The Strength of a Carboxylic Acid

The strength of a carboxylic acid is determined by the degree of ionisation of the molecule. 

Consider ethanoic acid (acetic acid, CH3COOH) is the equilibrium 

The larger the value of K, the more the equilibrium favours the product. The p in p. a means the negative log, so the lower the pK. value the stronger the acid. 

You can see from Table 7.2.2 that the usual carboxylic acids get weaker as the carbon chain gets longer. Benzoic acid (an aromatic acid) is slightly stronger than ethanoic acid. 

If you score less than 80%, then work through the text and re-test yourself at the end by using this same test. If you still get a low score then re-work the topic at a later date. 

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This partial electron withdrawal from one atom can affect not only a neighboring atom, but that atom s neighbor as well. Thus the strength of a carboxylic acid will be affected by the bonding environment of the carbon atom to which it is connected. This propagation of partial electron withdrawal through several adjacent atoms is known as the inductive effect) and is extremely important in organic chemistry. A very... 

Show what happens when ethanoic acid ionises, and how the structure of a carboxylic acid relates to the strength of the acid. 

Amines are considerably more basic than alcohols, ethers, or water. When an amine is dissolved in water, an equilibrium is established in which water acts as an acid and transfers a proton to the amine. Just as the acid strength of a carboxylic acid can be measured by defining an acidity constant Ka (Section 2.8), the base strength of an amine can be measured by defining an analogous basicity constant K. The larger the value of ifb (and the smaller the value of piTj,), the more favorable the proton-transfer equilibrium and the stronger the base. 

Table 9.4 shows typical values of bond dissociation enthalpies of some hydrogen bonds. The data in the table have been obtained from calculations on isolated species. These enthalpy values are therefore only approximate when applied to hydrogen bonds between molecules in a solid state lattice enthalpy values for these interactions cannot be measured directly. An example of how the strengths of hydrogen bonds can be obtained experimentally comes from the dissociation of a carboxylic acid dimer in the vapour state (equation 9.25). 

When a solution of a carboxylic acid and an alcohol in pyridine is treated with tosyl chloride, an ester is formed rapidly in excellent yield. This procedure is useful especially in the esterification of tertiary alcohols. The combination of a carboxylic acid and tosyl chloride serves as a convenient method of in situ preparation of symmetrical acid anhydrides for further formation of esters and amides (eq 19). The novelty of this protocol is that the acid can be recycled through the anhydride stage in the presence of the alcohol, thereby resulting in complete conversion to the ester (eq 20). Reactivity is determined hy the strength of the acid strong acids facilitate the esterifications. ... 

The binding potentials of molecular tweezers of this class were enhanced by the incorporation of a carboxylic acid functionality within the cavity of the molecular tweezer. Such species were used to bind nucleotide bases with extremely high affinities. Studies as to the origin of the strength and selectivity of the binding were conducted. "" ... 

X 10 to 1 X 10 pK 3.7 to 5). The explanation of the greater acid strength of the —OH group in the carboxylic acids than in the alcohols is given by the theory of resonance it is similar to that already given (Section 13-3) of the acid strength of the phenols. The dissociation of a carboxylic acid is represented by the equation... 

We have discussed the influence of substituents on acid strengths of simple carboxylic acids as though the full electrostatic effect of the substituent were exerted solely on the A// of ionization. However, careful thermodynamic analysis of acidities in aqueous solution show that entropy effects (Section 4-4B) are very important. This may seem surprising because entropy effects ought to be small for relative acid strengths, which can be assessed by the constants for simple equilibria such as Equation 18-3, in which (1) there are the same number of molecules on each side of the equation, and (2) the constraints on the species involved hardly seem different from one side of the equation to the other ... 

Controlled addition of a suitable proton donor or electrophile (reductions) or nucleophile (oxidations) is often useful in determining a reaction mechanism. The strength of a proton donor may vary from perchloric acid through acetic acid and a phenol to an alcohol C acids, such as malonic ester, or N acids, such as urea, may also be used. Used as bases may be pyridine, carboxylate ions, alkoxides, or salts of malonic ester. Sometimes it is of interest to determine whether it is the basic or the nucleophilic properties of the compound that are important. The use of two bases with approximately the same pK values but widely differing in nucleophilicity, such as pyridine and a 2,6-dialkylpyridine, might answer the question. 

Many factors affect solubility and must be considered. For example, pH, ionic strength, and temperature can significantly affect solubility. For example, the aqueous solubility of a carboxylic acid can be orders of magnitude higher at a pH above the pKa than below the pKa. This is due simply to changes in the polarity of the molecule. Conversely, the solubility of weak bases such as amines is higher when the pH is below the pKa of the base. For aqueous sample preparations, addition of a water-miscible solvent such as acetonitrile or alcohol can be used to enhance solubility. For example, the solubility of acetaminophen in water is approximately 11 mg/mL, but the solubility is doubled by adding 2% ethanol (19). 

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