Question

Explain the inductive affect in acids and bases

Answer 1

Compare the pKa values of acetic acid and its mono-, di-, and tri-chlorinated derivatives:

The presence of the chlorines clearly increases the acidity of the carboxylic acid group, but the argument here does not have to do with resonance delocalization, because no additional resonance contributors can be drawn for the chlorinated molecules. Rather, the explanation for this phenomenon involves something called the inductive effect. A chlorine atom is more electronegative than a hydrogen, and thus is able to ‘induce’, or ‘pull’ electron density towards itself, away from the carboxylate group. In effect, the chlorine atoms are helping to further spread out the electron density of the conjugate base, which as we know has a stabilizing effect. In this context, the chlorine substituent is called an electron-withdrawing group.

The inductive electron-withdrawing effect of the chlorines takes place through covalent bonds, and its influence decreases markedly with distance – thus a chlorine two carbons away from a carboxylic acid group has a decreased effect compared to a chlorine just one carbon away.

EDITED:

the inductive effect is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule, resulting in a permanent dipole in a bond.

When an atom or group of atoms is substituted for another atom or group of atoms in a molecule the distribution of electron density changes. The effect of the new atom or group of atoms on this electron density distribution is termed the "inductive effect" of the atom or group. If the new atom or group of atoms (the substituent) is more electronegative then the one that it replaced then electrons will be withdrawn from other parts of the molecule toward the new substituent. On the other hand if the new substituent is less electronegative than the group that it replaced then the electron density will increase in the rest of the molecule. Changes in substituents can have a profound effect on acidities and basicities of molecules and the relative strengths of structurally related acids and bases can be predicted on the basis of inductive effects of substituents. A more electronegative substituent X will increase the acidity of an oxy (or hydroxy) acid XO-H by greater withdrawal of electron density from the oxygen-hydrogen bond. This both weakens the O-H bond and increases the positive charge on hydrogen. A similar effect is present for the conjugate base XO ; here the presence of a more 1- electronegative substituent stabilizes the anion by withdrawing charge from the oxygen and effectively delocalizing the charge over more of the molecule. T

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