February, 1978 SHORT PAPERS 183

Titrimetric Determination of Halogens in Halo- and Dihalo-P-diketones

C. Muhammad Ashraf Chemistry Department, Makerere University, P.O. Box 7062, Kampala, Uganda

Riaz Ahmed, Amjad Aqeel and M. Saleem Khalid Institute of Chemistry, University of the Punjab, Lahore, Pakistan

Keywords : Halogen determination ; halo- and dihalo-/3-diketones ; titrimetry

The bromine in bromo-p-diketones is the oldest procedures for determining

\

\ ]-OH + Br2

L O L - H C

/ \

H2 c

?="

usually measured by Meyer's method, which is one of the enol content of /3-diketonesl:

\ L = O

Y2 /"="

+ l 2 + H B r /

+2HI _Icr

-HBr /C=O ---+ BrCH

\

A similar procedure has been reported by Macbeth2 for the determination of chlorine in chlorobenzoylacetone by iodimetric titration. However, when attempts were made to verify the results, required in connection with another investigation, the volume of the titrant (sodium thiosulphate solution) far exceeded the theoretically required volume. Similar errors have also been stated earlier to give absurdly high enol contents of certain compounds (in excess of These discrepancies led us to examine the determination of halogens in monochloro-, monobromo- and dibromobenzoylmethanes, mono- and dichlorobenzoyl- acetones and monobromo-, monochloro- and dichloroacetylacetones in 40% V / 'c' aqueous ethanolic solutions.

Experimental Reagents and Chemicals

Ethanol , about 80%. Hydrochloric acid, 10 N. Hydvobromic acid, 7.6 N. Acetic acid, 16 N. Potass ium iodide. AnalaR grade. Acetylacetone. Distilled before use. Benzoylaceto f ie. Dibenxoylmethane. Dibenzoylmethane was prepared6 and then used to prepare its halo

The halo derivatives of acetylacetone, benzoylacetone and dibenzoylmethane derivatives. were prepared by the reported procedure^.^^^^^-^

Procedure Solutions of the required concentrations (see Table I) of the halo and dihalo derivatives of

the three /3-diketones in 80% ethanol and of potassium iodide in water were prepared separately. Equal volumes of the halide and potassium iodide solutions were placed in flasks in a thermostatically controlled water-bath and, when these solutions attained the temperature of the bath, they were mixed together and 10-ml portions were withdrawn at different intervals of time and titrated against standard sodium thiosulphate solution. Blank runs (without any halo derivatives) were also carried out when necessary. The percentage completion of the reactions of the various halo derivatives of the three p-diketones were studied in the presence and absence of hydrochloric, hydrobromic and acetic acids. When

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184 SHORT PAPERS Analyst, Vol. 103 the effects of these acids were studied, different amounts of the acids (see Table I) were added to halo and dihalo derivatives of the P-diketones at the time their solutions were prepared. The remainder of the procedure was followed as described above.

Results and Discussion The percentage completion of the reactions in the presence and absence of the three acids

is shown in Table I. The results obtained indicate that the halogens in the halo derivatives (especially the monobromo and dibromo derivatives) of dibenzoylmethane were determined almost quantitatively in the presence of small amounts of hydrochloric, hydrobromic and acetic acids. Under similar conditions, these acids had no effect on the removal of halogens from monochloro- and dichlorobenzoylacetone. The halo derivatives of acetylacetones were least affected, and in fact were inert to the removal of halogen when similar concentra- tions were used. This necessitated the use oE higher concentrations of the reactants when monobromo- and mono- and dichloroacetylacetone were used. The latter experiments (the results for which have not been incorporated in Table I) revealed that the amount of halogen determined in these compounds far exceeded the theoretical values, probably owing to a direct reaction between potassium iodide and the acids to give hydriodic acid, which ulti- mately is oxidised to iodine.

TABLE I REACTION PARAMETERS FOR DETERMINATION OF HALOGENS IN HALO- AND

DIHALO-P-DIKETONES AT 40 "c Amount of halo derivatives reacted (moles per litre) = volume of 0.001 N Na,S,O, solution x 0.000 1,

except for dibromodibenzoylmethane, for which the amount reacted (moles per litre) = volume of 0.001 N Na,S,O, solution x 0.00005.

Concentration Concentration Volume of acid used/ml Volume of of halo of KI ,-A 7 0.001 N Percentage

Time/ derivative/ solution/ 10 N 16 N 7.6 N NaqS,O, completion Halo derivative min moll-' mol 1-l HCl CH,COOH HBr solution/ml of reaction

Monochlorodibenzoylme thane

Monobromodibenzoylmethane

Dibromodibenzoylmethane

Monochlorobenzoylace tone

Dichlorobenzoylacetone . .

Monochloroace t ylacetone

Dichloroacetylacetone . . Monobromoacetylacetone

*At 35 "C.

.. 120 30 15 30 30 .. 120

110 10 0

10 . . 16

0 4 4 4

.. 0 15 15 15

.. 30 30 30 30

. * 100 100 .. 110 110

. . 90 90

120 60 60

0.004 0.002 0.002 0.002 0.002 0.004 0.004 0.004 0.004 0.004 0.004 0.002 0.002 0.002 0.002 0.4 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.004 0.003 2 0.003 2 0.004 0.002 0.004 0.004 0.004

0.004 - - 0.002 0.022 - 0.002 0.22 - 0.002 - - 0.002 - 0.013 0.004 - - 0.004 0.11 - 0.004 2.5 0.004 5.0 - 0.004 - - 0.004 - - 0.002 0.22 - 0.002 0.022 - 0.002 - - 0.002 - 0.013 4.0 1.0 0.004 1 .o

1.0 0.004 - 0.004 - - 0.004 - - 0.004 1.0 0.004 - 1.0 0.004 - - 0.2 0.4 0.2 0.4 0.4 0.4 - 0.004 1 mol 1-1 - 0.4 - 0.1 0.072 - 0.1

-

- -

-

- - - - - - - - - -

0 - - 19.1 - 20.0

0.03 2.20 - 1.61 - 3.61 - 21.10 - 28.13 - 40.00

0.15 39.98 - 33.95 - 40.00 - 40.00

0.03 37.50 - 35.50 - > 100

6.5 6.5

1.0 5.5

- -

- 25.20 - 25.21 - 25.18

1.0 24.82 - 4.35 - 7.85 - 3.85 - 7.45 - 1.00 - 1.00

- 1.45 - 0.45

- 0

0 95

100 11 8 9

53 70

100 100 42

100 100

93 88

16 16 i 4 63 63 63 62 11 20 12 23 2.5 5 0 3.6* 1'

It is obvious from these results that the various halo- and dihalo-/3-diketones studied show considerable variations in their reactivities towards the removal of halogen. The reasons for these variations and the kinetics of the reactions involved are currently being studied and possible mechanisms operative in different instances will be reported later.

References 1. 2.

Meyer, K., Bey. Dt. Chem. Ges., 1912, 45, 2843; 1914, 47, 826. Macbeth, A. K., J . Chem. SOL, 1923, 123, 122.

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February, 1978 SHORT PAPERS

3. Park, J . D., Brown, H. A., and Lacher, J . R., J . A m . Chem. SOL, 1953, 75, 4753. 4 . Reid, J. C., and Calvin, M., J . Am. Chem. SOL, 1950, 72 , 2948. 6. Reiss, W., Chem. Ber. , 1954, 87, 92. 6 . Allen, C. F. H., Abell, R. D., and Normington, J . B., Org. Synth., 1941, Collect. Vol. 1, 205. 7. Marchi, E. L., Inorg. Synth., 1946, 2, 14. 8. Temnikova, T. I., Zh. Obshch. Khim. , 1964. 34, 2845. 9. Bigelow, L. A., and Hanslick, R. S., Org. Synth., 1943, Collect. Vol. 2, 244.

185

Received Apri l 26th, 1977 Accepted July 12th, 1977

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