Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The increasing order of the reactivity of the following compounds in nucleophilic addition reaction is: Propanal, Benzaldehyde, Propanone, Butanone [Main Sep. 03, 2020 (II)] (a) Benzaldehyde \(<\) Butanone \(<\) Propanone \(<\) Propanal (b) Butanone \(<\) Propanone \(<\) Benzaldehyde \(<\) Propanal (c) Propanal \(<\) Propanone \(<\) Butanone \(<\) Benzaldehyde (d) Benzaldehyde \(<\) Propanal \(<\) Propanone \(<\) Butanone

Step-by-Step Solution

Answer

Option (a): Benzaldehyde < Butanone < Propanone < Propanal

Step 1: Understand Nucleophilic Addition

Nucleophilic addition reactions involve a nucleophile attacking a compound with a positive or partially positive site. Aldehydes and ketones undergo nucleophilic additions due to the polarization of the carbonyl group, where the carbon atom is electrophilic.

Step 2: Analyze Reactivity Factors

The reactivity in nucleophilic addition is influenced by electronic and steric factors. Aldehydes are generally more reactive than ketones due to less steric hindrance and the greater positive charge on the carbonyl carbon. Additionally, aromatic aldehydes like benzaldehyde are less reactive due to resonance stabilization.

Step 3: Compare Aldehydes and Ketones

Propanal (an aldehyde) is expected to be more reactive than propanone and butanone (both ketones). Benzaldehyde, although an aldehyde, is less reactive due to resonance effects that stabilize the molecule, making the carbonyl carbon less susceptible to nucleophilic attack.

Step 4: Compare Propanone and Butanone

Both propanone and butanone are ketones, but propanone is less hindered than butanone, making propanone slightly more reactive than butanone in terms of nucleophilic addition.

Step 5: Order the Compounds

Based on the above factors, the order is: Benzaldehyde, Butanone, Propanone, Propanal. This order reflects increasing reactivity towards nucleophilic addition, with benzaldehyde being the least reactive and propanal being the most reactive.

Key Concepts

Reactivity of Aldehydes and Ketones

Aldehydes and ketones both feature the carbonyl group, which is crucial for nucleophilic addition reactions. The reactivity of these compounds plays a significant role in how they interact during chemical processes. One major factor in their reactivity is the presence of steric hindrance.

• Aldehydes generally exhibit higher reactivity compared to ketones.
• The reason is fewer surrounding bulky groups in aldehydes than in ketones, leading to less steric hindrance.

Another contributing factor is the electronic environment. Aldehydes have a greater partial positive charge on the carbonyl carbon, making them more prone to nucleophilic attack than ketones. Consequently, this makes aldehydes generally more reactive than ketones.

Stereoelectronic Effects

In chemistry, stereoelectronic effects play a major role in determining the reactivity of molecules, including aldehydes and ketones in nucleophilic addition reactions. Stereoelectronic effects involve interactions between the geometry of a molecule and its electron distribution.

• In aldehydes, the lack of bulky groups around the carbonyl group allows electrons to be more accessible.
• Ketones have two alkyl groups attached, creating more steric hindrance and therefore, reducing the accessibility of electrons for interaction.

Understanding these effects is key to predicting how molecules will behave in reactions. In essence, the arrangement of atoms and electrons can either enhance or hinder the ability of nucleophiles to attack the electrophilic carbon in the carbonyl group.

Polarization of Carbonyl Group

The carbonyl group, composed of a carbon double-bonded to an oxygen, is inherently polarized. This polarization is pivotal in the behavior of aldehydes and ketones during nucleophilic addition reactions. The electronegative oxygen atom attracts electron density, leaving the carbon atom with a partial positive charge.

• This partial positive charge is what makes aldehydes and ketones susceptible to nucleophilic attacks.
• The greater the positive charge on the carbon, the more reactive the compound is.

In aldehydes, the reduced hindrance allows for more polarization compared to ketones, which leads to higher reactivity. Hence, understanding carbonyl group polarization helps us anticipate reactivity patterns in various carbonyl-containing compounds.

Resonance Stabilization

Resonance stabilization is a phenomenon that plays an intricate role in the reactivity of carbonyl compounds, particularly in aromatic aldehydes like benzaldehyde. In resonance stabilization, electrons are delocalized across a molecule, which helps to stabilize the structure.

• This delocalization in aromatic aldehydes makes the carbonyl carbon less electropositive.
• As a result, it becomes less susceptible to attack by nucleophiles compared to non-aromatic aldehydes.

Despite being an aldehyde, benzaldehyde is less reactive than other aldehydes due to the resonance effect. This highlights the importance of electronic effects on reactivity, demonstrating that even within the same functional group, resonance can lead to varying reactivities.