Hydrolysis Of Tertiary Butyl Chloride

Introduction: The hydrolysis of tertiary butyl chloride (TBC) is a second-order reaction with the following stoichiometry:



When a large excess of water is taken, there will be the following consequences: (i) the rate of forward reaction becomes much greater than the rate of backward reaction, and hence the reaction becomes practically irreversible:



(ii) the concentration of water does not undergo much c principal reactant, is taken in the form of a 0.1N solution in acetone. (We can not take it in aqueous solution, since TBC then gets hydrolyzed). When water is added to this solution HCL is liberated as per the above reaction. An indicator, such as bromophenol blue, will therefore give an acidic indication in the reaction mixture.

Suppose that instead of starting the reaction in the above manner, we place some small amount of NaOH also is the reaction mixture at the beginning of reaction. Then, obviously, the reaction mixture will be alkaline as long as enough HCL is not liberated, and will later turn acidic when the quantity of HCL liberated is sufficient to neutralize the NaOH.
The reaction time is noted when the color of reaction mixture turns from alkaline indication to acidic indication. This time would correspond to a fractional conversion of:

XA = mole TBC converted/mole TBC initially taken
= mole HCL liberated/mole TBC initially taken
= mole Na0H used up/mole TBC initially taken
= mole Na0H initially taken/mole TBC initially taken

Aim of the Experiment: The aim of the experiment is to determine the pseudo-first-order rate constant and the second – order rate constant for the hydrolysis of TBC at ambient temperature.

Material required:
Solution of TBC in acetone (0.1N) : : 25ml
Na0H, aqueous (0.05N) . . : : 50ml
Bromophenol blue indicator
Conical flasks (25ml) . . : : 2 No.
Measuring pipette (5 ml) . . : : 3 No.

Stopwatch distilled water, standard oxalic acid.

Procedure:
1. Pipette precisely 3.0ml of 0.1N TBC into a clean and dry 25ml conical flask (I)
2. Pipette precisely 0.6 ml of 0.05N Na0H into another clean 25ml conical flask (II). To this, add 6.4ml of distilled water by means of a pipette. (Use different pipettes for the different solutions).
3. Add a drop Bromophenol blue into flask (II). This will give a deep blue indication. Observe the color by placing the flask on a glazed tile.
4. Add contents of flask (II) to contents of flask (I), starting a stopwatch at the same time. Swirl the solution for a few seconds, and then pour it back into flask (II) continue to swirl gently, and watch for change of colour.
5. When the solution changes color from deep blue to greenish yellow, stop the stopwatch, and note the time.
6. Repeat the experiment with different quantities of sodium hydroxide as follows:

Run No…. ………….... 1 2 3 4 5
ml of TBC Solution 3 3 3 3 3
ml of Na0H solution 0.6 0.8 1.0 1.2 1.4
ml of distilled water 6.4 6.2 6.0 5.8 5.6



7. Determine the exact normality of sodium hydroxide before starting the experiments. The values of XA given in the succeeding table correspond to NNa0H = 0.05. The values of fractional conversion XA will depend on the exact normality of Sodium Hydroxide used in the experiment.
8. Note the room temperature.
Readings and Calculations
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Run No. Fractional Time, t ln 1/(1-XA)
Conversion, XA sec
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
1 0.100
2 0.133
3 0.167
4 0.200
5 0.233
- - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Plot ln (1/1-XA) versus reaction time, t. Report the slope as the pseudo-first-order rate constant, .
sec-1(at oc)

Concentration of water, CB = CB 0
= 7ml/10ml of reaction mixture
= 7gm/10ml = 38.9 gm-mole/litre
Second-order rate constant, k = /CB 0
litre/(gm-mole)(sec)

Selected questions:
1. What initial concentration of TBC is used in runs 1 to 5?
2. What is the rate of reaction in run No.1 when the colour change is noted?
3. What is the actual difference between initial and final water concentrations in run No.5? Is this difference really negligible. How would you evaluate k if this difference were not to be neglected?
4. What is the half-life period of TBC under conditions of your experiment?
5. Suppose that a 0.1N solution of water is prepared in acetone. 5ml of this solution is added to 5ml of 0.1N of the TBC solution. What reaction time would be needed to obtain a 10 percent conversion?
Reference:
1. Bauman. E. “Experiments in Unit Processes and Reaction Engineering.