Stability of conjugate bases

The stability of the conjugate base is a good guide to acidity. More stable anions tend to be weaker bases, and their conjugate acids tend to be stronger acids. Some of the factors that affect the stability of conjugate bases are electronegativity, size, and resonance. 

There are four factors to consider when comparing the stability of conjugate bases ... 

Any factor that stabilizes the conjugate base of an acid increases the... 

A para-substituent may stabilize mesomerically either the conjugate acid of an acid-base pair rather more than it stabilizes benzoic acid, or it may stabilize the conjugate base rather more than it stabilizes the benzoate anion. The first situation is found in car-boxonium ions [13], where the delocalization of the positive charge on to a mesomerically electron-donating substituent stabilizes the cation. A similar resonance in the benzoic acid molecule [14] involves a separation of charge and affects the binding of the proton... 

Conversely, if a. para substituent stabilizes the conjugate base of an acid-base pair rather more than it stabilizes the benzoate ion, more positive substituent constants are required to achieve linearity in Hammett plots. Examples of this are acid dissociations of phenols and anilinium ions, where mesomerically electron-withdrawing substituents (Y = —NO2, —C N) are more effective in enhancing acid strength than they are in benzoic acid, because charge delocalization of the type [15] is not possible in the benzoate anion. 

The a-hydrogens of carboxylic acid derivatives show enhanced acidity, as do those of aldehydes and ketones, and for the same reasons, that the carbonyl group stabilizes the conjugate base. Thus, we can generate enolate anions from carboxylic acid derivatives and use these as nucleophiles in much the same way as we have already seen with enolate anions from aldehydes and ketones. 

This analysis suggests diat structural features which stabilize the conjugate base (often an anion) will therefore increase die acidity of an acid. While there are exceptions to diis general approach (e.g., comparison of die acidities of acids in die second and diird rows of die periodic table), it provides a sound basis for predicting what structural factors can increase or decrease the acidity of organic acids. 

Electron withdrawing groups on the a-carbon help to further stabilize the conjugate base and thus increase the acidity of the corresponding carboxylic acid. 

Acyl chlorides are Bronsted-Lowry acids, and, just like aldehydes, they donate an a-hydrogen. The electron withdrawing chlorine stabilizes the conjugate base more than the lone hydrogen of an aldehyde, making acyl chlorides significantly stronger acids than aldehydes. 

A is correct. The carbonyl group withdraws negative charge, stabilizing the conjugate base of pyruvate,... 

When a problem asks you to compare acidities or explain an acidity, you should consider the structure and stability of the conjugate base. Factors that stabilize the conjugate base will increase the strength of the acid. 

Delocalization of the negative charge stabilizes the conjugate base... 

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