R 2 CHNH 2
4.3 Ozone initiated oxidation of 2-chloroethanol in the presence of organic solvents
4.3.1 Effect of acetic acid on ozonation of 2-chloroethanol
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The percentage yield of major products namely acetaldehyde (AcAlde) and acetic acid (AA) and minor unknown products formed during the ozone initiated oxidation of 2-chloroethanol are presented in Figure 4.2. It is clear that yields for all products increase as ozonation time increase. At all time intervals the percentage yield of AA was significantly higher than AcAlde, suggesting that ozone has readily oxidized the acetaldehyde to its corresponding organic acid.
4.3 Ozone initiated oxidation of 2-chloroethanol in the presence of
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Chromatographic analysis of the product mixture showed the same pattern of product formation as for the oxidation of pure 2-chloroethanol with AcAlde and AA being the
major product. No new products peaks were observed during the ozonation of 2-chloroethanol in the presence of 5 % acetic acid.
Data for these experiments showing conversion of substrate and selectivity for major products AcAlde and AA and minor unknown products at different ozonation times are illustrated in Table 4.2 and Figure 4.3.
Table 4.2 Percent conversion and product selectivity data for the ozone initiated oxidation of 2-chloroethanol in 5 % acetic acid as a function of time
* Oxygen flow rate = 200 mL min-1 * Ozone concentration = 100 g mL-1
Figure 4.3 Percent conversion of 2-chloroethanol in the presence of 5% acetic acid and selectivity profile of products versus ozonation time
0 10 20 30 40 50 60 70
2 3 4 5 6 7 8 9 10 11 12 13
Conversion and Selectivity (%)
Ozonation time (h)
conversion - 2-CE S - AcAlde S - AA S - unknown
Ozonation
Time (h) Conversion
(%) Selectivity (%)
AcAlde AA Unknown
3 7.8 40.4 57.4 2.2
6 10.6 39.2 50.6 10.3
9 13.4 40.4 47.4 12.2
12 26.2 40.9 48.3 10.8
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Results presented in Table 4.2 show that the percent conversion of 2-chloroethanol increases gradually from 7.8 % after 3 h ozonation to 13.4 % after 9 h ozonation and thereafter increasing rapidly to 26.2 % after 12 h ozonation. This pattern of substrate conversion suggests that oxidation rates increase significantly when substrate molecules are exposed to ozone gas for longer periods.
The selectivity profiles for the ozonation of 2-chloroethanol in the presence of 5 % acetic acid showed similar trends when compared to the ozonation of 2-chloroethanol in the absence of solvent. Selectivity towards AcAlde did not change significantly from 40.4 % after 3 h ozonation to 40.9 % after 12 h, while selectivity towards AA decreased gradually from 57.4 % after 3 h to 48.3 % after 12 h ozonation.
Table 4.3 Solution conductivity, chloride content and product yields, for the ozonation of 2-chloroethanol in 5 % acetic acid as a function of time
Ozonation
Time (h) Conductivity
( S cm-1) Chlorides
(ppm) Yield (%)
AcAlde AA Unknown
3 332 92 3.1 4.5 0.2
6 2194 226 4.1 5.3 1.1
9 4068 461 5.4 6.3 1.7
12 5279 610 10.7 12.6 2.8
Table 4.3 shows the relationship between increasing chloride ion content and increasing conductivity of the reaction mixture after 3, 6, 9, and 12 h respectively. It is evident that the ozonation of 2-chloroethanol in the presence of 5 % acetic acid yields quantitative amounts of chloride ion as one of the major products and there is a steady increase of Cl- with an increase in ozonation time. The ppm chlorides obtained in the presence of acetic acid were slightly higher than in the absence of acetic acid. The difference in the chloride concentration ranged from 6 ppm after 3 h ozonation to 25 ppm after 12 h. It is also evident from the results of the conductivity measurements that there is a steady increase in conductivity of the reaction solution as the ozonation time is increased. The specific conductivity of the ozonated mixture in the presence of acetic acid was slightly higher
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than to the ozonated mixture in the absence of acetic acid. The difference in specific conductivity ranged from 14 S cm-1 after 3 h ozonation to 218 S cm-1 after 12 h.
The percentage yields of major products namely acetaldehyde (AcAlde) and acetic acid (AA) and minor unknown products are shown in Table 4.3. It is clear that product formation increases as ozonation time increases. At all time intervals the percentage yield of AA was significantly higher than AcAlde, indicating that the aldehyde is readily oxidized to its corresponding organic acid. The yields of the major products AcAlde and AA obtained in the presence of acetic acid were slightly higher than in the absence of the organic solvent. The difference in the yields for AcAlde and AA after 3 h ozonation ranged from 2.7 % and 3.9 % respectively, and after 12 h continuous ozonation ranged from 2.6 % to 1.9 % respectively.
The effect of a higher concentration of acetic acid on the ozonation of 2-chloroethanol was investigated. One hundred milligrams per litre ozone was steadily bubbled through 20 mL aliquot mixtures of 2-chloroethanol and 20 % acetic acid for 3, 6, 9 and 12 h under similar experimental conditions as for the ozonation of 2-chloroethanol in the absence of solvent. Analyzed data for these experiments showing conversion of substrate and selectivity towards major products AcAlde and AA and minor unknown products at different ozonation times are illustrated in Table 4.4 and Figure 4.4.
Table 4.4 Percent conversion and product selectivity data for the ozone initiated oxidation of 2-chloroethanol in 20 % acetic acid as a function of time
Ozonation
Time (h) Conversion
(%) Selectivity (%)
AcAlde AA Unknown
3 17.3 42.1 53.3 4.6
6 28.1 40.7 52.5 6.8
9 48.1 38.3 50.3 11.4
12 56.9 39.8 46.6 13.6
* Oxygen flow rate = 200 mL min-1 * Ozone concentration = 100 g mL-1
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Figure 4.4 Percent conversion of 2-chloroethanol in the presence of 20 % acetic acid and selectivity profile of products versus ozonation time
Results in Table 4.4 and Figure 4.4 show that the percent conversion of 2-chloroethanol increases from 17.3 % after 3 h ozonation to 56.9 % after 12 h ozonation. Most evident is that the percentage of conversion of 2-chloroethanol improved significantly when ozonation is carried out in the presence of 20 % acetic acid. The selectivity profiles for the ozonation of 2-chloroethanol in the presence of 20% acetic acid showed similar trends when compared to the ozonation of 2-chloroethanol in 5 % acetic acid. Selectivity towards AcAlde did not change significantly from 42.1 % after 3 h ozonation to 39.8 % after 12 h, while selectivity towards AA decreased gradually from 53.3 % after 3 h to 46.6 % after 12 h ozonation.
Table 4.5 shows the relationship between increasing chloride ion content and increasing conductivity of the reaction mixture after 3, 6, 9, and 12 h respectively. It is evident that the ozonation of 2-chloroethanol in the presence of 20 % acetic acid yields quantitative amounts of chloride ion as one of the major products and there is a steady increase of Cl- ions with an increase in ozonation time. The ppm chlorides obtained in the presence of acetic acid were slightly higher than in the presence of 5 % acetic acid. The difference in the chloride concentration ranged from 8 ppm after 3 h ozonation to 180 ppm after 12 h.
0 10 20 30 40 50 60
2 3 4 5 6 7 8 9 10 11 12 13
Conversion and Selectivity (%)
Ozonation time (h)
conversion - 2-CE S - AcAlde S - AA S - unknown
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Table 4.5 Solution conductivity, chloride ion content and product yields, for the ozonation of 2-chloroethanol in 20 % acetic acid as a function of time
Ozonation
Time (h) Conductivity
( S cm-1) Chlorides
(ppm) Yield (%)
AcAlde AA Unknown
3 370 100 7.3 9.2 0.8
6 2550 262 11.4 14.8 1.8
9 5431 615 18.4 24.2 5.5
12 6832 790 22.7 26.5 7.7
* Oxygen flow rate = 200 mL min-1 * Ozone concentration = 100 g mL-1
It is also evident from the results of the conductivity measurements that there is a steady increase in conductivity of the reaction solution as the ozonation time is increased. The specific conductivity of the ozonated mixture in the presence of 20 % acetic acid was slightly higher when compared to the ozonated mixture in 5 % acetic acid.
The percentage yields of major products namely acetaldehyde (AcAlde) and acetic acid (AA) and minor unknown products are shown in Table 4.5. It is clear that product formation increases as ozonation time increases. At all time intervals the percentage yield of AA was significantly higher than AcAlde, indicating that the aldehyde is readily oxidized to its corresponding organic acid. The yields of the major products AcAlde and AA obtained in the presence of 20 % acetic acid were slightly higher than that obtained in 5 % acetic acid suggesting that a higher concentration of solvent favors the ozone initiated oxidation of 2-chloroethanol.
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