Question

Complete the equation : \[ CH_3CHO + H_2NNH_2 \longrightarrow ? \]

Hint:

Important reactions of aldehydes and ketones: \[ RCHO + H_2NNH_2 \rightarrow RCH=NNH_2 + H_2O \] \[ R_2CO + H_2NNH_2 \rightarrow R_2C=NNH_2 + H_2O \] The products formed are called hydrazones.

Answer 1

Concept: Aldehydes and ketones react with hydrazine \((H_2NNH_2)\) to form hydrazones. This reaction is a nucleophilic addition-elimination reaction in which the nucleophilic nitrogen atom of hydrazine attacks the electrophilic carbonyl carbon of the aldehyde or ketone. After the formation of an unstable intermediate, a molecule of water is eliminated, resulting in the formation of a hydrazone. The general reaction is: \[ RCHO + H_2NNH_2 \longrightarrow RCH=NNH_2 + H_2O \]

Step 1: Identifying the reactants. The aldehyde given is acetaldehyde: \[ CH_3CHO \] The second reactant is hydrazine: \[ H_2NNH_2 \] Hydrazine contains two amino nitrogen atoms having lone pairs of electrons.

Step 2: Nucleophilic attack on the carbonyl carbon. The carbonyl carbon of acetaldehyde possesses a partial positive charge due to the high electronegativity of oxygen. Therefore, the nitrogen atom of hydrazine attacks the carbonyl carbon. An addition product is initially formed.

Step 3: Elimination of water molecule. The intermediate formed is unstable and loses one molecule of water. As a result, a carbon-nitrogen double bond \((C=N)\) is produced. The final product obtained is: \[ CH_3CH=NNH_2 \] which is known as acetaldehyde hydrazone (or ethanal hydrazone).

Step 4: Writing the balanced reaction. \[ CH_3CHO + H_2NNH_2 \longrightarrow CH_3CH=NNH_2 + H_2O \] Checking atoms: Carbon: \[ 2=2 \] Hydrogen: \[ 4+4=8 \] \[ 5+2=7 \] Wait carefully: Reactants: \[ CH_3CHO = C_2H_4O \] \[ H_2NNH_2 = N_2H_4 \] Total: \[ C_2H_8N_2O \] Products: \[ CH_3CH=NNH_2 = C_2H_6N_2 \] \[ H_2O = H_2O \] Total: \[ C_2H_8N_2O \] Hence the equation is balanced.

Step 5: Identification of the product. The product belongs to the class of compounds known as hydrazones. Therefore, \[ \boxed{ CH_3CH=NNH_2 } \] is called \[ \boxed{\text{Acetaldehyde Hydrazone (Ethanal Hydrazone)}} \]

Final Answer: \[ \boxed{ CH_3CHO + H_2NNH_2 \longrightarrow CH_3CH=NNH_2 + H_2O } \]