Inductive effect, acid strength

The strength of an acid depends on the atom to which the proton is bonded The two mam factors are the strength of the H—X bond and the electronegativity of X Bond strength is more important for atoms m the same group of the periodic table electronegativity is more important for atoms m the same row Electronegative atoms elsewhere m the molecule can increase the acidity by inductive effects... 

Fluorinated Acids. This class of compounds is characterized by the strength of the fluorocarbon acids, eg, CF COOH, approaching that of mineral acids. This property results from the strong inductive effect of fluorine and is markedly less when the fluorocarbon group is moved away from the carbonyl group. Generally, their reactions are similar to organic acids and they find apphcations, particularly trifluoroacetic acid [76-05-1] and its anhydride [407-25-0] as promotors in the preparation of esters and ketones and in nitration reactions. 

Mechanistically the rate-determining step is nucleophilic attack involving the hydroxide ion and the more positive siUcon atom in the Si—H bond. This attack has been related to the Lewis acid strength of the corresponding silane, ie, to the abiUty to act as an acceptor for a given attacking base. Similar inductive and steric effects apply for acid hydrolysis of organosilanes (106). 

Indolmycin, biosynthesis of, 864 Inductive effect. 37, 562 alcohol acidity and. 604 carboxylic acid strength and. 758 electronegativity and, 37 electrophilic aromatic substitution and, 562... 

The effect of introducing electron-withdrawing substituents into simple aliphatic acids is more marked. Thus halogen, with an inductive effect acting in the opposite direction to alkyl, might be expected to increase the strength of an acid so substituted, and this is indeed observed as pKa values show ... 

The inductive effect is also illustrated by the strengths of acids such as HN03 and HN02. The structures for the molecules are as follows ... 

Any substituent that stabihzes a negative charge enhances the dissociation process, i.e. increases the acidity. Electronegative elements can enhance the acid strength through inductive effects. The closer the substituent to the anion, the more profound the effects are. 

The inductive effect of one carboxyl group is expected to enhance the acidity of the other. In Table 18-4 we see that the acid strength of the dicarboxylic acids, as measured by the first acid-dissociation constant, K1, is higher than that of ethanoic acid (Ka = 1.5 X 10-5) and decreases with increasing number of bonds between the two carboxyl groups. The second acid-dissociation constant, K2, is smaller than Ka for ethanoic acid (with the exception of oxalic acid) because it is more difficult to remove a proton under the electrostatic attraction of the nearby carboxylate anion (see Section 18-2C). 

Generally, substituents located on saturated groups attached to nitrogen influence base strengths through their inductive effects in the same way that these substituents influence the strengths of carboxylic acids (see Section 18-2). 

Attempts were early made to explain the relative acidities of ammonium and methyl-substituted ammonium ions (Brown, 1946 Hammett, 1940 Palit, 1947). The order of decreasing acid strength in water at 25°C is NH4+ > Me3NH+ > MeN H3+ % Me2N H2+, an order which obviously cannot be rationalized on the basis of inductive effects. There were two difficulties with the explanations given for this order. First, there is nothing magic about the temperature 25° and comparisons at other temperatures lead to inversions of the order at 25°. Second, the specific effect of the solvent was not taken into account. Since there is... 

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