Electrophilic Aromatic Substitution

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Bromine Appearance

Diphenyl ether

4.45s

Colorless

Table 6. Rate of Bromination of Diphenyl Ether at room temperature @ 21oC

Time

Absorbance

15s

.999

30s

.999

45s

.999

60s

.999

90s

.715

120s

.574

150s

.517

180s

.456

210s

.452

240s

.408

Table 7. Rate of Bromination of Diphenyl ether

In Ice Water

Time

Absorbance

15s

.690

30s

.685

45s

.667

60s

.622

90s

.582

120s

.567

150s

.562

180s

.544

210s

.517

240s

.511

In 50oC

Time

Absorbance

15s

.507

30s

.507

45s

.454

60s

.440

90s

.334

120s

.288

150s

.227

180s

.173

210s

.136

240s

.105

Calculations:

1. Qualitative Rates of Bromination

Decreasing reactivity to Increasing reactivity

1. Nitrobenzene

2. Chlorobenzene

3. Benzene

4. Phenol

5. diphenyl ether

6.P-bromophenol

7. anisole

8. acetanilide

2. Bromination of Diphenyl Ether

1. at 50oC

ln(A)0 – ln(A)t = K1T

.105= e-k240

ln.105= e-k240

-2.2=-k(240)

2.2/240=k

9.1 x 10-3=k

2. at room temperature 21oC

ln (A)0 – ln(A)t = K1T

.999= e-k240

ln.999= e-k240

-.999=-k(240)

0.001/240=k

4.16 x 10-6=k

3. at 0oC

ln (A)0 – ln(A)t = K1T

.690= e-k240

ln.690= e-k240

-.0371=-k(240)

0.371/240=k

1.5 x 10-3=k

Activation Energy:

Temperature

-ln(k)

3.6 x 10-3

-4.0 x 10-4

3.4 x 10-3

-1.42 x 10-6

3.1 x 10-3

9.1 x 10-3

Activation energy: 83143 J/mol

Discussion:

Results showed that the rates of bromination of substituted benzenes had different rates i.e., from slowest to fastest the slowest deactivated aromatic compound was nitrobenzene, then chlorobenzene and the fastest benzene. This was expected as when chlorine is added on to a benzene ring it avoid an destabilizing effect by placing the substituent at the meta position. Benzene has no electronic effect and has substituents directing at the ortho and para postion1.

The rate constants for the bromination of diphenyl ether were determined at 0oC, room temperatures, and 50oC. The rate constants were 9.1 x 10-3 at 50oC, 4.16 x 10-6 at 21oC and 1.5 x 10-3 at 0oC. The activation energy was 83143 J/mol.

The commercial product methoxychlor, has a active ingredient DMDT, and has replaced DDT as an insecticide because in DDT there is a chlorine substituent on the aromatic compounds. Chlorination of the aromatic compounds is slow at reacting, and the methoxy is more reactive. Therefore, DMDT goes away faster than the DDT, hence it is good that the DDT is used as an insecticide.

Conclusion:

The