Kinetics: Hydrolysis of Tertiary Butyl Halides

Computer-Interfaced Experiments - Conductivity Measurement

Kinetics
Hydrolysis of Tertiary Butyl Halides - First Order Reaction
Objectives: Effect of Solvent Polarity and Leaving Group on Rate, Determination of Rate Constants

Peter Keusch

Datalogging using the Program CHEMEX and the Analog-Digital-Converter CHEMBOX
IBK electronic + informatic

German version

Chemicals:

2-chloro-2-methylpropane 99 % (m.w. = 92.57 g / mol d = 0.84 g / mL)
2-bromo-2-methylpropane 98 % (m.w. = 137.03 g / mol d = 1.216 g / mL)
acetone > 99.5 % (m.w. = 58.08 g / mol d = 0.783 g / mL)

Apparatus and glass wares:
magnetic stirrer hotplate
2 magnetic stirring bars
stirring bar remover
crystallizing dish d = 190 mm, h = 90 mm (for water bath)
beaker 200 mL
contact thermometer
temperature sensor
conductivity measuring cell
micropipette
volumetric pipettes 10 mL
volumetric pipettes 20 mL
volumetric pipettes 50 mL
3 pipette bulbs

Hazards and safety precautions:

Tert. butyl halides are harmful if inhaled. Skin, respiratory and eye irritants.

Acetone is highly flammable. Irritating to eyes.

Safety glasses, gloves and good ventilation required.

Theoretical background:

Tertiary butyl halides undergoes solvolysis in aqueous solvents.

0n this basis, conductometric measurement of the rate of formation of hydrogen halide indicates the course of the reaction. By determining the conductivity as a function of time, the rate constant can be found.

Kinetic equations (Download PDF file)

Experiment 1: Effect of solvent polarity on rate

Experimental procedure:

Fig. 1: Experiment set-up In addition to a conductivity measuring cell (1) a temperature sensor (2) is connected to the CHEMBOX via input Sensor2 (Fig. 1).

100 mL of 10 % aqueous solution of acetone (90 mL dist. water und 10 mL acetone) are pipetted into a beaker placed in a the water bath.

Using a hotplate stirrer and a contact thermometer the aqueous acetone solution is warmed up in the water bath to the desired temperature (approx. 30°C). The acetone solution is allowed to equilibrate in the constant-temperature water bath.

When thermal equilibrium has been reached, 88 ml of 2-chloro-2-methylpropane (0.8 mmol) are added to the aqueous acetone solution while vigorously stirring. Immediately the sensing software is started.

The data are logged at one-second intervals.

The change in the conductivity and the constancy of the temperature are displayed on the measuring screen (Fig. 2).

After data logging the measurement is stored.

In addition the experiment is carried out using 20, 30, 40 and 50 % aqueous acetone solution.

Fig. 2: Real time plot Hydrolysis of tert. butylchloride in an aqueous solution of acetone (water : acetone = 9 : 1) T = 30.2 °C

Data analysis using Excel (Download) - determination of the rate constants:

Fig. 3: Conductivity curves Hydrolysis of tert. butylchloride
in 10 % (1) and 20 % aqueous solution of acetone (2) T = 30.2 °C

Fig. 4: Conductivity curves Hydrolysis of tert. butylchloride
in 30 % (3) 40 % (4) and 50 % aqueous solution of acetone (5) T = 30.2°C

Measurement	1	2	3	4	5
Acetone concentration	10%	20%	30%	40%	50%
k_¥ [ mS ]	0.7832	1.0330	1.3790	1.3480	0.7589

Tab. 1: k_¥ = Conductivity at the end of the reaction

Fig. 5: Effect of acetone concentration on k_¥

According to equation (6) Kinetic equations(Download PDF file) the conductivity values are converted. (Tab. 2)

In so doing, a plot of
-ln (0.008 · (1 - k / k_¥))

versus t is allowed (Fig. 6).

Excel

Tab. 2: Measured values k (t) conversion according to y = -ln (0.008 · (1 - k / k_¥))

Fig. 6: Determination of the rate constant k
y = -ln (0.008 · (1 - k / k_¥))

Measurement	1	2	3	4	5
acetone concentration	10%	20%	30%	40%	50%
k [ s^-1 ]	0.0215	0.0152	0.0091	0.0036	0.0015

Tab. 3: Rate constants k

concentration effect

Fig. 7: Effect of acetone concentration on rate constant k

Discussion:

The more polar the solvent the faster takes place the nucleophilic substitution. The reason is that polar protic solvents do two things:

· They surround the carbocation which has the effect of stabilizing the intermediate.

· Polar protic solvents prevent a recombination of the carbocation with the leaving group. That is, the first step in the S_N1 reaction will not be reversed.

In the concentration interval of 10-30 % acetone, the reaction rate decreases linear with the acetone concentration. The S_N1 rate is proportional to the polarity of the solvent.

Experiment 2: Effect of leaving group on rate

90 ml 2-bromo-2-methylpropane (0.8 mmol) are hydrolyzed at approx. 30 °C using an aqueous acetone solution (70 mL water, 30 mL acetone).

Fig. 8: Conductivity curves Hydrolysis of tert. butylchloride (1) and tert. butylbromide (2)
in 30 % aqueous solution of acetone T = 30.2 °C

Fig. 9: Rate constant k tert. butylchloride (1) and tert. butylbromide (2) T = 30.2 °C
y = -ln (0.008 · (1 - k / k_¥))

Discussion:

The tertiary butylbromide reacts four times as fast as the tertiary butylchloride. The reason is:

· In the rate-determining step of S_N1 reactions, the alkyl halide (R - X) is cleaved into a positively charged carbocation and a negatively charged leaving group. The reaction rate depends not only on the polarity of the solvent and on the stability of the carbocation, but also on the stability of the leaving group. The more stable the leaving group is, the more easily the C - X bond is also cleaved, the higher the reaction rate is. Conjugated bases of strong acids are good leaving groups. The experiment above shows that bromide ion is a better leaving group than the chloride ion. Bromide is a weaker base than chloride. The weaker base is more stable and thus more easily formed.

Relative hydrolysis rateg of R - X (R = tertiary alkyl group)
X = I > Br > Cl

Note that S_N1 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).

Reference:
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 S_N1 Nucleophilic Substitution unimolecular - Animation
S_N1 Mechanism / S_N2 Mechanism

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