Carboxylic Acids and Their Derivatives PDF

# Carboxylic Acids and Their Derivatives ## 10 The Bark of the White Willow Tree (Salix Alba) is a Source of Salicylic Acid, From Which Aspirin (Acetylsalicylic Acid) Is Made. The image shows a tree with a caption that states the bark of the white willow tree (Salix alba) is a source of salicylic acid, from which aspirin (acetylsalicylic acid) is made. The image also contains a chemical structure of salicylic acid. ## 10.1 Nomenclature of Acids Because of their abundance in nature, carboxylic acids were among the earliest classes of compounds studied by organic chemists. It is not surprising that many of them have common names. These names usually come from some Latin or Greek word that indicates the original source of the acid. **Table 10.1:** Aliphatic Carboxylic Acids | Carbon atoms | Formula | Source | Common name | IUPAC name | |---|---|---|---|---| | 1 | HCOOH | ants | formic acid | methanoic acid | | 2 | CH3COOH | vinegar | acetic acid | ethanoic acid | | 3 | CH3CH2COOH | milk | propionic acid | propanoic acid | | 4 | CH3(CH2)2COOH | butter | butyric acid | butanoic acid | | 5 | CH3(CH2)3COOH | valerian root | valeric acid | pentanoic acid | | 6 | CH3(CH2)4COOH | goats | caproic acid | hexanoic acid | | 7 | CH3(CH2)5COOH | vine blossom | enanthic acid | heptanoic acid | | 8 | CH3(CH2)6COOH | goats | caprylic acid | octanoic acid | | 9 | CH3(CH2)7COOH | pelargonium | pelargonic acid | nonanoic acid | | 10 | CH3(CH2)8COOH | goats | capric acid | decanoic acid | Carboxylic acids are organic acids that contain the *carboxyl group*. In carboxylic acid derivatives, the -OH group is replaced by other groups. ## 10.2 Physical Properties of Acids The first members of the carboxylic acid series are colorless liquids with sharp or unpleasant odors. - Acetic acid constitutes about 4% to 5% of vinegar, and provides the characteristic odor and flavor. - Butyric acid gives rancid butter its disagreeable odor. - The goat acids (caproic, caprylic, and capric) smell like goats. - 3-Methyl-2-hexenoic acid, produced by bacteria, is responsible for the offensive odor of human armpits. **Table 10.3:** Physical Properties of Some Carboxylic Acids | Name | bp (°C) | mp (°C) | Solubility, g/100 g H2O at 25°C | |---|---|---|---| | Formic acid | 101 | 8 | | | Acetic acid | 118 | 17 | miscible (∞) | | Propanoic acid | 141 | -22 | | | Butanoic acid | 164 | -8 | | | Hexanoic acid | 205 | 1.5 | 1.0 | | Octanoic acid | 240 | 17 | 0.06 | | Decanoic acid | 270 | 31 | 0.01 | | Benzoic acid | 249 | 122 | 0.4 (but 6.8 at 95°C) | Carboxylic acids are polar. Like alcohols they form hydrogen bonds with themselves or with other molecules. Therefore, they have high boiling points for their molecular weights - higher even than those of comparable alcohols. For example, acetic acid and propyl alcohol, which have the same formula weights (60 g/mol), boil at 118°C and 97°C, respectively. Carboxylic acids form dimers. ## 10.3 Acidity and Acidity Constants Carboxylic acids (RCO2H) dissociate in water, yielding a *carboxylate anion* (RCO2-) and a *hydronium ion*. ``` R-C(=O)OH + H2O <===> R-C(=O)O- + H3O+ ``` Their acidity constants *Ka* in water are given by the expression ``` Ka = [RCO2-][H3O+]/[RCO2H] ``` **Table 10.4:** The Ionization Constants of Some Acids | Name | Formula | Ka | pKa | |---|---|---|---| | Formic acid | HCOOH | 2.1 x 10-4 | 3.68 | | Acetic acid | CH3COOH | 1.8 x 10-5 | 4.74 | | Propanoic acid | CH3CH2COOH | 1.4 x 10-5 | 4.85 | | Butanoic acid | CH3CH2CH2COOH | 1.6 x 10-5 | 4.80 | | Chloroacetic acid | ClCH2COOH | 1.5 x 10-3 | 2.82 | | Dichloroacetic acid | Cl2CHCOOH | 5.0 x 10-2 | 1.30 | | Trichloroacetic acid | Cl3CCOOH | 2.0 x 10-1 | 0.70 | | 2-Chlorobutanoic acid | CH3CH2CHCICOOH | 1.4 x 10-3 | 2.85 | | 3-Chlorobutanoic acid | CH3CHCICH2COOH | 8.9 x 10-5 | 4.05 | | Benzoic acid | C6H5COOH | 6.6 x 10-5 | 4.18 | | o-Chlorobenzoic acid | o-Cl-C6H4COOH | 12.5 x 10-4 | 2.90 | | m-Chlorobenzoic acid | m-Cl-C6H4COOH | 1.6 x 10-4 | 3.80 | | p-Chlorobenzoic acid | p-Cl-C6H4COOH | 1.0 x 10-4 | 4.00 | | p-Nitrobenzoic acid | p-NO2-C6H4COOH | 4.0 x 10-4 | 3.40 | | Phenol | C6H5OH | 1.0 x 10-10 | 10.00 | | Ethanol | CH3CH2OH | 1.0 x 10-16 | 16.00 | | Water | HOH | 1.8 x 10-16 | 15.74 | ## 10.4 What Makes Carboxylic Acids Acidic ? You might wonder why carboxylic acids are so much more acidic than alcohols, since each class ionizes by loosing H+ from a hydroxyl group. There are two reasons. From Table 10.4, we see that acetic acid is approximately 1011, or 100,000 million, times stronger an acid than ethanol. ``` CH3CH2OH <=> CH3CH2O- + H+ Ka=10-16 CH3C(=O)OH <=> CH3C(=O)O- + H+ Ka = 10-5 ``` The only difference between the structures of acetic acid and ethanol is the replacement of a CH2 group (in ethanol) by a carbonyl group (in acetic acid). - A carbonyl carbon atom carries a substantial positive charge (*δ*+). - This charge makes it much easier to place a negative charge on the adjacent oxygen atom, which is exactly what happens when we ionize a proton from the hydroxyl group. In ethoxide ion, the negative charge is localized on a single oxygen atom. However, in the acetate ion the negative charge can be delocalized through resonance. The negative charge is spread equally over the two oxygens so that each oxygen in the carboxylate ion only carries half the negative charge. The acetate ion is stabilized by resonance compared to the ethoxide ion, and this stabilization helps to drive the equilibrium more to the right. This means that more H+ is formed from acetic acid than from ethanol. - For both these reasons, the positive charge on the carbonyl carbon and delocalization of the carboxylate ion, carboxylic acids are much more acidic than alcohols. Physical data supports the importance of resonance in carboxylate ions. - In formic acid molecules, the two carbon-oxygen bonds have different lengths. - But in sodium formate, both carbon-oxygen bonds of the formate ion are identical, and their length is between those of normal double and single carbon-oxygen bonds. ## 10.5 Effect of Structure on Acidity; The Inductive Effect Revisited The data in Table 10.4 show that even among carboxylic acids (where the ionizing functional group is kept constant), acidities can vary depending on what other groups are attached to the molecule. - The Ka of acetic acid varies greatly with those of mono-, di-, and trichloroacetic acids. Notice that the acidity varies by a factor of 10,000. The most important factor operating here is the inductive effect of the groups close to the *carboxyl group*. This effect relays charge through bonds by displacing bonding electrons toward electronegative atoms, or away from electropositive atoms. - Electron- withdrawing groups enhance acidity. - Electron-releasing groups reduce acidity. Let us examine the carboxylate ions formed when acetic acid and its chloro derivatives ionize: - *Acetate* - *Chloroacetate* - *Dichloroacetate* - *Trichloroacetate* Because chlorine is more electronegative than carbon, the C-Cl bond is polarized with the chlorine partially negative and the carbon partially positive. Thus, electrons are pulled away from the carboxylate end of the ion toward the chlorine. - The effect tends to spread the negative charge over more atoms than in the acetate ion itself and thus stabilizes the ion. - The more chlorines, the greater the effect and the greater the strength of the acid. ## 10.6 Conversion of Acids to Salts Carboxylic acids, when treated with a strong base, form *carboxylate salts*. For example, ``` R-C(=O)OH + Na+OH- ==> R-C(=O)O-Na+ + H2O ``` The salt can be isolated by evaporating the water. - Carboxylate salts of certain acids are useful as soaps and detergents. - They are named as follows: - *Sodium acetate* - *Potassium benzoate* - *Calcium propanoate* The cation is named first, followed by the name of the carboxylate ion, which is obtained by changing the -ic ending of the acid to -ate. ## 10.7 Preparation of Acids Organic acids can be prepared in several ways. - **Oxidation of Primary Alcohols and Aldehydes:** The oxidation of primary alcohols and aldehydes to carboxylic acids has already been mentioned. These are oxidation reactions because going from an alcohol to an aldehyde to an acid requires replacement of C-H bonds by C-O bonds. - **Oxidation of Aromatic Side Chains:** Aromatic acids can be prepared by oxidizing an alkyl side chain on an aromatic ring. - **Reaction of Grignard Reagents with Carbon Dioxide:** Grignard reagents add to the carbonyl groups of aldehydes or ketones to give alcohols. In a similar way, they add irreversibly to the carbonyl group of carbon dioxide to give acids, after protonation of the intermediate carboxylate salt with a mineral acid like aqueous HCl. - **Hydrolysis of Cyanides (Nitriles):** The carbon-nitrogen triple bond of organic cyanides can be hydrolyzed to a carboxyl group. The reaction requires either acid or base. In acid, the nitrogen atom of the cyanide is converted to an ammonium ion. In base, the nitrogen is converted to ammonia and the organic product is the carboxylate salt, which must be neutralized in a separate step to give the acid. ## The mechanism of nitrile hydrolysis involves acid or base promoted addition of water across the triple bond. 1. This gives an intermediate *imidate* that tautomerizes to an *amide*. 2. The amide is then hydrolyzed to the carboxylic acid. - The addition of water to the nitrile resembles the hydration of an alkyne (eq. 3.52). - The oxygen of water behaves as a nucleophile and bonds to the electrophilic carbon of the nitrile. - Amide hydrolysis will be discussed in Section 10.20. Alkyl cyanides are generally made from the corresponding alkyl halide (usually primary) and sodium cyanide by an SN2 displacement.

Carboxylic Acids and Their Derivatives PDF

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