14: Substituent Effects

(9/94)(11,12/96)(11,12/04,01/05) Neuman Chapter 14
The subsituents I, Br, Cl, F, and NO2, increase the acidity of the CO2H group over that of
the unsubstituted compound (S = H). In contrast, the substituents CH3 or CO2 - decrease the
acidity of the CO2H group compared to the unsubstituted compound.
Acidity Constants. Ka values of acids directly reflect the acidity of acids. The larger the Ka value,
the stronger the acid and vice-versa. pKa values also describe acidity. Since Ka = 10 -pKa , pKa values
decrease as Ka values increase.
Origin of the Substituent Effect. While substituent effects can be transmitted by
resonance or by inductive effects, S affects CO2H acidity in these carboxylic acids only by
inductive effects. Resonance effects are not possible because the S group and the CO2H group
are not conjugated (Figure 14.05) [see below]. The CH2 group intervening between S and
CO2H has a tetrahedral carbon, with no π orbitals, that prevents conjugation between S and
CO2H.
Figure 14.05 Figure 14.06
When the Substituent is F. Inductive effects often result from σ bond polarization that is
the result of electronegativity differences between bonded atoms as we illustrate for C-F
bonds (Figure 14.06) [see above]. F is much more electronegative than H, so C-F bonds are
highly polarized (Chapter 3) as we show for fluoroacetic acid (fluoroethanoic acid). The
inductive effect of F on the acidity of the CO2H group is a result of the positively polarized
CH2 carbon to which the CO2H group is attached.
How C-F Polarity Affects Acidity. Fluoroacetic acid is an acid because it donates a
proton to water or other bases (Figure 14.07).
Figures 14.07
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