FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. Title of the invention - "AN IMPROVED PROCESS FOR THE
PREPARATION OF VINYLCHLOROFORMATE"

2. Applicant(s)
(a) NAME:
(b) NATIONALITY:
(c) ADDRESS:

ALEMBIC LIMITED
An Indian Company.
Alembic Campus, Alembic Road, Vadodara - 390 003, Gujarat, India.

3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

Field of the invention:
The present invention relates to an improved process for preparation of vinyl chloroformate (VCF) represented by a structural formula (I) as given below.

Background of the invention:
The molecular formula of vinyl chloroformate (VCF) is C3H3CIO2 and molecular weight is 106.51. CAS registry No. of VCF is [5130-24-5].
VCF is used in preparation of various monomers like Hema VC, Tris VC, Vinal acid, V2D25. These monomers are used as key ingredients for making contact lenses.
Frederick et al. first prepared VCF by thermal decomposition of glycol bis (chloroformate) such as ethylene glycol bis (chloroformate). The detailed process is given in US2377085 which is as given below in the scheme.

In this process, the temperature require for decomposition is very high and preferably greater than 460°C which otherwise give rise to formation of byproducts. Moreover, the yield is 33.3% and purity is also low.
2

US4210598 discloses a process for preparation of VCF which is as given below in the scheme.

The above process suffers with several drawbacks such as mercuric diacetaldehyde is not stable and it turns black in most of the events. In the process of phosgenation in second step, acetone-dry ice condenser to get -78°C temperature is required. After completion of reaction, the reaction mixture is distilled through vigreux column at 140°C. Distillation at such high temperature generates many impurities which may arise due to reaction between VCF and phosgene. The purity of VCF obtained by this process is very low.
Another process given in the example 14 discloses the process of preparation of VCF from chloromercuric acetaldehyde using phosgene and nitrobenzene as solvent. In this process, after completion of the reaction, the volatile constituents (VCF and phosgene) of mixture were evaporated off in vacuo and collected in a trap at -78°C. Distillation of trapped material gives VCF. This process is highly inconvenient because recovering VCF from mixture of VCF and phosgene is very difficult. Handling of phosgene while distillation, is very difficult at plant scale.
Therefore, these both processes are not feasible at industrial scale. Moreover, in this process impurity is generated and VCF obtained is never highly pure.
One more process is reported for the preparation of VCF in Journal of organic chemistry. (1978), 43(4), 752-4 that is as shown in the scheme below.
3

US4242280 discloses a process of preparation of chloromercuric acetaldehyde as disclosed in the following scheme.

This process use mercuric oxide and mercuric chloride. These both are costly reagent which overall increase the cost of preparation of chloromercuric acetaldehyde.
The routes reported hereinabove gives purity in the range of 85 to 90%. The given purity level is not acceptable for the preparation of contact lenses.
Hence, there is a need to develop a process which not only overcomes disadvantages of the prior art but also provide a process which is economical, operationally simple and industrially applicable.
The present inventors have directed their research work towards developing a process which increases purity and which is also feasible at industrial scale. Surprisingly they found an improved process for preparation of VCF which overcomes problems of existing art and also gives purity level higher than 99%.
Object of the invention:
The primary object of the present invention is to provide an improved process for the preparation of VCF.
4

Another object of the present invention is to provide an improved process for the preparation of VCF having purity greater than 99%.
A further object of the present invention is to provide a process which is easy to handle at an industrial scale.
Detailed description of the invention:
Accordingly, the present invention provides a process for preparation of VCF of formula (I)

comprising a step of
(i) reacting chloromercuric acetaldehyde (IV)

with chlorine free phosgene in the presence of solvent for time sufficient for the
reaction to take place;
(ii) removing phosgene from the reaction mixture before distillation of VCF; (iii) distillation of VCF.
The synthetic reaction scheme of the present invention is as shown below.
5

In the process of present invention, in first step, vinyl acetate (II) is reacted with mercuric chloride (III) in large excess of water and in the presence of base to give chloromercuric acetaldehyde (IV). The reaction is conducted at room temperature. Suitable base are alkali metal and alkaline earth metal salts of organic acids or of weak inorganic acids such as Na2C03, K2C03, CaC03, NaOAc, KOAc, alkali metal or alkaline earth metal hydroxides such as NaOH, KOH, slaked lime or basic inorganic oxides such as quicklime.
In second step, chloromercuric acetaldehyde is reacted with phosgene in a suitable solvent to give the VCF. Suitable organic solvent should be such that it can absorb phosgene. The example of suitable organic solvent includes but not limited to aromatic hydrocarbon such as nitrobenzene, toluene and the like or the mixture thereof. The most preferred organic solvent is nitrobenzene. The most important criteria for this process are that phosgene should be chlorine free. The reaction is carried out at temperature from about 50°C to 60°C. Generally, the reaction is carried out for 10-15 hr. The reaction is monitored on Gas chromatography (GC). After completion of the reaction, nitrogen gas is purged in order to remove phosgene from the reaction mixture. This process should be continued till at least most of the phosgene is removed from the mixture. Phosgene content in the reaction mixture should be less than 2.0%. VCF is fraction distilled. Butylated hydroxytoluene (BHT) is added in very small quantity to VCF before final distillation which acts as stabilizer. Final purification of VCF is obtained by distillation through glass packed column. The process of the present invention provides highly pure VCF having purity greater than 99%. This process is feasible on industrial scale.
6

The following examples illustrate the invention further. It should be understood, however, that the invention is not confined to the specific limitations set forth in the individual example but rather to the scope of the appended claims.
Example-1
Preparation of chloromercuric acetaldehyde (IV)
Mercuric Chloride (252.3Kg) was charged to DM water (735.0L) at 20°C and pH of slurry was checked which was around 3.0. CaC03 (46.95Kg) was added to the reaction mixture and stirred for 30 min. The pH of the reaction mixture was around 4.0 and colour of the reaction mixture turned pink from grey. Vinyl acetate (104.8Kg) was added to the reaction mixture slowly during 3-4 hr. The reaction was slightly exothermic and the colour of the reaction was changed to white from pink with the evolution of carbon dioxide. The reaction mixture was stirred for 3h at room temperature and then filtered. The solid was first washed with DM water till the filtrate was neutral and then with acetone (90.0L). The solid was suck dried and then dried in tray drier at less than 50°C to give the title compound (192.OKg) Yield: 80.53% Melting point: 125-135°C (dec)
Example-2
Preparation of vinyl chloroformate (VCF) (I)
To a stirred and cooled at 8°C solution of chloromercuric acetaldehyde (IV) (192 kg) in Nitrobenzene (810 kg) was purged phosgene gas till phosgene concentration was obtained 10 to 13 % within 6-8 hr. The reaction mixture was brought to room temperature and then heated to 55°C for 10-13 hr by steam heating under GC monitoring of the reaction till VCF content minimum 1% and phosgene content 3-4% was obtained. The reaction mixture was cooled to 25-30°C and nitrogen was purged under cooling system till phosgene content in reaction mixture was less then 2.0 %. VCF was fraction distilled from reaction mixture at 250-300mm Hg and at 30-120°C to get the pure VCF fraction (vapor temp 66-68°C) to which butylated hydroxytoluene (BHT) (200ppm) was added and again fraction distilled using glass packed column. The pure VCF was collected (33Kg) Yield: 42.94% Purity: 99.2% by HPLC
7

We claim:
1. A process for preparation of VCF of formula (I) having purity greater than 99%

comprising a step of:
(i) reacting chloromercuric acetaldehyde (IV)

with chlorine free phosgene in the presence of solvent for time sufficient for the reaction to take place;
(ii) removing phosgene from the reaction mixture before distillation of VCF; (iii) distillation of VCF.
2. The process as claimed in claim 1, wherein solvent is selected from aromatic hydrocarbon such as nitrobenzene, toluene or mixture thereof.
3. The process as claimed in claim 1, wherein phosgene removal is effected by purging nitrogen into the reaction mixture containing VCF and phosgene
Ashwini Sandu OfS. Majumdar & Co. Applicant's Agent
Dated this 31st day of May 2007

8

ABSTRACT
Title. " AN IMPROVED PROCESS FOR THE PREPARATION OF VINYLCHLOROFORMATE"
The present invention relates to an improved process for preparation of vinyl chloroformate (VCF) represented by a structural formula (I) as given below.

9