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:
A process for preparation of 3,4-dimethylbenzaldehyde
2. APPLICANT(S)
(a) NAME: Excel Industries Limited
(b) NATIONALITY: An Indian Company
(c) ADDRESS:
184-87, Swami Vivekanand Road, Jogeshwari(West), Mumbai 400 102, Maharashtra, India.
3. PREAMBLE TO THE DESCRIPTION:
The following COMPLETE specification particularly describes the nature of this invention and the manner in which it is to be performed.

A NOVEL PROCESS FOR PREPARATION OF
3,4-DIMETHYLBENZALDEHYDE
FIELD OF INVENTION
This invention relates to a novel process for preparation of 3,4-dimethylbenzaldehyde. More particularly, the present invention relates to a process for preparation of 3,4-dimethylbenzaldehyde by conversion of 4-bromo-o-xylene into corresponding 3,4-dimethylbenzaldehyde by Grignard reaction.
BACKGROUND AND PRIOR ART
3,4-Dimethylbenzaldehyde is a key intermediate for a variety of specialty chemicals, polymer additives, pharmaceuticals, agrochemicals, dyes etc.
Zhang Guofu et al. have reported preparation of 3,4-dimethylbenzaldehyde from 3,4-dimethyl benzyl alcohol [Chem. Comm. (Cambridge, UK) 49(72), 7908-7910; 2013]. Iodine catalyzed aerobic oxidation of alcohols to corresponding aldehydes in acetic acid has been reported in Fenzi Cuihua, 24(5), 406-410; 2010. Reaction of o-xylene with paraformaldehyde to yield 3,4-dimethylbenzaldehyde has been reported in the literature [J. Iran Chem. Soc, 7(2),487-494; 2010].
Hu, Y.L. et al have reported preparation of 3,4-dimethylbenzaldehyde from 3,4-dimethylbenzylchloride [J. Iran Chem. Soc, 7(2), 487-494;

2010]. Bio-oxidation of primary alcohols to aldehydes through hydrogen transfer to produce title compound has been reported in Eu. J. Org. Chem., (18), 3445-3448; 2010.
Preparation of title compound from 3,4-dimethylbenzylbromide has been reported in Syn. Comm., 40(8), 1106-1114; 2010. Method for selective oxidation of substituted toluenes by enzymatic biotransformation has been reported in PCT Int. Appl., 2003031634.
Although the outcome of enzymatic reactions is highly selective, the method is very sensitive and requires demanding process setup and expertise for implementing the same on a commercial scale. Sensitivity of biocatalytic reactions is a limiting factor and hence chemical methods are preferred over the same.
According to the inventors of present invention, the abovementioned methods for preparation of 3,4-dimethylbenzaldehyde reported in the prior art are not suitable for production on a commercial scale and hence there is a need for a more simple method for the same.
Present invention offers a simple chemical method for production of 3,4-dimethylbenzaldehyde by formylation of 4-bromo-o-xylene employing Grignard reaction, which is not reported in the prior art.

OBJECTS OF INVENTION
Main object of present invention is to provide a novel process for preparation of 3,4-dimethylbenzaldehyde which is suitable for production on a commercial scale.
Another object of invention is to provide a simple process for preparation of 3,4-dimethylbenzaldehyde which gives consistent results.
Another object of invention is to provide a simple process for preparation of 3,4-dimethylbenzaldehyde by formylation of 4-bromo-o-xylene by Grignard reaction.
DETAILED DESCRIPTION OF THE INVENTION
As per the invented process, 3,4-dimethylbenzaldehyde is prepared from 4-bromo-o-xylene by Grignard reaction using magnesium turnings, N,N-dimethylformamide which is an electrophile and a solvent. Iodine is used as an initiator.
The preferred solvent is tetrahydrofuran. Other ethereal solvents such as dimethyl ether, diethyl ether and methyl tetrahydrofurans can also be employed.
Grignard reaction is initiated using catalytic iodine. However, other standard Grignard initiators such as methyl iodide, ethyl iodide, dibromoethane etc. can also be used.

Grignard reagent is formed by treating 4-bromo-o-xylene with magnesium turnings in presence of initiator in a solvent such as tetrahydrofuran at 50-70°C.
Notably, the Grignard reaction initiation is observed only if the reaction is carried out in following sequence: Magnesium turnings, some quantity of tetrahydrofuran and initiator are taken in the reactor under inert atmosphere. The reaction mass is heated to 50-70°C., preferably at 55-60°C and a mixture of some quantity of 4-bromo-o-xylene & tetrahydrofuran is added dropwise with stirring to initiate the Grignard reaction. Then remaining quantity of 4-bromo-o-xylene and tetrahydrofuran mixture is added to the reaction mass at 55-60°C over a period of 5 hrs. After addition, the reaction mixture is heated to and maintained at 70°C with stirring for another 8 hrs to ensure completion of the reaction. The reaction mass is then cooled to -5°C. A mixture of N,N-dimethylformamide and tetrahydrofuran is added thereto over a period of 10 min maintaining the temperature below 0°C. Thereafter, the reaction mass is stirred for 1 hr at room temperature. The reaction mass is made acidic with aqueous HC1. The aqueous and organic layers are separated. The product from aqueous layer is extracted with tetrahydrofuran and mixed with the organic phase. The organic phase is then washed with 5% NaHC03 solution. Solvent is removed by distillation under reduced pressure to obtain 3,4-dimethylbenzaldehyde. Overall reaction can be presented as given below:


Grignard compound of formula 1 reacts with N,N-dimethylformamide to form compound of formula 2 as given below:

Upon treatment with aqueous HCL, the compound of formula 2 yields 3,4-dimethylbenzaldehyde.

Example disclosed herein is illustrative and not limitative to the scope of the claimed invention.

EXAMPLE-1
4.33 gm magnesium turnings, 75 ml tetrahydrofuran and 0.15 gm iodine were taken in the reactor. The mixture was stirred and heated to 57°C. A mixture of 5.6 gm 4-bromo-o-xylene & 5 ml tetrahydrofuran was added dropwise to the reactor to initiate Grignard reaction which initiates in about 15-20 min. Thereafter, a mixture of 23 gm 4-bromo-o-xylene & 20 ml tetrahydrofuran was added dropwise at 58-68°C over a period of 5 hrs. The reaction mass was maintained at about 70°C for 8 hrs. The reaction mass was then cooled to -5°C. A mixture of 9.71 gm N,N-dimethylformamide & 25 ml tetrahydrofuran was added in 10 min maintaining the temperature below 0°C. The reaction mass was stirred for 1 hr at room temperature. It was then cooled to 10°C and it was made acidic by adding 108 ml aqueous HC1(1:3). Aqueous and organic layers were separated. Aqueous layer was equilibrated with tetrahydrofuran and the extract was mixed with organic layer. The organic layer along with extract was washed with 5% NaHCO3 solution. Solvent was then removed by distillation under reduced pressure to obtain 20 gm 3,4-dimethylbenzaldehyde with 84% purity.

CLAIMS:
1. A process for preparation of 3,4-dimethylbenzaldehyde comprising steps of: (i) reacting 4-bromo-o-xylene with magnesium metal in presence of an initiator to obtain Grignard compound of formula 1; (ii) formylating the compound of formula 1 by reaction with N,N-dimethylformamide to form compound of formula 2; (iii) treating compound of formula 2 with aqueous HC1 to obtain 3,4-dimethylbenzaldehyde.

2. A process as claimed in claim 1, wherein the reaction in step (i) is carried out by:
(a) taking magnesium turnings, solvent and initiator in the reactor, stirring and heating the mixture to 50-70°C; (b) gradually adding 10-25% of the total quantity to be added of a mixture of 4-bromo-o-xylene and solvent to the reactor to initiate Grignard reaction; (c) after initiation of Grignard reaction, gradually adding remaining portion of mixture of 4-bromo-o-xylene and solvent at 50-70°C.; (d) maintaining the reaction mass at reflux temperature for about 8 hrs.

3. A process as claimed in claim 2, wherein the solvent used is selected from tetrahydrofuran, dimethyl ether, diethyl ether, methyl tetrahydrofuran.
4. A process as claimed in claim 2, wherein the initiator used is selected from iodine, methyl iodide, dibromoethane.
5. A process as claimed in claim 1, wherein the formylation reaction
in step (ii) is carried out by:
(a) cooling the reaction mass to -5°C; (b) adding a mixture of N,N-dimethylformamide & solvent maintaining the temperature below 0°C; (c) stirring the reaction mass for 1 hr at room temperature and then cooling to 10°C.
6. A process as claimed in claim 1, wherein the reaction in step (iii)
is carried out by:
(a) acidifying the reaction mass with aqueous HC1; (b) separating aqueous and organic layers and equilibrating aqueous layer with solvent to extract product dissolved therein; (c) washing organic layer and extract with NaHC03 solution; (d) removing solvent by distillation to obtain 3,4-dimethylbenzaldehyde.