What are the products of the following reactions? (A trace amount of acid is present in each case.)
a. cyclopentanone + ethylamine
b. cyclopentanone + diethylamine

Verified step by step guidance

1. Identify the type of reaction occurring in each case. Cyclopentanone is a ketone, and the presence of an amine (ethylamine or diethylamine) along with a trace amount of acid suggests that these are nucleophilic addition-elimination reactions, potentially forming imines or enamines.
2. For part (a), cyclopentanone reacts with ethylamine (a primary amine). The reaction mechanism involves the nucleophilic attack of the amine's nitrogen on the carbonyl carbon of cyclopentanone, forming a tetrahedral intermediate.
3. In the presence of a trace amount of acid, the intermediate undergoes protonation and dehydration (loss of water), leading to the formation of an imine. The general structure of the imine will be: \( \text{C}_5\text{H}_8=\text{N}-\text{CH}_2\text{CH}_3 \).
4. For part (b), cyclopentanone reacts with diethylamine (a secondary amine). Secondary amines cannot form imines because they lack a hydrogen atom on the nitrogen to facilitate dehydration. Instead, the reaction forms an enamine. The mechanism involves nucleophilic attack, followed by proton transfer and elimination of water, resulting in a compound with a double bond adjacent to the nitrogen atom.
5. The enamine product for part (b) will have the general structure: \( \text{C}_5\text{H}_7=\text{CH}-\text{N}(-\text{CH}_2\text{CH}_3)_2 \). This structure arises from the alpha-hydrogen of cyclopentanone being removed during the enamine formation process.

Key Concepts

Nucleophilic Addition

Nucleophilic addition is a fundamental reaction in organic chemistry where a nucleophile attacks an electrophilic carbon atom, typically found in carbonyl compounds like ketones and aldehydes. In the case of cyclopentanone reacting with amines, the nitrogen atom of the amine acts as the nucleophile, attacking the carbonyl carbon, leading to the formation of an intermediate that can further react to form an imine or amine product.

Acid-Base Catalysis

In the presence of a trace amount of acid, acid-base catalysis can facilitate the reaction between cyclopentanone and amines. The acid can protonate the carbonyl oxygen, increasing the electrophilicity of the carbonyl carbon, making it more susceptible to nucleophilic attack by the amine. This process can enhance the rate of reaction and influence the products formed.

Imine Formation

Imine formation is a specific reaction that occurs when a primary amine reacts with a carbonyl compound, resulting in the creation of an imine. In the reactions involving cyclopentanone and ethylamine or diethylamine, the nucleophilic attack leads to the formation of a tetrahedral intermediate, which can lose water to yield an imine. The nature of the amine can affect the stability and reactivity of the resulting imine.