EXPERIMENTAL PROCEDURE

Difluoromethane, which has a very low boiling point (- 52^0),

was stored in a large bulb (G) fitted with a side arm which could .;3

'1

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be cooled with liquid nitrogen. Bromotrichloromethane was,stored in a small tube attached to the main line*. This tube was also surrounded by liquid nitrogen. Each component was individually

'degassed* by allowing the frozen compound to warm until liquid. It was then frozen again and the tap to the tube or storage bulb opened to the main line and the pumps. This procedure was repeated several times for each compound.

Finally, the 'degassed' difluoromethane was allowed to expand

into the main line and bulb B, When a suitable pressure was registered, the bulb tap was closed and the difluoromethane in the line was r e ­

distilled back into the storage bulb arm by cooling in liquid nitrogen. The measured amount was then distilled from bulb B into the evacuated reaction vessel by cooling the reaction vessel with liquid

nitrogen. After sufficient time for complete distillation, a measured amount of bromotrichloromethane was added in a similar manner to the reaction vessel. The pre-heated furnace was then raised around the reaction vessel and the temperature allowed to stabilize. During this time the mercury arc lamp was struck and allowed to reach its normal working range before the shutter was removed from the illumination aperture in the furnace wall.

Because of the highly volatile nature of the difluoromethane more consistent injections into the analytical gas chromatogi*aph were obtained by adding a ballast liquid to t*he final reaction mixture. The ballast liquid must be well resolved in the final chromatogram and must not interact with any of the reaction products. A few series were carried out with trichloroothylene as ballast. This was w o11 separated from reaction components but in tho liquid

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phase, in the sample tube, the bromine formed in the reaction probably added to the trichloroothylene. This would account for the appearance of a peak in the chromatogram with a longer

retention time than hexachloroethane. With successive injections this unidentified peak enlarged. The use of trich1oroethy1ene as ballast was therefore abandoned in favour of tetrachloromethane

(carbon tetrachloride) which was adequately resolved from the reaction products and unlikely to take part in any solution phase reactions.

During the reaction .a measured amount, generally 10 ram. pressure in bulb B, of 'degassed' ballast was taken. At the end of the

reaction the reaction mixture was distilled into a small tube attached to the main line. After about five minutes the tap of bulb B was also opened and the ballast compound allowed to distil into the sample tube. Samples could then be taken from the.tube and injected into the chromatographic apparatus.

ANALYSIS

A Griffin and George D6 gas chromatograph was used for all quantitative analytical work. The instrument employs a Martin and James gas density balance for which the relationship q = KAM/M-m holds for all compounds (q = weight of compound with molecular

weight M, A = peak area on chromatogram, K = constant and m = molecular weight of the carrier gas which was nitrogen). Hence the concentration of any material is given by q/M = ICA/M-m and thus from a chromatogram where all the peaks are identified, relative concentrations are readily obtained. The most suitable stationary

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*

phase for the separation of halomethanes was found to be silicone oil which was used in 20% proportions supported on 60-100 mesh

'Embacel', The stainless steel columns used were 6' x 3/16". By a suitable choice of column temperature and nitrogen flow rates

a well resolved chromatogram was obtained.from the 4 (il injections made onto the column with the stainless steel capillary needle.

The chromatogram peak areas were, for a few experiments, measured by a fixed arm planimeter. Later the peak areas were measured using a Honeywell Precision Integrator with a threshold of 2% full scale deflection. A Du Pont 310 Curve resolver was

t

also employed to measure the area of the hexachloroethane and bromotrichloromethane peaks.

Identification of Products

A typical analytical chromatogram showed five peaks (fig 11 ), but the mass spectra chromatogram showed nine peaks. By comparing

the retention times of these peaks with those for authentic

difluoromethane, chloroform, bromotrichloromethane and hexachloro- ethane,- the peaks corresponding to these compounds could be

assigned in the analytical chromatogram. Retention time comparisons for dichloromethane, carbon tetrachloride and tetrachloroethylene further identified three of the extra peaks occurring in the mass spectra chromatogram. The fourth unknown peak in this chromatogram had a retention time which was comparable with that for the known dibromodichloromethane impurity in authentic bromotrichloromethane.

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of 60 Iran, of CH^Pg in 149 ml, with 22 mm., of CClgBr in 62 ml. This mixture was photolysed for 25 hours at 176°C with a high

pressure mercury arc lamp. Approximately 2 pi. of the final reaction mixture was injected into a Perkin-Elmer Ell chromatograph fitted with a 150 metre silicon oil capillary column held at 70°C with a helium pressure of 11 Ib./sq.in. The separated mixture was led

to the ionization chamber of an A.E.I, M.S.12 mass spectrometer, where the spectra were recorded with the following settings;- magnet range 5; decrease 9; ionization current 22 ev; acceleration potential 8 Kv; band width 500 c/s and chart speed 1.5 in;/s.

Peak 1

This was shown to be difluoromethane by comparison of retention time with that of an authentic sample.