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Acidity of Carboxylic Acids, Effect of Substituents on Acidity

Carbaryl acid is the name given to organic compounds that contain carboxylic acid groups. Hydrogen and metals combined in an alkaline solution.

The Acidity of carboxylic acid

Carbaryl acid is the name given to organic compounds that contain carboxylic acid groups. Hydrogen and metals combined in an alkaline solution produce carboxylate ions. Similarly, acidic carboxylic acids are affected by these reactions. When carboxylic acids reacted with weak bases, such as carbonates and bicarbonates, carbon dioxide was formed, which was a stronger acidity than that of simple phenols. As a rule, a carboxylic acid donates its electron to a hydrogen atom. Carbonates are acidic because of the -COOH group of hydrogen ions.


The carboxylate ion and the hydronium ion are formed during the dissociation of carboxylic acids in water. In a resonance effect, the negative charge of the carboxylate ion is effectively delocalized, stabilizing it.

Organic compounds with carboxylic acids are stronger than their mineral counterparts, but that's not the case in the reverse. Carboxylic acids have a higher acidity than alcohols and phenols. Two equivalent resonance structures of the carboxylate ion transfer the negative charge to two further electronegative oxygen atoms as a carboxylate ion.

Conversely, the phenoxide ion has a smaller area where the electrons are delocalized and the carbon atoms are less electronegative. The stability of the phenoxide ion is thus greater. Because acidic carboxylic acids are acidic, whereas alkaline phenols are alkaline. Metals and alkalis are capable of forming carboxylate ions only by resonating with carboxylic acids. The carboxyl group makes the group more acidic when electron withdrawal occurs, while electron donation makes the group less acidic.

Moreover, the acidity of carboxylic acids depends on the substituents attached to the carboxyl group, such as alkyl or aryl. Negative charges can be effectively delocalized by electron-withdrawing groups through resonance and inductive effects. By adding the electron-withdrawing group to carboxylic acids, the conjugate base becomes more stable and, as a result, more acidic. However, electron-donating groups destabilize the conjugated bases those carboxylic acids form, resulting in reduced acidity.

An example of this is:

CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC-CH2COOH

The resonance effect causes the phenyl or vinyl groups on carboxylic acids to increase the acidity even though the inductive effect causes the acidity to decrease.



Effects of substituents on acidity

Substituent effects

In the context of discrete atoms or molecules, a substituent is known as an atom or molecule fragment. Changes in the properties or reactions of a molecule due to variations in its substituents are called substituent effects.

Depending on their strength, weak acids are ranked on the pKa scale. An acid that has a lower number is considered stronger. The following are three compounds whose pKa values we'll be examining:

Acids with a lower pKa value have a higher pKa value. The pKa of phenol is approximately 10.00, while the pKa of ethanol is approximately 16. While ethanol is hardly acidic, phenol is. Acetic acid and ethanol have different basicity constants: for acetic acid, the constant is 5, while that for ethanol is 17. Oxygen is not bonded to the proton of any compound. There is no oxygen atom attached to the proton in any compound.

Conjugate bases have a negative charge due to the presence of oxygen atoms in both species. Thus, neither species exhibits a periodic trend. When acetic acid has a negative charge on its second oxygen, a resonance contribution occurs. There is an equal amount of energy in both resonance forms. Acetate cannot have a single negative charge since its collective rather than a discrete charge makes it incapable of having one. Charge delocalization is a result of this. By contrast, the negative charge attached to an oxygen atom makes up ether ions, which have no other purpose.

An atom with concentrated charges is more unstable than an electrostatically charged surface. It does not matter what size the atoms are in terms of how they spread out (i.e., delocalize), as this is controlled primarily by resonance rather than their size. It is possible to increase the reactivity of organic compounds by up to 12 pKa units with resonance delocalization. Ethanol and acetic acid, for example, each have a reactivity increase of around 12 pKa. Because of the resonance delocalization in acetate, it is more stable than ethoxide.

Carboxylic acids' acidity is influenced by resonance

Is there a reason why molecules with carbonyl groups adjacent to hydroxyl groups have a significantly higher acidity than molecules with hydroxyl groups? These equations can be used to establish how acid-base equilibrium works and what pKa is.

Thermal equilibrium exhibits the highest degree of stability, and the energy difference between the two sides determines the balance value. Acid and base reach equilibrium when the less acid wins (i.e., the less acid). Water can be used to determine PKa values. Acid's conjugate base (A:(-)) will be stabilized, which will result in a pH rise and a shift of the equilibrium toward the right. As is shown in the below diagram, anions carboxylate are stabilized by resonance at a higher level than neutrals. The carboxylate anion is made up of C-O bonds with different lengths and weights.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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