Hydrolysis of tert-butyl Chloride in different solvents

Hydrolysis of tert-butyl Chloride in different solvents
Practical conducted on 5 March, 2013
Reported by Pham Vu Hung on 10 March, 2013
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Introduction:
This practical is meant to measure the rate of reaction of the hydrolysis of tertiary-butyl chloride –a colorless, liquid organic compound at room temperature that is sparingly soluble in water - in water/acetone and water/isopropanol mixtures. Since there are many influencing factors for the rate of reaction, all are kept constant but the intended variable: the solvents. If the reactant is more stabilized by interaction with solvent molecules than is the transition state, the activation barrier for the reaction will be increased, and the rate will decrease. Conversely, if the transition state is more stabilized by solvation than is the reactant, the activation barrier for the reaction will be decreased, and the rate will increase.
In this experiment, the kinetics of a unimolecular substitution reaction — the SN1 reaction between water and tert-butyl chloride (2-chloro-2-methylpropane) in various solvents will be measured in order to explore the rate of reaction on different solvents’ effects Materials:
Apparatus: 50.00 mL burette Chemicals: 0.04M NaOH 10.00 mL pipette t-butyl chloride 100 cm3 volumetric flask Acetone Conical flask Isopropanol Measuring cylinder Bromothymol blue Procedures:
1. 150 cm3 of 0.04M NaOH(aq) is placed in a beaker.
2. 100 cm3 of a 50/50 acetone/watermixture (by volume) is put into a stoppered flask or bottle and is mixed well.
3. 1.00 cm3 (±0.05 cm3) of t-butyl chloride liquid is added in the acetone/water mixture prepared in step 2.
4. The flask is stoppered and shaken well, and time is measured. The flask is to be kept stoppered except when removing aliquots of solution for titration.
5. An “infinity time” sample is prepared by adding 10.00 cm3 sample of the reaction mixture to a stoppered flask containing 50 cm3 of water. Then the flask is stoppered and is left standing for at least an hour.
6. At time intervals of 10 minutes, 10 cm3 aliquots of the reaction mixture is removed from the flask and placed in an Erlenmeyer flask containing 15 cm3 of acetone. 3 drops of bromothymol blue indicator are added to the flask. Standardized NaOH solution is then used to titrate the mixture to a blue end point that persists for 10 seconds. The volume of NaOH used is recorded.
7. After 50 minutes, the “infinity time” sample is titrated in the same way and the volume of NaOH used is recorded.
8. Repeat steps 2-7 using a 50/50 water/isopropanol mixture instead.
Results:
Calculation of [HCl]∞: Equation of the reaction:
(CH3)3C-Cl + H2O → (CH3)3C-OH + HCl
For the “infinity time” sample, volume of t-butyl chloride liquid added: 0.100cm3
Mass of t-butyl chloride liquid: m = dV = 0.851g/cm3 x 0.100cm3 = 0.0851g
Number of mols of t-butyl chloride used: n = m/M = 0.0851g ÷ 92.616g/mol = 9.19x10-4mol
⇒ 9.19x10-4mol of HCl was produced
[HCl]∞= n/V = 9.19x10-4mol ÷ 0.01dm3 = 0.092M

Therefore, we obtain the data:

Volume of t-butyl chloride: 1.0 cm3
For 50/50 acetone/water
[HCl]∞
0.092
Time (minutes)
10
20
30
40
50
Volume of NaOH (cm3)
3.2
6.1
9.4
10.8
12.3
[HCl]t (mol/dm3)
0.0128
0.0244
0.0376
0.0432
0.0492
ln([HCl]∞-[HCl]t)
-2.536
-2.694
-2.911
-3.020
-3.151
Volume of NaOH(aq) used for the 10 cm3 infinity time sample: 23 cm3

Graph of ln([HCl]∞-[HCl]t) against Time

Pseudo first order rate coefficient of the reaction: 0.0156
For 50/50 water/isopropanol
[HCl]∞
0.092
Time (minutes)
10
20
30
40
50
Volume of NaOH (cm3)
2.0
2.5
3.0
3.4
4.3
[HCl]t (mol/dm3)
0.008
0.01
0.012
0.0136
0.0172
ln([HCl]∞-[HCl]t)
-2.477
-2.501
-2.526
-2.546
-2.593
Volume of NaOH(aq) used for the 10 cm3 infinity time sample: 23 cm3

Graph of ln([HCl]∞-[HCl]t) against Time

Pseudo first order rate coefficient of the reaction: 0.0028

Sample calculation of [HCl]t: Equation of the titration:
NaOH + HCl → NaCl + H2O
For 10 minutes sample, volume of 0.04M NaOH used: 2.0cm3 = 0.002dm3
Number of mols of NaOH used: n = Molarity*V = 0.04M x 0.002dm3 = 8.0x10-5mol
⇒ 8.0x10-5mol of HCl was produced
[HCl]10 = n/V = 8.0x10-5mol ÷ 0.01dm3 = 0.008M

Mechanism:
The hydrolysis involves 3 steps:
1. the ionization of the t-butyl chloride to form an intermediate;
2. the formation of a high energy transition state between the intermediate and water;
3. the formation of product, t-butyl alcohol by a hydrogen ion leaving the transition state complex.
In the first transition state, the carbon-chlorine bond is partially broken through a slow step, after which the carbon has partial positive charge while the chlorine bears a partial negative charge. Since the transition state is charge-separated, it is better stabilized by polar solvents than by nonpolar solvents. Thus, increasing the polarity of the solvent – which solvate the polarized transition state even better and thereby lower the activation barrier — should increase the rate of this hydrolysis reaction. Similarly, moving to a better hydrogen bond-donating solvent should allow stabilization of this transition state through hydrogen bonding with the chlorine, bearing a partial negative charge; again, the rate of the hydrolysis reaction should be enhanced.

These are the hydrolyses of t-butyl chloride in 2 different solutions, protic and aprotic. Theoretically, the protic solvent would lower both the activation energy and the intermediates’ potential energy, allowing faster reaction. This is verified in the experimental methods: pure water drives the reaction to completion quickly while pure acetone stops the reaction. Thus, 50/50 isopropanol/water mixture should drive a quicker reaction than the equally proportional mixture of acetone/water.
The results obtained do not correspond well to what would be expected; in fact, they vary greatly. One would expect 50/50 isopropanol/water to have the higher rate constant but it does not.
For improvements:
One possible reason for the varying results is the inaccuracy in the ratios of the solvent mixtures. Since only small quantities were used, a difference of one or two milliliters of one solvent or the other could change the percentages almost proportionally. Thus, a larger amount is recommened.
Imprecision in titration would also lead to errors. Intra-rater and inter-rater error is the result of differences between individuals doing the measuring and differences between each measurement done by the same individual performing the titration.
In order to start the timing of the reaction, t-butyl chloride has to be taken from the apparatus, put into the prepared solution and shaken well in the stoppered flask. To do all those procedures quickly and carefully before timing, one man is hardly enough. Thus, this action should be 2-man carried out.
The concentration of HCl in the infinity time sample was not experimentally obtained, but rather theoretically calculated. In fact, the amount experimentally was too small, indicating an unfinished experiment, not to mention the 2 different amounts for 2 reactions. Thus, there should be some other way to drive the reaction more quickly. Conclusions:
Although it takes quite an amount of time, through a number of errors to measure all the concentrations, the results came out unsatisfactorily: The overall effect of solvent on the rate of an SN1 reaction cannot be defined by the results of these experiments. The data is too inconsistent. No final conclusions can be drawn about what solvents in what solvents will speed or slow the reaction.