The efficient method for preparing 2,4-dinitrophenylhydrazine from hydrazine and 2,4-dinitrochlorobenzene is through nucleophilic aromatic substitution. Here's a step-by-step process for the reaction:1. Dissolve an appropriate amount of 2,4-dinitrochlorobenzene in a suitable solvent, such as ethanol or acetone.2. Add an excess of hydrazine hydrate N2H4H2O to the solution. The excess hydrazine will ensure that the reaction goes to completion and increases the yield of the product.3. Heat the reaction mixture under reflux for several hours. This will allow the reaction to proceed at a faster rate and increase the yield of the product.4. After the reaction is complete, cool the mixture to room temperature. The product, 2,4-dinitrophenylhydrazine, will precipitate out of the solution.5. Filter the precipitate and wash it with a small amount of cold solvent to remove any unreacted starting materials or byproducts.6. Dry the product to obtain the final 2,4-dinitrophenylhydrazine.The yield of the reaction depends on various factors, such as the purity of the starting materials, the efficiency of the reaction, and the effectiveness of the purification steps. In general, the yield of this reaction can range from 70% to 90%. However, it is important to note that the actual yield may vary depending on the specific conditions and techniques used in the laboratory.