Acidity of acetic acid

Field Effects. These were discussed on page 16. As an example of the influence of field effects on acidity, we may compare the acidity of acetic acid and nitroacetic acid ... 

Figure 3.8 Two resonance structures that can be written for acetic acid and two that can be written for acetate ion. According to a resonance explanation of the greater acidity of acetic acid, the equivalent resonance structures for the acetate ion provide it greater resonance stabilization and reduce the positive free-energy change for the ionization.

The pATa of ethanol is 16, and that of acetic acid is 4.8. The increased acidity of acetic acid relative to ethanol can be rationalized in terms of delocalization of charge in the acetate anion, whereas in ethoxide the charge is localized on oxygen. Even more delocalization is possible in the methanesulfonate anion, and this is reflected in the increased acidity of methanesulfonic acid (pAla — 1-2). 

The acidity can be increased by adding electron-withdrawing groups to the R (electron donors have the opposite effect). For example, the acidity of acetic acid increases as chlorine atoms replace hydrogen atoms. Acetic acid has = 1.76 X 10 chloroacetic acid has = 1.40 x 10 , dichloroacetic acid has K = 3.32 X 10 2 and trichloroacetic acid has K = 2.00 x 10 . ... 

Figure 6-12 compares calculated acidities for carboxylic acids (relative to the acidity of acetic acid) to experimental aqueous-phase pKa s. The overall correlation is good, and suggests a high level of cancellation of solvent effects in these closely-related systems. 

K. Hiraoka, R. Y. Yamdagni, and P. Kebarle, "Effects of Halogen Substituents on the Intrinsic Acidity of Acetic Acids Determined by Measurements of Gas-Phase Ion Equilibria, J. Amer. Chem. See. 95, 6834 (1973). 

Bassam Z. Shakhashiri, "Effect I of Acetate Ion on the Acidity of Acetic Acid The Common Ion Effect," Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 3 (The University of Wisconsin Press, Madison, 1989) pp. 155-157. 

The other factor that is contributing to the dramatic increase in the acidity of acetic acid is resonance stabilization. Neither ethanol nor its conjugate base, which is called ethoxide ion, is stabilized by resonance. The following resonance structures can be written for acetic acid and its conjugate base, acetate anion ... 

Rablen chose fcrf-butanol as the reference system, arguing that C-C bonds are less polar than C-H bonds and thereby eliminates polarization effects that may be inherent in smaller alcohol reference molecules, like methanol or ethanol. The proton exchange shown in Reaction 3.3 defines the relative acidities of acetic acid versus t rt-butanol. Using the CBS-Q method, acetic acid is 27.9 kcal mol" more acidic than t rt-butanol. 

Headley, A.D., Starnes, S.D., Wilson, L.Y. and Famini, G.R. (1994). Analysis of Solute/Solvent Interactions for the Acidity of Acetic Acids by Tlieoretical Descriptors. J.Org.Chem., 59, 8040-8046. 

The existence of more than one resonance structure stabilizes the acetate ion and contributes to the greater acidity of acetic acid. 

A high NH-acidity is observed for 4-acetylamino-l-R-l,2,4-triazolium salts (258) (72ZC250). Values are similar to the acidity of acetic acid, which is some 10 orders of magnitude greater than that for acetamide. 

What would be a suitable indicator for the titration of ammonia with hydrochloric acid Of acetic acid with sodium hydroxide ... 

The +7 character of the group is most simply shown by its effect on the acidity of acetic acid. The apparent Ka value of ethynylacetic acid at an ionic strength /== 0.1 (NaCl) is 3.32 (water, 25 This is a so-called mixed constant, involving activity of hydrogen... 

Table 1 Acidities of acetic acids and proton transfer from the acids to acetate ion...

The acid-strengthening effect of other electron-attracting groups (other than the carbonyl group) can be shown by comparing the acidities of acetic acid and chloroacetic acid ... 

The inductive effect is the polarization of electron density transmitted through covalent bonds by a nearby atom of higher electronegativity. We see the operation of the inductive effect when we compare the acidities of acetic acid (p 4.76) and trifluoroacetic acid (pA 0.23). Fluorine is more electronegative than carbon and polarizes the electrons of the C—F bond, creating a partial positive charge on the carbon of the —CF3 group. The... 

Substitution at the a-carbon of an atom or a group of atoms of higher electronegativity than carbon increases the acidity of carboxylic acids, often by several orders of magnitude (Section 2.5C). Compare, for example, the acidities of acetic acid (pii 4.76) and chloroacetic acid (p7 2.86). A single chlorine substituent on the a-carbon increases acid strength by nearly 100 Both dichloroacetic acid and trichloroacetic acid are stronger acids than phosphoric acid (p7i 2.1) ... 

Thorp MA, Kruger J, Oliver S, Nilssen EL, Preseott CA (1998) The antibacterial acidity of acetic acid and Burow s solutimi as topical otological preparations. J Laryngol Otol 112 925-928... 

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