March 13, 2025 | UR Gate

Halogenation Reaction of Acetone with Iodine: An Experimental Study

Definition of Halogenation

The halogenation reaction is an important chemical process in which one or more halogen elements (such as fluorine, chlorine, bromine, or iodine) are added to another chemical compound. This reaction is widely used in industrial and laboratory fields to prepare organic and inorganic compounds. It also plays a key role in the manufacturing of pharmaceuticals, pesticides, plastics, and various other chemical applications.

Purpose of the Experiment:

To determine the rate constant (k) of the reaction.
To find the half-life period (t½).
To determine the order of the reaction.

Theoretical Principle

The halogenation of acetone with iodine occurs in an acidic medium according to the following equation:

CH₃COCH₃ + I₂ → CH₂ICOCH₃ + HI

The acid acts as a catalyst, and the reaction rate is independent of the iodine concentration because it is involved in the fast step. When the concentration of acetone is much larger than that of iodine, the reaction becomes zero-order, in which the rate of the reaction is constant over time and independent of the concentration of the reactant.

dx/dx = k_o

x = kt

Where (x) is the concentration of the product at time (t).

Materials Used:

Acetone
10% sodium acetate
1 M sulfuric acid
0.1 M sodium thiosulfate
0.5 mol/L iodine dissolved in 10% potassium iodide
Starch indicator
Distilled water

Apparatus Used:

25 ml graduated pipette
Burette
250 ml volumetric flask
500 ml reaction vessel (solution bottle)
Stopwatch
250.25 ml graduated cylinder

Procedure:

Step 1: To find the equivalent volume of iodine at the beginning of the reaction (V◦):

Prepare a solution containing 5 ml of iodine solution (0.5 mol/L iodine dissolved in 10% potassium iodide) and 45 ml of distilled water to make a total volume of 50 ml.
Using a pipette, take 25 ml of this solution and titrate with 0.1 M sodium thiosulfate (calibration process). The sodium thiosulfate is placed in the burette, and the volume is V◦. The reaction ends when iodine changes from brown to colorless.

Step 2: To find the equivalent volume of remaining iodine (V◦):

In a 250 ml volumetric flask, place:

25 ml of acetone
10 ml of 1 M sulfuric acid
190 ml of distilled water
Add 25 ml of the undiluted iodine solution, making the total volume 250 ml. Mix vigorously while starting the stopwatch.

After 5 minutes, take 25 ml of the mixture and place it in a conical flask (titration flask) containing 10 ml of 10% sodium acetate solution and starch indicator (sodium acetate is used to stop the reaction).
Titrate the flask using sodium thiosulfate solution, and the reaction ends when iodine changes from brown to blue.
Repeat steps 3 and 4 at times (10, 15, 20, 25, 30, and 35 minutes).

Results and Calculations:

Note:

This is an illustrative example performed in the laboratory.

Step 1: 5 ml iodine solution + 45 ml distilled water = 50 ml

0.1 N sodium thiosulfate

V◦ = 7.5

Step 2: 190 ml distilled water

10 ml sulfuric acid

25 ml acetone

25 ml concentrated iodine

Discussion:

Why is the reaction considered zero-order?

Because iodine is involved in the fast step.
When the concentration of acetone is much larger than that of iodine, the reaction becomes zero-order.

Why does the order of the reaction differ from its molecularity?

The molecularity of the reaction is three, as three molecules are involved, but the order is zero because the reaction rate does not depend on the concentration of the reactants.

Why is the reaction considered generally acidic?

The reaction is considered generally acidic because any acid can be used, as it only speeds up the reaction by providing positive protons that accelerate the process.