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General Chemistry Principles, Patterns, and Applications, 2011

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phosphoric acid H3PO4 2.16 7.21 12.32<br />

phosphorous acid H3PO3 1.3 6.70<br />

succinic acid HO2CCH2CH2CO2H 4.21 5.64<br />

sulfuric acid H2SO4 −2.0 1.99<br />

sulfurous acid* “H2SO3” 1.85 7.21<br />

Polyprotic Bases Formula p K b1 p K b2<br />

ethylenediamine H2N(CH2)2NH2 4.08 7.14<br />

piperazine HN(CH2CH2)2NH 4.27 8.67<br />

propylenediamine H2N(CH2)3NH2 3.45 5.12<br />

*H2CO3 <strong>and</strong> H2SO3 are at best minor components of aqueous solutions of CO2(g) <strong>and</strong> SO2(g),<br />

respectively, but such solutions are commonly referred to as containing carbonic acid <strong>and</strong><br />

sulfurous acid, respectively.<br />

Inductive effects are also observed in organic molecules that contain electronegative substituents. The<br />

magnitude of the electron-withdrawing effect depends on both the nature <strong>and</strong> the number of halogen<br />

substituents, as shown by the pKa values for several acetic acid derivatives:<br />

pKaCH 3CO2H 4.76 < CH 2ClCO2H 2.87<br />

< CHCl2CO2H1.35 < CCl3CO2H 0.66 < CF3CO2H 0.52<br />

As you might expect, fluorine, which is more electronegative than chlorine, causes a larger effect than<br />

chlorine, <strong>and</strong> the effect of three halogens is greater than the effect of two or one. Notice from these data<br />

that inductive effects can be quite large. For instance, replacing the –CH3 group of acetic acid by a –<br />

CF3 group results in about a 10,000-fold increase in acidity!<br />

E X A M P L E 5<br />

Arrange the compounds of each series in order of increasing acid or base strength.<br />

a. sulfuric acid [H 2SO 4, or (HO) 2SO 2], fluorosulfonic acid (FSO 3H, or FSO 2OH), <strong>and</strong><br />

sulfurous acid [H 2SO 3, or (HO) 2SO]<br />

b. ammonia (NH 3), trifluoramine (NF 3), <strong>and</strong> hydroxylamine (NH 2OH)<br />

The structures are shown here.<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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