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Computer-Interfaced Experiments - Conductivity Measurement

Kinetics
Hydrolysis of Tertiary Butyl Chloride - First Order Reaction


Objectives: Determination of Rate Constants and Activation Parameters

Peter Keusch



Datalogging using the Program "Conductivity"
and the Analog-Digital-Converter CASSY-E - LEYBOLD DIDACTIC



German version



Chemicals:
2-chloro-2-methylpropane ·99 %
(m.w. = 92.57 g /mol, d = 0.84 g / mL)

Apparatus and glass wares:
magnetic stirrer hotplate
2 magnetic stirring bars
stirring bar remover
crystallizing dish 190 · 90 ·(for water bath)
200 mL beaker
contact thermometer
thermometer 0 - 50 °C ·(resolution: 0.1 °C)
conductivity measuring cell
volumetric pipette 50 mL
micro pipette
pipette bulb


Hazards and safety precautions:

Tert-butylchloride is harmful if inhaled. Skin, respiratory and eye irritant.

Safety glasses, gloves and good ventilation required.


Theoretical background:

The tertiary butyl halides are already hydrolyzed by water.


equation

Hence the hydrolysis of t-butyl chloride is monitored by following the change in conductance of the reaction mixture with time. By determining the conductivity as a function of time, the rate constant can be found. If the reaction is carried out at different temperatures, the activation energy can also be found.


Kinetic equations (Download PDF file)


Experimental procedure:

The conductivity measuring cell is connected to the conductivity box plugged at input A of CASSY INTERFACE.

100 mL of distilled water are pipetted into a beaker placed in a water bath  (Fig. 1).  A thermometer (resolution: 0.1 °C) is immersed in the water.

Using a hotplate stirrer and a contact thermometer the water of the water bath is warmed up to the desired temperature. The hydrolysis experiment is carried out at three different temperatures in the range from 18 °C to 28 °C.

A reaction temperature below the room temperature is obtained and maintained by careful addition of ice or cold water to the water bath.
Experiment set-up
Fig. 1: Experiment set-up

The water in the beaker is allowed to equilibrate in the constant-temperature bath.

When thermal equilibrium has been reached, the temperature is read to the nearest 0.1°C. The program  'Conductivity'  is started and the subprogramm  'Kinetics'  is activated.

The cell constant (e.g. 1.02 cm-1) and the reaction temperature are entered under the menu item  <F3>'Calibrate conductivity meter'. Under <F2>'Select measuring range '  a measuring range of  0 .. 2 mS/cm  is selected. The recording time is set to  500 s.

88 mL of 2-chloro-2-methylpropane (0.8 mmol) are pipetted into the water while vigorously stirring. Immediately the sensing software is started by pressing the fuction key  <F1>.

The individual measurements are stored in the main menu.



Fig. 2: Hydrolysis of des tert. butylchloride - overlay of the conductivity curves
18.2 °C  (1)    22. 9 °C  (2)    28.4 °C  (3)
( The temperature data are rounded in the overlay.)


Data analyis:


Evaluation of the data measured using the cassy-program Conductivity/Kinetics - determination of the rate constants and the activation energy

Analog to the procedure described under  Alkaline Hydrolysis of Esters  the data analysis can be conducted using the available cassy-program.

According to equation (6)  First Order Reaction (Download PDF file)  a plot is created allowing the determination of rate constants.

The conductance measured at the end of the reaction  k¥ , corresponds to  [ A ]0  (initial concentration of t-butyl chloride) and  k¥ - k  corresponds to  [ A ]   (concentration of
t-butyl chloride at time t).

Equation (6)  First Order Reaction (Download PDF file)  becomes ln( k¥ - k) / k¥)) = - kt.

The subprogram   'Kinetics'   provides a formula editor, which allows the integration of the measured conductivity values into a formula. With the aid of the instruction  < F4>'Select formula'  a submenu opens which permits the entering of a formula.  c  (concentration)  is entered for   'Physical symbol' . After setting the   'No. decimal places '  the beginning of the formula  c(n,t,æ)=  is completed by entering  

0.8 · (1 - æ / æ¥)

where  æ  is the conductance at time t and  æ¥   the conductance at completion of the reaction.  æ ¥   is found in the data table by activating the menu point  <F5>'Output measured values'. The program is switched to  <F7>'Select representation'® 'Representation'   and  'Graph order: 1'   is selected. Now the program is switched to the main menu by activating   <F6>'Evaluate in graph'.



Fig. 3: Determination of the rate constant    ( 22.9 °C)
according to equation (6)   Kinetic equations (Download PDF file)



Fig. 4: Determination of the activation energy


Data analysis using Excel - determination of the rate constants and the activation parameters:

First k is plotted versus t  (Fig. 3).  Afterwards, according to equation (6) Kinetic equations (Download PDF file)  the conductivity values are converted  (Tab. 1)  Plotting

-ln(0.008 · (1 - k / k ¥))

versus t the rate constant can be found  (Fig. 3).


spread sheet
Tab. 1: Measured values k(t), conversion according to y = -ln(0.008 · (1 - k / k¥))


conductivity curves
Fig. 5: Conductivity curves
18.2 °C  (1)    22. 9 °C  (2)    28.4 °C  (3)


rate constants
Fig. 6: First order kinetics plot - determination of the rate constant k
y = -ln(0.008 · (1 - k / k¥))


18.2 [ °C ] 22.9 [ °C ] 28.4 [ °C ]
k [ s -1 ] 0.0107 0.0192 0.0372
Tab. 2: Rate constants k

If the reaction temperatures and the corresponding rate constants are entered into the table of the Excel file   Activation parameter (Download), all activation parameters  (Table 2)  will be calculated and the plots according to the ARRHENIUS and EYRING relation  (Fig. 5)  will be generated.


activation parameters
Tab. 3: Calculation of the activation parameters


Arrhenius and Eyring
Fig. 7: ARRHENIUS (1) and EYRING plot (2)



Solvent DH ‡
[ kcal · mol -1 ] 		DH ‡
[ kJ · mol -1 ] 		DS ‡
[ J / (mol · K) ] 		Page
H2O - - 14.4 220
H2O / Dioxane   90/10 - - 8 250
EtOH / H2O 27   /73 - - 2.7 220
EtOH / H2O   27/63 - - -3.2 220
EtOH / H2O   50/50 - - -2.9 220
EtOH   80 % 22.6 94.9 -6.2 147
EtOH   50 % 22.3 93.7 0.6 147
Tab. 4: Literature values - C.H. Bamford und C.F.H Tipper. Chemical Kinetics Vol 10, 1972


Note that SN1 reactions in which the nucleophile is also the solvent are commonly called solvolysis reactions. Solvent as the nucleophile makes kinetic order indeterminate (pseudo-first-order because [solvent] is ~ constant).


References:
Computer-Interfaced Experiments   Kinetics: Hydrolysis of tertiary Butyl Halides - First Order Reaction
Microscale Projection Experiments   Hydrolysis of tertiary Butyl Halides
  Demonstration Experiment ob Video   Hydrolysis of tertiary Butyl Halides - First Order Reaction
Rod Beavon   SN1 Nucleophilic Substitution unimolecular - Animation


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