The Grignard Method

The use of the Grignard reagents of the type RMgX for the production of alkyl-and aryl-chlorosilanes was pioneered by Kipping in 1904 and has been for a long time the favoured laboratory method for producing these materials. 

The reaction is carried out by first reacting the alkyl or aryl halide with magnesium shavings in an ether suspension and then treating with silicon tetrachloride (prepared by passing chlorine over heated silicon). With methyl chloride the following sequence of reactions occur  

The reaction proceeds in a stepwise manner but because of the differences in the reactivities of the intermediates a high yield of dichlorodimethylsilane is produced. 

The products are recovered from the reaction mixture by filtration to remove the magnesium chloride, followed by distillation. It is then necessary to distil fractionally the chlorosilanes produced. The fractional distillation is a difficult stage in the process because of the closeness of the boiling points of the chlorosilanes and some by-products Table 29.1) and 80-100 theoretical plates are necessary to effect satisfactory separation. 

The Grignard method was the first route used commercially in the production of silicone intermediates. Its great advantage is its extreme flexibility since a wide range of organic groups may be attached to the silicon in this method. Because of the need to use ether or other inflammable solvents considerable production hazards arise. On economic grounds the main drawbacks of the process are the multiplicity of steps and the dependence on silicon tetrachloride, which contains only 16% Si and is thus a rather inefficient source of this element. 

The most straightforward solution to the problem of producing methyl-, ethyl-, and phenylchlorosilanes would be to adapt the classical laboratory methods of synthesis to large-scale operation. A logical choice would be the Grignard reaction, long a laboratory favorite because it is so universally applicable. For the preparation of dimethyl silicone from inethyl chloride by the Grignard method, the steps would be  

In order to get back to the ultimate raw materials, which might be sand, coke, chlorine, and methane or methanol, some preliminary steps  

To be complete, the process also should include the preparation of magnesium from magnesium chloride or oxide  

No argument need be presented here for one or anothei of the optional methods given for the three preparations involved in equations 4 through 10. The choice of method usually is determined by local conditions and does not influence the general consideration of the 

Grignard process. Six steps remain necessary the three operations given in equations 1, 2, and 3, plus the preparations of magnesium, silicon tetrachloride, and methyl chloride. 

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Silicones and Other Heat-resisting Polymers 29.2.1 The Grignard Method... 

The synthesis and mechanism of formation of a triazene from an arenediazonium ion and an amine with one or two aliphatic substituents (see Scheme 13-1, R = alkyl, R = H or alkyl) will be discussed in Section 13.2. Here we will briefly mention Dimroth s method (1903, 1905 a) for synthesis of wholly aliphatic triazenes (Scheme 13-6, R and R = alkyl). Dimroth obtained these by the action of Grignard reagents on alkyl azides followed by isolation via copper(i) salts. The Grignard method can also be applied for the synthesis of triazenes with an aromatic substituent by using an aryl azide. 

Ansuer The Grignard method will be best for making (27) from secondary halide (28) as it avoids elimination reactions likely with cyanide ion. Disconnection (29a) now leads back to an alcohol (30) which can be made in a similar manner as (24). 

Advanced Example In research on violet perfumes, aldehyde (33) became a key Intermediate for a new series of compounds. Dlels-Alder disconnection gives acrolein and diene (34) which can he made by the Grignard method from available enone (36),... 

Previously published methods for the synthesis of dimethylzinc, a useful alkylating agent, include the reaction of dimethylmercury with metallic zinc,1 the reaction of a zinc-copper couple with methyl iodide,2 and the Grignard method.3 The reaction of trimethylaluminum with zinc(II) halides or alkoxides can be used,4 but it is more convenient to use zinc(ll) acetate, which is very readily obtained by dehydrating the commercial dihydrate with boiling acetic anhydride or by the reaction5 ... 

Formamide and other Derivatives of Formic Acid (Grignard).—This is the Grignard method of preparing aldehydes. The same remarks apply to this reaction as to the other Grignards dealt with. The equations illustrate its course. 

The direct hydrogenation to deoxidize fatty acid esters results in hydrocarbons with one carbon atom less than the number present in the hydrocarbon chain of the original ester. The reaction can be carried out quantitatively, but it is a relatively drastic chemical operation. In the Grignard method the hydrocarbon chain of the molecule is lengthened the reactions are executed at relatively low temperatures the overall yield amounts to 70-90% of the theory. 

Samples of triphenylborane are obtained readily by thermal decomposition of trimethylammonium tetraphenylborate prepared by the method described. Preparation of triphenylborane by this method eliminates the tedious and somewhat hazardous treatment of the reactive residues produced during the formation of the triphenylborane by the Grignard method.3... 

The Grignard method is generally applicable to the preparation of ji-allyl complexes, e.g. ... 

Given a carboxylic acid, write an equation for its synthesis by hydrolysis of a nitrile (cyanide) or by the Grignard method. 

The synthesis of 18 exemplifies the Grignard method. The reagent is made1 from the alkyl halide with magnesium metal in dry ether and combined, without isolation, with the electrophile— all steps being carried out under strictly anhydrous conditions. 

From 1914, the Grignard method of synthesis completely displaced the organozinc method and was widely used674. Up to 1960, fifty publications reporting the use of this method appeared125,675. 

Added importance centers on the dialkyldialkoxysilanes in view of the fact that they are satisfactory intermediates for the manufacture of silicon resins. These compounds may be made by a modification of the Grignard method without the use of ether as a solvent,58 and many of the alkyltriethoxysilanes listed in the compound index at the end of this chapter also were made this way.69 The allyltri-ethoxysilane so prepared apparently did not polymerize, but the phenylethynylsilanes prepared by the following reactions... 

With this discussion as a basis, the entire process for the preparation of a silicone polymer by the Grignard method can be shown in diagrammatic form in Fig. 1. This flow sheet does not specify the type of... 

If attention at first is confined to the production of methyl silicone from the previously accepted raw materials, the chemical processes must include reduction of silica to silicon, preparation of the methyl chloride from methane or methanol, reaction of the methyl chloride with silicon, and hydrolysis of the methylchlorosilanes. If the same conventions are used as in the discussion of the,Grignard method, and the methanol process for methyl chloride is elected, the steps are ... 

It is evident that this process is simpler in its requirements than the Grignard method and involves fewer chemical operations. Since the silicon need not be chlorinated, no free chlorine is required. The hydrochloric acid produced by the hydrolysis of dimethyldichloro-silane is exactly consumed by the reaction with methanol, so that two... 

As in other preparative methods for organosilicon compounds, the direct synthesis produces a mixture of methylchlorosilanes rather than the single compound shown in equation 3. Besides dimethyl-dichlorosilane, the mixture usually contains silicon tetrachloride, tri-chlorosilane, methyltrichlorosilane, methyldichlorosilane, trimethyl-chlorosilane, and even silicon tetramethyl. Under proper conditions, dimethyldichlorosilane is the principal product. Of the other compounds, methyltrichlorosilane usually is next in abundance this substance finds use in the cross-linked methyl silicone resins, or it can be methylated further by the Grignard method to increase the yield of dimethyldichlorosilane. There is no way of recycling it in the direct process, and so supplemental operations are required for the conversion. The interconversion of this and the other minor products of the direct synthesis, involving the exchange of methyl and chlorine groups as desired, has been a special study in itself.10... 

As was pointed out in the discussion of the Grignard method, a larger part of the chlorobenzene molecule appears in the finished silicone product than is true of the methyl chloride molecule. At the same price per pound for raw materials, the basic material cost for phenyl silicone therefore would be less than that for methyl silicone. The difference is accentuated by the fact that chlorobenzene is produced in very large volume at low cost, so that it becomes an inexpensive source of phenyl groups for phenyl silicone. On the other hand, the factors which act to increase the relative cost of phenyl silicone by the direct method are (1) the cost of recovering the silver catalyst, and (2) the possible uneconomical disposition of the hydrochloric acid, which cannot easily be recirculated in the process. 

Since moderate amounts of trihalosilane are useful in the cross-linked siloxane polymers, some or all of the quantity produced in the direct process can be used in this way. If large amounts of alkyl-or aryltrichlorosilancs are required, it would seem more economical to prepare them by the Grignard method or by the action of a mixture of chlorine or hydrogen chloride and organic chloride on silicon. 

The two disadvantages of the Grignard method, i.e, synthesis of the aromatic dibromide and the difficulty of initiating the reaction of an aryl chloride with magnesium, have led to a search for other routes to... 

Since the tin heterocycle has been generated by the Grignard method, the exchange reaction can be utilized to convert the tin derivative to either a Group III derivative or a Group V derivative. 

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