Study On The Halogenation Of Aromatic Compounds And Synthesis Of Aromatic Aldehydes In The Water Phase

The halogenation of aromatic compounds which has a extensive applications in organic synthesis is a very important reaction in organic chemistry, particularly, in the application of industrial production of pharmaceutical intermediates. So its in-depth study will contribute to improve the production process, cost savings and the environment protection and so on, and has a great of significance.Traditional halogenation of aromatic compounds uses halogen as halogenating agent and the reaction is in organic solvents, that results in the high cost and serious pollution. In this paper, an unconventional method which was based on the think of green chemistry was selected without directly using halogen as halogenating agent,and the whole reaction process was carried out in the water phase.Firstly, in this paper, the halogenation of benzyl position of aromatic compounds in the water phase was studied, using methyl benzene, p-xylene and p-methyl benzoic acid as substrates. In order to carry on this reaction, we use visible light as the initiator of free radical chain process, water as reaction medium ,and hydrogen peroxide and hydrogen halide as the halogenating agent. And we investigated the influence of the light strength and the ratio of raw materials to this reaction. The results show that the quantity of the products enlarge gradually with the increase of the light strength; When the ratio of substrate, H₂O₂, HX or NaX is 1:2:1.5, in the irradiation of 125W UV lamp, the reaction was carried out at room temperature and continued for 15 hours, the yields of the products of benzyl halogenation(only the benzyl halide) are the highest namely 43%. From this experiment we also can see that it is very easy to carry on the reaction with methyl benzene and its activited derivatives. However, it is very hard to carry on this reaction with deactivated derivatives of methyl benzene.Secondly, the halogenation of aromatic ring in the water phase was studied, using p-toluenesulphonic acid as the starting material in the water phase through the bromide or chloride and desulphonation, then we got 2,6-dibromotoluene and 2,6-dichlorotoluene, respectively. Among them,we focused on studing the synthetic route and the optimum conditions of 2,6-dichlorotoluene, the experiments show that the best ratio of reactants (p-toluene sulfonic acid, sodium hypochlorite and hydrochloric acid ratio) was 1:4:8, reaction temperature should not be too high and must be controlled below 30℃, reaction time should not be longer than 24h, and a yield of 56.1% reached under such conditions .Thirdly, the synthesis of aromatic aldehydes in the water phase was studied. 2,6-dichlorotoluene was used as the raw material to synthesize 2,6- dichlorobenzaldehyde through two different methods:direct oxidation of methyl and methyl light halogenated,hydrolysis.One was chosen because of its simple operation with a higher yield of 85.2% .A novel method of the synthesis of 2,6-dichlorobenzaldehyde was proposed: hypochlorite used as chloride agent,then 2,6-dichlorotoluene was synthesized in the water phase via chlorination of the benzene ring and desulphonation,so, 2,6-dichlorotoluene was attained, then the product was used to synthesize 2,6-dichlorobenzaldehyde.The whole process was completed in the water phase,and the method provided with so many advantages such as:mild reaction conditions, high yields and environmental-harmless, and so on.Lastly, all of the products was characterized by spectral analysis and GC-MS analysis.