Kinetics of Aromatic Bromination

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Figure 2. Graph of quantitative rates of bromination on diphenyl ether at room temperature (23ºC)

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Figure 3. Graph of quantitative rates of bromination on diphenyl ether at 50ºC

Determination of the Activation Energy

For the bromination of diphenyl ether

Temperature (T)

1/T

k (rate constant)

-ln(k)

273.16

0.00366

0.0015

6.502

296.16

0.00338

0.0086

4.756

323.16

0.00309

0.0207

3.878

Include the graph of the above data, and determine the slope of the line.

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Figure 4. Graph of –ln(K) vs. the inverse of temperature at 273.15K, 298.16K, and 323.16K for the bromination of diphenyl ether.

Show your calculation of the activation energy (∆G‡).

Discussion

In this experiment, the absorbances of benzene, nitrobenzene and chlorobenzene were recorded at 435nm during 40 minute intervals at a temperature of approximately 75ºC. The change in absorbance of benzene from start to finish was 0.316, nitrobenzene was 0.215, and chlorobenzene was 0.186.Benzene had the greatest change in absorbance due to it reacting by ortho-para substitution which is activating and therefore the fastest reaction. Nitrobenzene undergoes a meta directing substitution because the resonance contributors are especially unstable. Chlorobenzene is weakly deactivating and had the least change in absorbency because of this.

The rates of bromination on phenol, diphenyl ether, p-bromophenol, acetanilide, and anisole were measured at 0ºC, 23º, and 50ºC. Phenol was the fastest reacting reagent, because of its hydroxyl group, which is a very strong activating substituent. The second fastest reacting agent was acetanilide, followed by p-bromophenol, and anisole. Diphenyl ether showed no colour change because its substituent is strongly deactivating.

The rate constants for the bromination of diphenyl ether were determined using spectroscopy at 0º, 23º and 50º. Measurements were taken at 15 second intervals for the first minute and 30 second intervals for the next three minutes at 435nm. These intervals were plotted on a graph of –ln(A) vs time. These graphs were used to find the slope, which is the rate constant (k) at each temperature. The rate constants at 0ºC, 23ºC and 50ºC were 0.0015, 0.0086 and 0.0207, respectively. From determining the rate constants, a graph of –ln(k) vs 1/T (temperature in Kelvin). The slope from this graph plus the constant R determined the activation energy (∆G‡) to be 38195.3.

The insecticide DDT was replace by the commercial product containing 50% of the activate ingredient DMDT because DDT contains a chlorine group attached to either side of the two rings. Chlorine is a deactivating substituent and causes inductive withdrawal. Due to this, DDT does not break down into the environment easily. DMDT contains two methoxide groups attached to either side of the two rings. Methoxide is an activating substituent and therefore DMDT reacts a lot faster than DDT.

Conclusion

From this experiment, benzene was found to have the greatest change in absorbency (0.316) compared to nitrobenzene and chlorobenzene. Phenol was the fastest reacting aromatic compound due to its highly activating hydroxyl substituent. The rate constants of the bromination of diphenyl ether at 0ºC, 23ºC and 50ºC were 0.0015, 0.0086 and 0.0207, respectively The activation energy was found to be 38195.3.

References

1. Marrs, P. Organic Chemistry Laboratory Manual. pp. 25-30. (University of Victoria, BC). Fall 2015.