Question

Which of the following represent the correct order of nucleophillic addition for (1) HCHO, (2) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\), (3) \(\mathrm{CH}_{3} \mathrm{CHO}\), (4) \(\mathrm{CH}_{3} \mathrm{COC}_{2} \mathrm{H}_{5}\) (a) \(1>2>3>4\) (b) \(1>3>2>4\) (c) \(4>2>3>1\) (d) \(1>4>3>2\)

Short answer

The correct order of reactivity is option (b) 1 > 3 > 2 > 4.

Step-by-step solution

Evaluate Steric Hindrance

Nucleophilic addition reactions are influenced by steric hindrance. Smaller groups around the electrophilic carbon make a compound more reactive to nucleophilic attack. Among the given compounds, HCHO (formaldehyde) and CH₃CHO (acetaldehyde) have one or no alkyl groups, while the others have two. Hence, minimize steric hindrance consideration: HCHO > CH₃CHO > CH₃COCH₃ ≈ CH₃COC₂H₅ based on steric hindrance.

Consider Electronic Effects

Carbonyl groups are affected by electronic effects. Alkyl groups are electron-donating, making the carbonyl carbon less electrophilic. HCHO, having no alkyl groups, is the most reactive, followed by CH₃CHO and then CH₃COCH₃ and CH₃COC₂H₅, which have two alkyl groups. This order primarily reflects electronic factors and is similar to the steric order.

Determine the Overall Reactivity Order

Combine both steric and electronic effects to determine the overall order. Both effects lead to the same conclusion: HCHO is most reactive due to the minimal steric hindrance and maximal positive charge on the electrophilic carbon. Thus, the order is HCHO > CH₃CHO > CH₃COCH₃ > CH₃COC₂H₅.

Key concepts

Steric Hindrance in Organic Chemistry

Understanding steric hindrance is crucial for predicting the reactivity of carbonyl compounds in nucleophilic addition reactions. Steric hindrance arises when bulky groups around a reactive site impede the approach of a nucleophile. In simpler terms, more crowded molecules are harder for other chemicals to attack.
Formaldehyde (HCHO) is the least hindered because it lacks any alkyl groups, providing the nucleophile easier access to the carbonyl carbon. This makes it very reactive.

• HCHO (Formaldehyde) - No alkyl groups: Minimal hindrance
• CH₃CHO (Acetaldehyde) - One alkyl group: Some hindrance
• CH₃COCH₃ (Acetone) - Two alkyl groups: Significantly more hindrance
• CH₃COC₂H₅ (Ethyl Methyl Ketone) - Two larger groups: Even greater hindrance

Steric hindrance generally decreases the reactivity of molecules in nucleophilic addition, with formaldehyde being the most reactive due to its minimal steric bulk.

Electronic Effects in Carbonyl Chemistry

Electronic effects explain how the electron density around the carbonyl group influences reactivity. Carbonyl compounds have partially positive carbon and partially negative oxygen, creating an electrophilic center. The number and type of groups attached to the carbon affect its electrophilicity.
Alkyl groups are known for their electron-donating ability, which "shields" the carbonyl carbon, making it less electrophilic.

• HCHO - No alkyl groups: Maximum electrophilicity
• CH₃CHO - One alkyl group: Moderate electron donation
• CH₃COCH₃ - Two alkyl groups: High electron donation reduces reactivity
• CH₃COC₂H₅ - Similar electron donation as acetone

Hence, the absence of electron-donating groups enhances the carbon's positive character, increasing nucleophilic attack likelihood, as seen in formaldehyde.

Reactivity Order of Aldehydes and Ketones

The reactivity of aldehydes and ketones is determined by combining steric and electronic factors. These properties collaboratively affect how easily a nucleophile can add to the carbonyl carbon.
Both steric and electronic effects show that:

• HCHO > The most reactive, least crowded, and electronically exposed.
• CH₃CHO > Moderately reactive due to some steric hindrance and electron donation.
• CH₃COCH₃ > Less reactive due to more steric bulk and significant electron donation.
• CH₃COC₂H₅ > Least reactive, having both steric and electronic factors reducing reactivity.

This combines both steric hindrance and electronic effects, leading to a clear understanding that smaller and less electron-donated environments in aldehydes, like in formaldehyde, enhance their overall reactivity in nucleophilic addition processes.