Question

Consider the following aldehydes with respect to their reactivity toward addition reaction with a given nucleoppile (I) O=Cc1ccccc1 (II) O=Cc1ccc([N+](=O)[O-])cc1 (III) (IV) N#Cc1cc([N+](=O)[O-])ccc1C=O The order of reactivity is (A) III > I > II > IV (B) IV > II > I > III (C) II > IV > I > III (D) \(\mathrm{IV}>\mathrm{III}>\mathrm{II}>\mathrm{I}\)

Short answer

The order of reactivity of these aldehydes towards an addition reaction with a given nucleophile is (C) II > IV > I > III, considering both the effects of electron-donating and electron-withdrawing groups and the steric hindrance of the functional groups.

Step-by-step solution

Identify the functional groups in each aldehyde

: Aldehyde (I): Benzaldehyde - phenyl group attached to the carbonyl group Aldehyde (II): 4-Nitrobenzaldehyde - nitro group attached in the para position of the phenyl group Aldehyde (III): Formaldehyde - just the aldehyde group without any additional substituents Aldehyde (IV): 4-Nitro-α-picoline N-oxide – nitro group in the para position of the phenyl ring and nitrile group

Effects of electron-donating and electron-withdrawing groups on reactivity

: Aldehydes with electron-withdrawing groups, such as the nitro group in aldehydes (II) and (IV), increase the electrophilicity of the carbonyl carbon by decreasing the electron density on the carbonyl group. It makes the carbon more electrophilic and hence more reactive towards nucleophiles. On the other hand, electron-donating groups, such as the phenyl group in aldehyde (I), decrease the electrophilicity of the carbonyl carbon by increasing the electron density. Therefore, the carbon becomes less electrophilic and less reactive toward nucleophiles. Aldehyde (III) does not have any additional functional group, so it has a more electrophilic carbonyl carbon than aldehyde (I), but less electrophilic than aldehydes (II) and (IV).

Steric hindrance and reactivity

: Steric hindrance refers to the size or bulkiness of the functional groups near the carbonyl group. Larger, bulkier groups can hinder the approach of the nucleophile to the carbonyl carbon and make the reaction slower. In this case, aldehyde (III) has the least steric hindrance because it has no additional functional groups, while aldehyde (IV) has the most steric hindrance due to the presence of the bulky nitrile group.

Determine the order of reactivity

: Taking into account both the effects of the electron-donating and electron-withdrawing groups and the steric hindrance, we can determine the order of reactivity of these aldehydes as follows: 1. Aldehyde (II) - Electron-withdrawing nitro group makes it the most reactive 2. Aldehyde (IV) - Electron-withdrawing nitro group, but with more steric hindrance than (II) 3. Aldehyde (I) - Electron-donating phenyl group makes it less reactive than (II) and (IV) 4. Aldehyde (III) - Least reactive due to the absence of any additional functional groups Therefore, the order of reactivity is II > IV > I > III, which corresponds to answer option (C).

Key concepts

Aldehyde Reactivity

Aldehydes are organic compounds featuring a carbonyl group (a carbon double-bonded to an oxygen) bonded to a hydrogen atom. This structure makes them quite reactive. Their reactivity in nucleophilic addition reactions largely depends on the presence and type of substituents attached to the carbonyl group.
In nucleophilic addition reactions, a nucleophile attacks the positively charged carbon atom of the carbonyl group, forming intermediates that eventually lead to product formations. Aldehydes generally exhibit greater reactivity than ketones due to having fewer steric hindrance issues and less electron-donating groups around the carbonyl.

Electron-withdrawing Groups

Electron-withdrawing groups (EWGs) play a significant role in enhancing the reactivity of aldehydes. These groups, such as nitro (-NO2) groups, reduce the electron density around the carbonyl carbon. By doing so, they increase the carbon atom's partial positive charge, making it a better target for nucleophiles.

• EWGs withdraw electron density via resonance or inductive effects.
• They increase electrophilicity by making the carbon more deficit in electrons.

In the given exercise, aldehydes (II) and (IV) contain nitro groups. These substituents profoundly affect their reactivity in nucleophilic addition reactions by making the carbonyl carbon more electrophilic and thus more reactive towards nucleophiles compared to others without such groups.

Steric Hindrance

Steric hindrance refers to the obstruction of reactions due to the spatial arrangement of atoms. In aldehyde reactions, it's critical because bulky groups around the carbonyl carbon can impede the approach of nucleophiles.

• Aldehyde (III) experiences the least steric hindrance due to its simple structure.
• The presence of multiple bulky groups in aldehyde (IV) increases steric hindrance, affecting reactivity.

Hence, steric hindrance can decrease the overall reaction rate by physically blocking nucleophiles from reaching the carbonyl carbon effectively.

Electrophilicity of Carbonyl Carbon

The carbonyl carbon's electrophilicity is pivotal in determining the reactivity of aldehydes in nucleophilic addition reactions. Electrophilicity refers to the tendency of an atom to attract and react with nucleophiles.

• Greater partial positive charge on the carbon makes it more electrophilic.
• Less steric hindrance and more electron-withdrawing groups enhance electrophilicity.

Overall, the more electrophilic the carbonyl carbon, the more likely it will engage vigorously in reactions. In the given aldehyde examples, the electrophilicity is modulated by both steric and electronic factors, guiding the order of reactivity observed in the exercise solution.