Introduction Acetylacetone (Hacac) is the simplest of the beta-diketones. Hacac commonly exists in t — Analytical Chemistry Chemistry Question

Introduction

Acetylacetone (Hacac) is the simplest of the beta-diketones.

Hacac commonly exists in two tautomeric forms, ketone and enol, which can rapidly interconvert in solution as well as in the gas phase.

[VISUAL]

In aqueous solutions, Hacac is in equilibrium with hydrogen ions (H⁺) and with acetylacetonate (enolate) ions (acac^-). The dissociation constant Ka is 1.51 ⋅ 10-9.

Acetylacetone can form stable complexes with many transition metal ions. In most of the structurally characterized complexes, Hacac that is singly deprotonated acts as a bidentate ligand, and binds to the metal via both two oxygen donor atoms to form six-membered chelate ring.

[VISUAL]

Acetylacetone is known to form neutral complexes with about 60 metals and is among the most versatile and most used chelating ligand in coordination chemistry.

Copper (II) acetylacetonate can be synthesized in a ligand exchange reaction:

2 acac^- + [Cu(H₂O)₆] 2+ [Cu(acac)₂]. n H₂O + (6 – n) H₂O

Chemicals and Reagents
* Acetylacetone
* 0.2 mol dm^-3 CuSO₄ aqueous solution
* 25 % NH₃ solution
* Diluted HCl solution
* Diluted NH₃ solution
* Universal indicator papers
* Chemicals for determination of Cu content.

Apparatuses and Glassware
* Glass beakers, 100 cm₃, 50 cm₃
* Pipette, 5 cm₃
* Graduated cylinder, 50 cm₃
* Rubber bulb
* Watch glass
* Glass rod
* Magnetic stirrer
* Sintered glass funnel
* Vacuum pump
* Wash bottle
* Analytical balance with readability of 0.001 g.

Experimental procedure

Step 1. Preparation of copper (II) acetylacetonate
1. Use 25 cm₃ of 0.2 mol dm^-3 CuSO₄ aqueous solution to calculate the quantities of chemicals required for synthesis of the complex.
2. Prepare the ammonium acetylacetonate (NH4acac) solution: Add slowly 25% NH₃ aqueous solution (ρ = 0.90 g cm^-3) into liquid acetylacetone in a 50 cm₃ glass beaker with a molar ratio Hacac : NH₃ of 1.0 : 0.9. A white solid appears for seconds and then dissolves completely to obtain a clear solution.
3. Prepare the copper(II) acetylacetonate: Add the freshly prepared NH4acac solution (with a 50% excess) into a 100 cm₃ glass beaker in which 25 cm₃ of 0.2 mol dm^-3 CuSO₄ aqueous solution and a stirring bar are placed. Start stirring the solution on a magnetic stirrer and adjust the pH of the mixture to 3-4 by the addition of dilute solutions of HCl and NH₃. The pale blue solid of copper (II) acetylacetonate is precipitated. The reaction mixture is stirred for additional 30 min to complete the precipitation. The formed solids are collected on sintered glass funnel by a vacuum filtration, washed 3 times with small portions of distilled water and then transported to a watch glass and dried at 120oC for 30 min. Weigh the product on the analytical balance with the readability of 0.001 g.

Step 2. Determination of the Cu content in the complex
1. Students propose an appropriate procedure to determine the Cu content in the product. (Hints: The Cu²⁺ concentrations can be determined by iodometric titration, complexometric titration with EDTA…).