The Hydrolysis of Tert-Butyl Chloride

This paper covers a calculation of the reaction rate during the hydrolysis of t-butyl chloride using isopropanol/water mixtures containing 50% by mass of isopropanol. The mixed solvent was used due to the slow rate of reaction that was appropriate for the analytical process used. The progress of the reaction was monitored by measuring the concentration of H+ in the solution, as the pH of the solution was likely to decrease due to the proton released during the reaction. Neutralization occurred as the pink hue of phenolphthalein faded and the solution became acidic when nHCL= nNaOH=nRCL was absorbed. When NaOH was applied, the solution turned colorless, but when more NaOH was added, the solution turned pink. However, after some time, the pink color was expected to turn to colorless.

Introduction

t-butyl chloride belongs to halogenoalkanes and is thus a tertiary halogenoalkane. Because of the polar nature of this compound, it undergoes solvolysis in water. Solvolysis denotes a simple nucleophillic substitution reaction. For polar molecules of water, the C-CL bond breaks and is substituted with C-OH bonds during hydrolysis. Starting a chemical reaction requires molecules to possess activation energy (Mcmurry, 2007). The rate of a chemical reaction is;

rate=-d/dt[t-BuCL]=+d/dt[t-BuOH]=+d/dt[HCL]

On the other hand, the rate constant can be established as;

Rate=k[t-BuCL]1

Overall, the reaction shows that for every mole of t-BuCL used during the reaction, I mole of HCL is the product.

Materials and Methods

Materials

60ml of standardized 0.25M NaOH , 50% isopropyl alcohol/water, beaker, buret, stand and clamp, test tube, graduated cylinder, Erlenmeyer flask, phenolphthalein.

Procedure

60 ml of standardized 0.25MNaOH was placed in a beaker containing 50% of isopropyl alcohol/water solution. 3ml portions of the solution rinsed the buret, which was then filled with the same solution.

100ml of the 50% solution of isopropyl alcohol was then put into the Erlenmeyer flask.

Few phenolphthalein indicator drops were put in the solution.

1ml of CH3)3-CL) was then added to the solution and swirled immediately.

Time for the addition of tert-butyl chloride was recorded to the nearest 10 seconds.

Another 1.0 ml of NaOH solution was added to the solution, swirled to mix until there was a pink color. The time that pink color appeared was recorded.

NaOH solution was then added again until the pink color changed to colorless and time recorded.

This was repeated until 7 time readings were observed as pink color disappeared.

The solution temperature was taken and recorded. 600ml beaker was half-filled with distilled water and heated to about 400 C.

Warm water (50ml) was added to the flask and mixed thoroughly.

The stoppered flask was placed in a hot water bath for one hour.

The temperature of the bath was sustained between 350C and 400C using the bunsen burner. Titration was then carried out to the flask after it was unstoppered until a faint pink color was formed indicating an endpoint using the standardized NaOH solution.

Buret reading at the end of the experiment was recorded.

Results and Discussion

Time taken for color to disappear

NaOH) reading on the buret

t(S)

Total Volume of NaOH at t

n moles of (OH)

n moles of (RCL)

In(n(RCL)

11.53.35

49

0

0.000025

6.25*10-9

6.25*10-9

-18.89

11.55.45

48

130

0.000050

1.25*10-8

1.25*10-8

-18.02

11.56.46

47

61

0.000075

1.875*10-8

1.875*10-8

-17.79

12.00.23

46

229

0.000100

2.5*10-8

2.5*10-8

-17.50

12.04.25

45

242

0.000125

3.125*10-8

3.125*10-8

-17.28

12.08.01

44

216

0.000150

3.75*10-8

3.75*10-8

-17.10

12.12.09

43

248

0.000175

4.375*10-8

4.375*10-8

-16.94

(CH3)3-CL) undergoes a reaction with a solvent(water) yielding ((CH3)3-OH)

The process leading to hydrolysis has two steps, ionization yielding CL- and (CH3)3C+)

(CH3)3C-CL (CH3)3C+ +CL- (slow)

and the second step is the reaction of the cation and the solvent to yield alcohol and the aqueous proton.

(CH3)3C+ +H2O (CH3)3COH+H+ (fast)

Addition of the two equations gives the overall reaction.

Rate=-d[RCI]/dt=k[RCI] where [RCI] is the molar concentration of tert-butyl chloride and k the rate constant

Thus, rate=3.375*10-8/1.5*10-4=2.25*10-4

Rate=k[RCI]

K=Rate/[RCI]

=2.25*10-4 /4.375*10-8

=5.14*103

Conclusion

The reaction rate depends on the energy of activation required, the concentration of reactants and temperature. A useful indicator of rate equation is the half-life. The half-life means time taken for reactants concentration to reduce by 50% during the reaction.

Questions

The half-life of tert-butyl chloride;

T1/2 =In2/k

=0.693/5.14*103

=1.3482*10-4 s

If 1.3482*10-4 was time taken to hydrolyzed 100% of tert-butyl chloride, then the time taken to hydrolyze 95% will be;

(1.3482*10-4 )*95/100=1.2808*10-4s

3. Second order reaction and first order reaction as shown in the equation d[RCI]/dt=k[RCI][H2O] can be distinguished. For first order reaction, the rate constant k have units of second-1 while in the second order reaction, k have units of L.mole-1.second-1.

4. i. Th difference in molarity is due to the fact the rate of reaction and hence the rate constant will only depend on the concentration of t-BuCL that is independent of NaOH. Thus, if the true morality of NaOH is used, then the rate constant would be the same.

ii. tert-butyl chloride is a tertiary alcohol and thus relatively stable. Therefore, it undergoes SN 1 mechanism to form alcohol, that is followed by a SN2 mechanism that the primary alcohol undergoes.

iii. The room temperature changes due to changing pressure as a result of moving wind.

iv. The flask remains uncorked to prevent possible reaction with carbon from the air.

References

Mcmurry, J. (2007). Reactions of Alkyl Halides: Nucleophilic Substitution and Eliminations. In: Organic Chemistry (7th ed.). Brooks Cole.