CH3(CH2 | palmiti | 63 ºC | | | | | )14CO2H | c | | | | | | | acid | | | | | +---------+---------+---------+ | | | | CH3(CH2 | stearic | 69 ºC | | | | | )16CO2H | acid | | | | | +---------+---------+---------+ | | | | CH3(CH2 | arachid | 76 ºC | | | | | )18CO2H | ic | | | | | | | acid | | | | | +---------+---------+---------+ | | +-----------------------------------+-----------------------------------+ | +---------+---------+---------+ | | | | #### Un | | | | | | | saturat | | | | | | | ed | | | | | | +---------+---------+---------+ | | | | Formula | Common | Melting | | | | | | Name | Point | | | | +---------+---------+---------+ | | | | CH3(CH2 | palmito | 0 ºC | | | | | )5CH=CH | leic | | | | | | (CH2)7C | acid | | | | | | O2H | | | | | | +---------+---------+---------+ | | | | CH3(CH2 | oleic | 13 ºC | | | | | )7CH=CH | acid | | | | | | (CH2)7C | | | | | | | O2H | | | | | | +---------+---------+---------+ | | | | CH3(CH2 | linolei | -5 ºC | | | | | )4CH=CH | c | | | | | | CH2CH=C | acid | | | | | | H(CH2)7 | | | | | | | CO2H | | | | | | +---------+---------+---------+ | | | | CH3CH2C | linolen | -11 ºC | | | | | H=CHCH2 | ic | | | | | | CH=CHCH | acid | | | | | | 2CH=CH( | | | | | | | CH2)7CO | | | | | | | 2H | | | | | | +---------+---------+---------+ | | | | CH3(CH2 | arachid | -49 ºC | | | | | )4(CH=C | onic | | | | | | HCH2)4( | acid | | | | | | CH2)2CO | | | | | | | 2H | | | | | | +---------+---------+---------+ | | +-----------------------------------+-----------------------------------+ 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. ### 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. +-----------------------------------------------------------------------+ | ### Physical Properties & Acidity | +-----------------------------------------------------------------------+ ### Properties of Carboxylic Acids ### 1. Physical Properties of Carboxylic Acids The [[table]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/crbacid1.htm#crbacd2) 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]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/crbacid1.htm#crbacd2b) 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 To refresh your understanding of these principles [[Click Here]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/suppmnt1.htm#exp4). 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. A structural formula for the dimer of acetic acid is shown here. When the mouse pointer passes over the drawing, an electron cloud diagram will appear. 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.  ### 2. Acidity of Carboxylic Acids The pKa \'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 (pKa = 16) and 2-methyl-2-propanol (pKa = 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 pKa , illustrated by the general equations given below. These relationships were described in an [[previous section]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/react1.htm#rx1b) 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 pKa 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, as illustrated by the energy diagram displayed by clicking the \"[Toggle Display]\" button.  ------------------------------------------------------------------------------------------------------------------------------------------ **Vinylagous Acids**\ Compounds in which an enolic hydroxyl group is conjugated with a carbonyl group also show enhanced acidity.\ To see examples of such compounds [[Click Here]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/special2.htm#top4) ------------------------------------------------------------------------------------------------------------------------------------------ The resonance effect described here is undoubtedly the major contributor to the exceptional acidity of carboxylic acids. However, [[inductive effects]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/intro2.htm#strc3b) also play a role. For example, alcohols have pKa\'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]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/addene1.htm#add1bc). 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]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/benzrx1.htm#benz3b). The methoxy group is electron donating and the nitro group is electron withdrawing (last three entries in the table of pKa values). ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ For additional information about substituent effects on the acidity of carboxylic acids [[Click Here]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/suppmnt2.htm#top6) ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ +-----------------------------------------------------------------------+ | ### Preparation of Carboxylic Acids | +-----------------------------------------------------------------------+ ### 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 SN2 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.**.\ To learn about this useful method [[Click Here]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/rearrang.htm#top8). ---------------------------------------------------------------------------------------------------------------------------------------------- +-----------------------------------------------------------------------+ | ### Reactivity | +-----------------------------------------------------------------------+ ### Reactions of Carboxylic Acids ### 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 SN2 reactions, as in equations 2 & 3, has been [[described elsewhere]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/alhalrx1.htm#hal2e). 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. Examples of these reactions will be displayed by clicking the \"[Other Examples]\" button. ### 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]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/crbacid1.htm#crbacd3). The alcohols provide a useful [[reference chemistry]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/alcohol1.htm#alcrx2) 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]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/crbacid2.htm#react1b) 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]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/crbacid1.htm#crbacd6b) 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  +   | H-substitut | --------- | HO-substitu | CH3CO-OCH2C | | CH3CO-OCH2C | ion\ | ----------- | tion\ | H3 | | H3 |  | CH3CO-OH | | | | | |  +   CH3CH2 | | | | | | -OH | | | | | | --------- | | | | | | ----------- | | | | | | ----- | | | +-------------+-------------+-------------+-------------+-------------+ In order to classify this reaction correctly and establish a plausible mechanism, the oxygen atom of the alcohol was isotopically labeled 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 will be displayed on clicking the \"[Esterification Mechanism]\" button; also, once the mechanism diagram is displayed, a reaction coordinate for it can be seen by clicking the head of the green \"[energy diagram]\" arrow. 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. +-----------------------------------------------------------------------+ | ### Reduction & Oxidation Reactions | +-----------------------------------------------------------------------+ ### 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]](https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/aldket1.htm#rx2a) for aldehydes and ketones. The resulting salt of a carbonyl hydrate then breaks down to an aldehyde that undergoes further reduction.