Question

Aldehydes are generally more reactive than equivalent ketones to nucleophiles. This is likely due to differences in: (A) steric hindrance. (B) leaving group ability. (C) resonance stabilization. (D) electron-withdrawing character.

Short answer

Steric hindrance makes aldehydes more reactive than ketones to nucleophiles.

Step-by-step solution

Understand the Concept

Both aldehydes and ketones undergo nucleophilic addition reactions, but their reactivity differs due to structural differences. Consider what might affect the reactivity of these carbonyl compounds.

Examine Steric Hindrance

Aldehydes have only one alkyl group attached to the carbonyl carbon, while ketones have two. This means aldehydes have less steric hindrance, making them more accessible to nucleophiles.

Evaluate Leaving Group Ability

Neither aldehydes nor ketones contain good leaving groups directly attached to the carbonyl carbon. Leaving group ability is not significant in explaining the reactivity difference.

Consider Resonance Stabilization

Both aldehydes and ketones have a similar carbonyl resonance structure. Resonance stabilization does not explain the difference in their reactivity.

Assess Electron-Withdrawing Character

A ketone's two alkyl groups provide electron-donating inductive effects, stabilizing the molecule more than an aldehyde with only one alkyl group. This means aldehydes have a slightly more electrophilic carbonyl carbon due to less electron donation.

Conclusion

The primary reason aldehydes are more reactive is the reduced steric hindrance, making it easier for nucleophiles to attack the carbonyl carbon.

Key concepts

nucleophilic addition

Nucleophilic addition is a fundamental reaction mechanism in organic chemistry. It involves a nucleophile, which is an electron-rich species, attacking an electrophile, often a carbonyl carbon in aldehydes and ketones.
The carbonyl carbon is electrophilic due to the difference in electronegativity between the carbon and oxygen. Oxygen is more electronegative, attracting electron density away from the carbon, making the carbon susceptible to nucleophilic attack.
In aldehydes and ketones, the carbonyl carbon is sp2 hybridized, leading to a planar structure. When a nucleophile attacks, it converts the carbon into an sp3 hybridized structure, breaking the π bond of the carbonyl and forming a new σ bond with the nucleophile.
Here are the steps involved in nucleophilic addition reactions with aldehydes and ketones:

• The nucleophile attacks the electrophilic carbonyl carbon.
• The π bond between the carbon and oxygen breaks, transferring electron density to the oxygen atom.
• A tetrahedral intermediate is formed.
• If a proton source is available, the oxygen can form a hydroxyl group, completing the reaction.

This mechanism is crucial in organic synthesis, forming various important compounds like alcohols.

steric hindrance

Steric hindrance refers to the physical presence of groups around a reactive site, which can block or hinder the accessibility of nucleophiles.
In the context of aldehydes and ketones, steric hindrance plays a significant role in their reactivity. Aldehydes have one alkyl group attached to the carbonyl carbon. Ketones, however, have two alkyl groups.
Here's how steric hindrance affects reactivity:

• For aldehydes, the single alkyl group allows the carbonyl carbon to be more exposed. This makes it easier for nucleophiles to approach and attack the carbonyl carbon.
• In ketones, the two alkyl groups create more steric hindrance around the carbonyl carbon. This makes it less accessible, reducing the rate of nucleophilic attack.

Consequently, the reduced steric hindrance in aldehydes makes them more reactive towards nucleophiles compared to ketones. This is a critical factor in many chemical reactions where the reactivity of these compounds is involved.

aldehyde vs ketone reactivity

The reactivity of aldehydes and ketones towards nucleophiles can be compared based on several factors, but primarily steric hindrance and electronic effects.
While both aldehydes and ketones have a carbonyl group, their surrounding environment differs, influencing their reactivity.
Here’s a detailed comparison:

• Aldehydes have one hydrogen atom and one alkyl group attached to the carbonyl carbon. This reduces steric hindrance, allowing easier nucleophilic attack.
• Ketones possess two alkyl groups attached to the carbonyl carbon. These groups create steric hindrance, making the carbonyl carbon less accessible to nucleophiles.
• Electronically, the alkyl groups in ketones donate electron density through inductive effects, stabilizing the carbonyl carbon more than in aldehydes. This electron donation decreases the electrophilicity of the carbonyl carbon, making nucleophilic attack less favorable.

Because of these reasons, aldehydes are generally more reactive towards nucleophiles compared to ketones. The combination of reduced steric hindrance and higher electrophilicity of the carbonyl carbon makes aldehydes more susceptible to nucleophilic addition reactions.