Question

Draw a structural formula for the product of each acid-catalyzed reaction. (a) Phenylacetaldehyde \(+\) hydrazine \(\longrightarrow\) (b) Cyclopentanone + semicarbazide \(\longrightarrow\) (c) Acetophenone \(+2,4\)-dinitrophenylhydrazine \(\longrightarrow\) (d) Benzaldehyde \(+\) hydroxylamine \(\longrightarrow\)

Short answer

Question: Draw the structural formulas for the products of acid-catalyzed reactions involving the following compounds: a) Phenylacetaldehyde + hydrazine b) Cyclopentanone + semicarbazide c) Acetophenone + 2,4-dinitrophenylhydrazine d) Benzaldehyde + hydroxylamine Answer: a) Phenylacetaldehyde hydrazone: C6H5CH2CH=NNH2 b) Cyclopentanone semicarbazone: C5H9N3O c) 2,4-Dinitrophenylhydrazone of acetophenone: C8H8N4O4 d) Benzaldehyde oxime: C6H5CH=NOH

Step-by-step solution

Understand the Reaction Mechanism

In all four reactions, we deal with the reaction of a carbonyl compound (either an aldehyde or a ketone) with a compound carrying an N-H group. The acid-catalyzed mechanism proceeds through the formation of an imine or hydrazone, which is a condensation reaction. The resulting product will have a double bond between the nitrogen and the carbonyl carbon, and the carbonyl oxygen is replaced by the nitrogen-containing group from the second reactant.

Reaction (a) Phenylacetaldehyde + hydrazine

For part (a) of the given exercise, we need to draw the product of the reaction between phenylacetaldehyde and hydrazine (N2H4). During the reaction, the carbonyl oxygen of phenylacetaldehyde will be replaced by one of the nitrogen atoms in hydrazine. Product (a): Phenylacetaldehyde hydrazone \[\ce{C6H5CH2CHO + N2H4 -> C6H5CH2CH=NNH2}\]

Reaction (b) Cyclopentanone + semicarbazide

For part (b), the reactants are cyclopentanone and semicarbazide. The carbonyl oxygen of cyclopentanone will be replaced by the nitrogen atom of semicarbazide. Product (b): Cyclopentanone semicarbazone \[\ce{C5H8O + H2NNHCONH2 -> C5H9N3O}\]

Reaction (c) Acetophenone + 2,4-dinitrophenylhydrazine

In part (c), acetophenone and 2,4-dinitrophenylhydrazine are the reactants. Upon reaction, the carbonyl oxygen of acetophenone will be replaced by the nitrogen atom of 2,4-dinitrophenylhydrazine. Product (c): 2,4-Dinitrophenylhydrazone of acetophenone \[\ce{CH3COC6H5 + H2NNHC6H3(NO2)_2 -> C8H8N4O4}\]

Reaction (d) Benzaldehyde + hydroxylamine

Finally, in part (d), benzaldehyde reacts with hydroxylamine. The carbonyl oxygen of benzaldehyde is replaced by the nitrogen atom of hydroxylamine. Product (d): Benzaldehyde oxime \[\ce{C6H5CHO + H2NOH -> C6H5CH=NOH}\] Now that we have found the products for all four reactions, we have completed the task of drawing the structural formulas for the products of each acid-catalyzed reaction.

Key concepts

carbonyl compounds

Carbonyl compounds are a group of organic chemicals that include a carbon atom double-bonded to an oxygen atom, commonly represented as C=O. This carbon-oxygen double bond is called a carbonyl group. Carbonyl compounds primarily include aldehydes and ketones.

• Aldehydes have the carbonyl group bonded to at least one hydrogen atom and are often represented as RCHO, where R can be any hydrocarbon chain.
• Ketones, on the other hand, have the carbonyl group bonded to two carbon atoms, represented as RCOR'.

In reactions, the carbonyl carbon is a primary site of reactivity due to the polarity of the C=O bond, where the carbon has a partial positive charge and the oxygen a partial negative charge. This makes carbonyl compounds particularly susceptible to nucleophilic attacks, a key aspect of many reaction mechanisms, including acid-catalyzed condensation reactions.

imine formation

Imines are nitrogen-containing compounds formed by the reaction of carbonyl compounds with ammonia or primary amines. This process typically involves a nucleophilic addition followed by the elimination of water, facilitated by acid catalysis.
Acid catalysis intensifies the electrophilic nature of the carbonyl group by protonating the oxygen, making it more susceptible to attack by the amine.

• The primary amine attacks the carbonyl carbon, forming a tetrahedral intermediate.
• This intermediate is unstable and undergoes dehydration (loss of water), establishing a C=N double bond characteristic of imines.

The general reaction can be represented as: \[\ce{R1CHO + R2NH2 ->[H^+] R1C=NR2 + H2O} \] Where R1 is the carbon side chain of the aldehyde and R2 is the side chain of the amine. Imines are valuable intermediates in organic synthesis, often used in the formation of more complex structures.

hydrazone formation

Hydrazones are formed by the reaction of carbonyl compounds with hydrazines, a family of compounds with the general formula \\[\ce{R2NNH2}\].
In an acid-catalyzed environment, the process shares similarity with imine formation. The hydrazine attacks the carbonyl carbon, resulting in the production of a hydrazone after dehydration.

• The mechanism starts with the protonation of the carbonyl oxygen, enhancing the electrophilicity of the carbon atom.
• A hydrazine molecule then attacks the carbon, forming a hydrazone.

The reaction achieves equilibrium quickly, forming a stable double bond between the nitrogen and the carbon originally holding the carbonyl group. Hydrazones are important for various applications, including as precursors in the synthesis of pharmaceuticals and other nitrogen-rich compounds. The example reactions in the original exercise illustrate this formation process, yielding specific hydrazone derivatives.

reaction mechanism

A reaction mechanism in chemistry provides a step-by-step path that dictates how reactants transform into products during a chemical reaction. In the context of the discussed acid-catalyzed condensation reactions, understanding the mechanism is crucial for predicting the structural outcomes.
The generalized mechanism for acid-catalyzed formation of imines and hydrazones is as follows:

• Protonation: The process begins with the protonation of the carbonyl oxygen, increasing the electrophilic nature of the carbonyl carbon.
• Nucleophilic Attack: An amine or hydrazine forms a bond with the carbonyl carbon, creating a tetrahedral intermediate.
• Dehydration: The intermediate loses a molecule of water, forming the double bond between nitrogen and carbon.

Each step is pivotal, aiding in understanding why the carbonyl oxygen is replaced and how the nitrogen atom assumes that position. Intricate solvent and temperature conditions can influence the reaction’s pace and success, something detailed in specific laboratory settings for these reactions.