IMChO 2007, Експеримент — Задача 1. Kinetics of iodination of acetone in acidic medium

Reagents, labware and equipment

Part 1. Standardization of iodine solution

a) Using the pipette, transfer a 5.00 mL aliquot of the iodine aqueous solution into the 100 mL conical flask labeled A. Using the measuring cylinder, add 10 mL of distilled water.

b) Titrate the obtained iodine solution with the standard sodium thiosulfate solution to the pale-yellow coloration. Then add 3 drops of the starch solution and continue titrating until the blue coloration disappears (fig. 1/2007).

c) Repeat the titration as needed. Write down the titration results and the accepted titrant volume, mL.

d) Wash the flask A.

Questions and assignments

1. Calculate the iodine concentration in the given solution.

Part 2. Study of dimethyl ketone iodination kinetics in acidic medium

e) Bring up the dimethyl ketone solution to the mark in the 100 mL volumetric flask with distilled water and mix well. Caution: the solution warms up upon mixing. Hold the stopper tightly, periodically taking it out to release the pressure inside the flask.

f) Pour a portion of dimethyl ketone solution into the beaker. Add the hereunder given volumes of distilled water, 0.1000 M hydrochloric acid and dimethyl ketone solution to the conical flasks labeled A, B, C, and D (stopper the flasks after adding the solutions).

Volume, mL

g) Using the pipette, transfer a 5.00 mL aliquot of the iodine solution into the flask A. Switch on the stopwatch exactly when you start adding the iodine solution. Stopper the flask tightly, mix the flask contents well and incubate for 15 min.

h) Meanwhile, prepare the sodium acetate solution. Weigh ~5.0 g of sodium acetate in the 50 mL beaker. Using the measuring cylinder, add 80 mL of water and stir the mixture to completely dissolve the salt.

i) In 15 min, add 10 mL of the sodium acetate solution to the reaction mixture using the cylinder and stir.

j) Titrate the iodine excess in the reaction mixture with the standard thiosulfate solution. Write down the titration results.

k) Repeat i. f), g), i), and j) for the flasks B, C, and D, adding the solutions in the amounts shown in the above table.

Questions and assignments

2. Calculate the initial rate of iodine consumption $-\frac{d[\mathrm{I_2}]}{dt}$ (in $\mathrm{mol\cdot L^{-1}s^{-1}}$).

3. Using the initial reaction rates and initial concentrations, calculate the reaction orders $x$, $y$, and $z$.

4. Write down the kinetic reaction.

Part 3. Standardization of the alkali

l) Fill the second burette with the alkali solution. Titrate a 10.00 mL aliquot of 0.1000 M HCl solution in the presence of Methyl orange with the alkali solution. The titration endpoint is determined by the color transition of the indicator from pink to yellow.

m) Repeat the titration as needed. Write down the titration results and the accepted titrant volume, mL.

Questions and assignments

5. Calculate the concentration of the alkali solution.

Determination of the dimethyl ketone concentration

n) Transfer a 4.00 mL aliquot of the dimethyl ketone solution into the 250 mL volumetric flask and bring up to the mark with water.

o) Transfer a 10.00 mL aliquot of the obtained dimethyl ketone solution from the 250 mL flask into a 100 mL conical flask. Add 10 mL of 0.2 M hydroxylamine hydrochloride solution, stopper the flask, stir the obtained mixture and store it for 5 min.

p) Titrate the mixture with the alkali solution in the presence of Methyl orange until the indicator color changes from pink to yellow.

q) Repeat the titration as needed. Write down the titration results and the accepted titrant volume, mL.

Questions and assignments

6. Calculate the dimethyl ketone concentration in the initial solution.

7. Knowing molar concentrations of all the reagents, calculate the rate constant $k$ for the flasks A – D using the kinetic equation you have derived earlier.

8. The catalytic iodination of dimethyl ketone you have studied includes the following steps:

The rate of the entire process is determined by that of enol formation (2).

Use the above scheme to derive the theoretical kinetic equation of dimethyl ketone iodination (show dimethyl ketone as A).

9. Write down the condition when the derived equation coincides with the experimental kinetic equation $w=k'[\mathrm{A}][\mathrm{H^+}]$ that you had to obtain doing when the experimental parts.

10. Iodination of dimethyl ketone can be also carried out in an acid-free medium, the initial rate being considerably lower in this case than in an acidic medium. Draw the kinetic curves in the acid-containing and acid-free media.

11. Compare the rate constants of bromination and iodination of dimethyl ketone with $\mathrm{Br_2}$ and $\mathrm{I_2}$ under the conditions of your experiment (encircle one of the following):

a) $k_{\mathrm{Br}}<k_{\mathrm{I}}$; b) $k_{\mathrm{Br}}>k_{\mathrm{I}}$; c) $k_{\mathrm{Br}}\cong k_{\mathrm{I}}$; d) $k_{\mathrm{Br}}\gg k_{\mathrm{I}}$

12. Messinger’s method is another procedure for dimethyl ketone determination. The method involves addition of an excess of iodine to the alkaline aqueous solution of dimethyl ketone with subsequent titrimetric determination of the unreacted iodine after acidification of the mixture. Write down the equation of the reaction occurring upon iodine addition to dimethyl ketone.

13. Is the Messinger’s method selective towards dimethyl ketone? Write down the reaction supporting your answer.

14. Write down the structures of the products of the following reactions.