1- Anions are solvated by hydrogen – bonding,
2- Cations are solvated by nucleophilic sites on water molecule (oxygen). And in this case of t-butyl carbonium ion the nucleophiles form strong covalent bond to carbon and converting the intermediate to a substitution product.
The reaction mechanism is a sequential account of each transition state and intermediate in a total reaction, the over all rate of reaction is determined by the transition state of highest energy in the sequence, so the rate determining step is the rate determining step for both the Sn1 and E1 for t – butyl chloride.
(“Note 6”: the water soluble organic solvent acetone is used to keep a reasonable concentration of t-butyl chloride in solution)
The balance equation for t-butyl chloride solvolysis in water-acetone solvent is:
The effect of concentration on the solvolysis of t-butyl chloride in 70 %water – 30 %acetone solvent.
As the reaction proceeds the solution becomes increasingly acidic until all of the t –butyl chloride has reacted and all HCl that can form has formed. So we will monitor the reaction by allowing HCl formed to neutralize a predetermined amount of NaOH. An indicator dye (bromo-phenol blue) will change color when the NaOH has been neutralized, and clocking of the reaction should begin at the instant.
So according to kinetic measurements:
Rate of reaction = K [t – butyl chloride]
Where K is the specific rate constant in S -1 and [t – butyl chloride] is the concentration of t-butyl chloride in M.
Our kinetic measurement will depend on the determination of the amount of HCl produced by the reaction, so by monitoring the color change of the acid – base indicator, we will determine the time required for 10% of t-butyl chloride to hydrolyze by having 10 % as much NaOH present as T-butyl chloride.
Rate = - d [Rcl]
dt
; Where Rcl =
-dt [Rcl] = K [Rcl]
dt
Rearranging,
d [Rcl] = -K dt
[Rcl]
And integrating for t=0 to t=t will give;
=
Ln [Rcl] t - Ln [Rcl] 0 = - Kt
- 2.303 Log [Rcl] 0 = - Kt
[Rcl] t
2.303 Log [Rcl] 0 = Kt
[Rcl] t
Where [Rcl] 0: is the molar concentration at time t = 0
[Rcl] t: is the molar concentration at time t = t
Two methods to calculate K
- since the equation
Kt = 2.303 Log [Rcl] 0
[Rcl] t
Is an equation of a straight line (y=mx+b) with slope k. and intercept =0, a plot of 2.303 log [Rcl] 0 / [Rcl] t versus t should yield a straight line with slope k.
- if the solvolysis reaction run to 10% completion
Then,
[Rcl] = 0.90 [Rcl] 0
Kt = 2.303 Log [Rcl] 0 = 2.303 log (1.11)
0.90 [Rcl] 0
And therefore,
K = 0.104
T
So by finding the value of K and compensate it in the rate of reaction equation “Rate = K[Rcl]” where the concentration of Rcl is known we can calculate the value of the rate of reaction and we will see it’s effect on the solvolysis of t – butyl chloride in 70% water – 30 % acetone solution.
The effect of temperature on the solvolysis of t -butyl chloride in 70%water – 30%acetone solvent.
In nearly every instance an increase in temperature causes an increase in the rate of reaction, “because the total fraction of all of the t – butyl chloride 1molecules having energies equal to or greater than activation energy (Ea)
Corresponds to the shaded portion of the area under the curve increases by increasing the temperature” and by comparing the area for two different temperature, we see that the total fraction of t- butyl chloride molecules with sufficient kinetic energy to undergo reaction increases with increasing temperature and consequently, so does the reaction rate.
“Note7: changing the concentration affects the rate of reaction changing the temperature affects the rate constant as well as the rate.”
By finding the values of reaction rate constant K for different concentration of t-butyl chloride and different reaction temperature, we will find the effect of temperature on the solvolysis of t-butyl chloride in water acetone solvent.
Quantitatively, K (s-1) is related to Ea and T by the equation
K1 = Ae-Ea/RT1 ……1
Ea is the activation energy, in joule / mole. (Jmol-1)
A is a proportionality constant, in s-1
R is the gas constant = 8.314 Jmol-1K-1
e is the base of the natural logarithms.
T is temperature in Kelvin.
This relation ship is known as Arrhenius equation
We measure Ea by taking the natural logarithm of eq.1
Ln K = ln A - Ea
RT
Thus, a plot of ln k versus 1/T gives a straight line whose slope is equal to –Ea/R and whose intercept with coordinate is ln A
“Note8: Ea is the activation energy, a constant characteristic of the reaction”
We can calculate the rate constant at some specific temperature if Ea and K at some other temperature are known.
For any temp. T1 (known), Ea (known), K1 (known)
K1 = A e –Ea/RT1
For any other T2 (known); (K2 unknown)
K2 = A e –Ea/RT2
By dividing K1 over K2
K1 = A e –Ea/RT1
K2 A e –Ea/RT2
Taking natural logarithm of both sides, we get
Ln K1 = Ea (1/T2 - 1/T1).
K2 R
Or in common logarithms (base 10 logarithms) gives:
Log K1 = Ea (1/T2 - 1/T1)
K2 2.303 R
And by finding the value of K2 we will be able to find the rate of reaction at T2 and we will find the effect of temperature on the rate of solvolysis of t – butyl chloride in 70 % water – 30 % acetone solution.
By finding the values of reaction rate constant K for different concentration of t-butyl chloride and different reaction temperature, we will find the effect of concentration and temperature on the solvolysis of t-butyl chloride in water acetone solvent.
Procedure:
Part A: the effect of concentration on the rate of solvolysis of t – butyl chloride in 70%water – 30%acetone solvent.
a-
Experimental procedure: to measure the time necessary for 10 % solvolysis of t – butyl chloride (0.1 M concentration) in 70 % water – 30% acetone solvent at room temperature.
A, a, I:-
- Prepare 500 ml of 0.1 M t– butyl chloride in acetone only and put it in an Erlenmeyer flask and label it #1.
- Prepare 100 ml of 0.1 M NaOH solutions (in water) and put it in an Erlenmeyer and label it #2.
- Using a burette take 30 ml of the solution in flask #1 and put it in another Erlenmeyer and label it #3.
- By a graduated pipette take 3 ml of sodium hydroxide 0.1 M in an Erlenmeyer flask and label it #4.
- Using a graduated cylinder measure 67 ml of distilled water added to an Erlenmeyer flask #4.
- Add two drops of Bromo-phenol blue indicator to flask #4.
A, a, II:-
- Add quickly the solution in Erlenmeyer flask #4 to solution in flask #3 and start the stop watch to count for time in seconds.
- Swirl the mixture and after one or two seconds immediately pour the combined solutions back into Erlenmeyer flask #4 to minimize the errors in the results.
- The color of the mixed solutions is blue, so continue swirling the solution in Erlenmeyer flask #4 till the instant color of the solution start changing to yellow, then we stop the stopwatch and record the time.
- Repeat the procedure at least three times and calculate the average.
- Tabulate the results in record A.
b-
Experimental procedure: to measure the time necessary for 10 % solvolysis of t – butyl chloride (0.2 M concentration) in 70 % water – 30% acetone solvent at room temperature.
A, b, I:-
- Prepare 500 ml of 0.2 M t– butyl chloride in acetone only and put it in an Erlenmeyer flask and label it #1.
- Prepare 100 ml of 0.1 M NaOH solutions (in water) and put it in an Erlenmeyer flask and label it #2.
- Using a burette take 30 ml of the solution in Erlenmeyer flask #1 and put it in another Erlenmeyer flask and label it #3.
- By a graduated pipette take 3 ml of sodium hydroxide 0.1 M in an Erlenmeyer flask and label it #4.
- Using a graduated cylinder measure 67 ml of distilled water added to an Erlenmeyer flask #4.
- Add two drops of bromo-phenol blue indicator to Erlenmeyer flask #4.
A, b, II:-
- Add quickly the solution in an Erlenmeyer flask #4 to solution in flask #3 and start the stop watch to count for time in seconds.
- Swirl the mixture and after one or two seconds immediately pour the combined solutions back into an Erlenmeyer flask #4 to minimize the errors in the results.
- The color of the mixed solutions is blue, so continue swirling the solution in Erlenmeyer flask #4 till the instant color of the solution start changing to yellow, then we stop the stopwatch and record the time.
- Repeat the procedure at least three times and calculate the average.
- Tabulate the results in record A.
Part B: the effect of temperature on the rate of solvolysis of t – butyl chloride in 70%water – 30%acetone solvent.
a-
Experimental procedure: to measure the time necessary for 10 % solvolysis of t – butyl chloride (0.1 M concentration) in 70 % water – 30% acetone solvent at zero Celsius degree.
B, a, I:-
- Prepare 500 ml of 0.1 M t– butyl chloride in acetone only and put it in an Erlenmeyer flask and label it #1.
- Prepare 100 ml of 0.1 M NaOH solutions (in water) and put it in an Erlenmeyer flask and label it #2.
- Using a burette take 30 ml of the solution in Erlenmeyer flask #1and put it in an Erlenmeyer flask and label it #3.
- By a graduated pipette take 3 ml of sodium hydroxide 0.1 M in an Erlenmeyer flask and label it #4.
- Using a graduated cylinder measure 67 ml of distilled water added to Erlenmeyer flask #4.
- Add two drops of bromo-phenol blue indicator to Erlenmeyer flask #4.
B, a, II:-
- Suspend the Erlenmeyer flasks in a water bath full with ice and water, allowing the temperature of the Erlenmeyer flasks and their contents to equilibrate for ten minutes.
- Adding quickly the solution in Erlenmeyer flask #4 to solution in Erlenmeyer flask #3 and start the stop watch to count for time in seconds.
- Swirl the mixture and after one or two seconds immediately pour the combined solutions back into Erlenmeyer flask #4 to minimize the errors in the results.
- The color of the solution after that will become blue, so continue swirling the solution in Erlenmeyer flask #4 till the instant color of the solution start changing to yellow we stop the stop watch and record the time
- Repeat the procedure at least three times and calculate the average.
- Tabulate the results in record B.
b-
Experimental procedure: to measure the time necessary for 10 % solvolysis of t – butyl chloride (0.1 M concentration) in 70 % water – 30% acetone solvent at a temperature greater than room temperature by ten degrees.
B, b, I:-
- Prepare 500 ml of 0.1 M t– butyl chloride in acetone only and put it in an Erlenmeyer flask and label it #1.
- Prepare 100 ml of 0.1 M NaOH solutions (in water) and put it in an Erlenmeyer flask and label it #2.
- Using a burette take 30 ml of the solution in Erlenmeyer flask #1 and put it in an Erlenmeyer flask and label it #3.
- By a graduated pipette put 3 ml of sodium hydroxide 0.1 M in an Erlenmeyer flask and label it #4.
- Using a graduated cylinder measure 67 ml of distilled water added to Erlenmeyer flask #4.
- Add two drops of bromo-phenol blue indicator to flask #4.
B, b, II:-
- Suspend the flasks #3 and #4 in a water bath full with ice and water, allowing the temperature of the flasks and their contents to equilibrate for ten minutes.(to reach the temperature of the water bath)
- Adding quickly the solution in flask #4 to solution in flask #3 and start the stop watch to count for time in seconds.
- Swirl the mixture and after one or two seconds immediately pour the combined solutions back into flask #4 to minimize the errors in the results.
- The color of the mixed solutions is blue, so continue swirling the solution in flask #4 till the instant color of the solution start changing to yellow we stop the stopwatch and record the time
- Repeat the procedure at least three times and calculate the average.
- Tabulate the results in record B.
Record A
Record B
References;
-
E. Brady, James. E. Humiston, Gerard., General Chemistry Principles and Structure, second edition, SI version, john Willy and sons, Inc.
-
Brewester, Vaderwerf and McEwen. “Unitized Experiments in Organic Chemistry”, 3rd Ed.
-
Streitwieser, Andrew. H. Heathcock, Clayton. Introduction to Organic Chemistry.
-
H. Reusch, William. An Introduction to Organic Chemistry.
-
J. Laidler, Keith. Chemical kinetics. 2nd ed.
- Search engines that where used:
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Goldwhite, Harold. R. Spielman, John. College Chemistry, 1984.
