Question:

Although phenoxide ion has more number of resonating structures than carboxylate ion,carboxylic acid is a stronger acid than phenol.Why?

Updated On: Sep 28, 2023
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Solution and Explanation

Resonance structures of phenoxide ion are:


It can be observed from the resonance structures of phenoxide ion that in II,III and IV,less electronegative carbon atoms carry a negative charge.Therefore,these three structures contribute negligibly towards the resonance stability of the phenoxide ion.Hence,these structures can be eliminated.Only structures I and V carry a negative charge on the more electronegative oxygen atom.
 

Resonance structures of carboxylate ion are:


In the case of carboxylate ion,resonating structures I”² and II”² contain a charge carried by a more electronegative oxygen atom.


Further,in resonating structures I”² and II”²,the negative charge is delocalized over two oxygen atoms.But in resonating structures I and V of the phexoxide ion,the negative charge is localized on the same oxygen atom.Therefore, the resonating structures of carboxylate ion contribute more towards its stability than those of phenoxide ion.As a result,carboxylate ion is more resonance-stabilized than phenoxide ion. Hence,carboxylic acid is a stronger acid than phenol.

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Notes on Aldehydes, Ketones and Carboxylic Acids

Concepts Used:

Aldehydes, Ketones, and Carboxylic Acids - Physical Properties

The following are the Physical Properties of Aldehydes, Ketones, and Carboxylic Acids:

The physical properties of aldehydes and ketones are stated below:

Physical State:

While Ethanal is a volatile liquid, Methanal is a gas at room temperature. Similarly, other aldehydes and ketones are either gas or liquid at room temperature.

Boiling point: The boiling point of methanal is -19o C and for ethanal it is +21o C. From this we can say that the boiling point of ethanal is close to room temperature. Generally the boiling point of aldehydes and ketones increases with increase in molecular weight. Boiling point depends upon the strength of the intermolecular forces.

  • Vander Waals Dispersion Force: The boiling point of aldehydes and ketones depend on the carbon atoms. When the molecules lengthen and the number of electrons increases, the bond increases and the boiling point increases.
  • Van Der Waals dipole-dipole attraction: Due to the presence of the double bond between carbon and oxygen, aldehydes and ketones are polar to each other. This leads to an attraction between the permanent dipoles and other molecules near it. This is what makes the boiling point of these compounds high.

Solubility:

Aldehydes and ketones are soluble in water. The solubility decreases when the length of the alkyl chain increases. Aldehydes and ketones like methanol, ethanal and propanone are miscible in water of all quantities. These compounds cannot form hydrogen bonds on their own but are able to do so with water due to the dipole-dipole attraction.

Smell:

All lower aldehydes have a strong and unpleasant smell. Other aldehydes and ketones have a pleasant smell. When the molecule size of the compounds increases, the smell becomes less pungent. Naturally occurring aldehydes and ketones are used regularly in flavouring agents and perfumes like vanilla flavoring.

The physical properties of carboxylic acids are stated below:

Physical state:

Carboxylic acids, at room temperature, are colourless liquids. These acids have nine carbon atoms or less. Higher acids are more waxy and are solid.

Boiling Point:

Compared to aldehydes, ketones and other compounds of similar molecular masses, carboxylic acids have high boiling points. The reason behind such high boiling points is for the acid molecules ability to substantially associate with each other through intermolecular hydrogen bonding. The hydrogen bonds do not break up completely even in the vapour state as a result. Most carboxylic acids are present as dimers in the vapour state.

Smell:

At room temperature, carboxylic acids have shown to possess unpleasant odours.

Solubility:

The more simple aliphatic components of carboxylic acids have four carbon atoms and are miscible or soluble in water. This is due to its ability to form hydrogen bonds with water. When the number of atoms in the carboxylic acids is increased, the solubility nature decreases. For higher members of the same group, the immiscibility nature can be attributed to its hydrophobic communication of the hydrocarbon part of the acid. However, they are able to become soluble in organic solvents that are less polar like alcohol, benzene, ether etc.