Which one of the following is not formed when acetone reacts with 2-pentanone in the presence of dilute NaOH followed by heating?
The Correct Option is B
Approach Solution - 1
To solve this problem, we need to consider the reaction mechanism between acetone and 2-pentanone in the presence of dilute NaOH followed by heating. This is an example of an aldol condensation, which involves the following steps:
- Deprotonation: Under basic conditions, acetone, being an alpha-beta unsaturated ketone, undergoes deprotonation at the alpha carbon, forming an enolate ion.
- Nucleophilic Attack: The enolate ion acts as a nucleophile and attacks the carbonyl carbon of 2-pentanone, leading to the formation of a β-hydroxy ketone (aldol addition product).
- Dehydration: Upon heating, the β-hydroxy ketone undergoes dehydration (loss of water) to form an α,β-unsaturated ketone through the elimination of the hydroxyl group and adjacent hydrogen.
The structures of the possible products are analyzed to determine which cannot be formed from this reaction.
| Product | Formation Possibility |
|---|---|
| Possible | |
| Not Possible | |
| Possible | |
| Possible |
Explanation: The product cannot be formed because it is not an α,β-unsaturated ketone but instead resembles an isomeric structure that would require a different reaction mechanism, likely involving structural rearrangement that is not accounted for by a simple aldol condensation.
Approach Solution -2
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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.