Wurtz reactions

One of the most widely investigated process in organic chemistry is oxidation. A large number of reactions involving oxidation are used in industries besides being of interest in the laboratory. A number of oxidizing agents with different substrates have been described.  

Oxidations have been known to be carried out in aqueous medium for a long time. The well known oxidation of arenes with KMnO in aqueous alkaline medium is very well known. However, the yields are considerably increased 

In the present unit, some innovative and recent oxidations by chemical reagents in aqueous media are given. Enz)miatic oxidations have also been known to occur in water. However, this subject will be discussed in a separate section. Following are given some of the important reactions in aqueous medium. 

A number of peracids can be used. The reaction takes place in nonpolar solvents such as dichloromethane and benzene. The above epoxidations are stereoselective and take place by syn addition to the double bond, as established by x-ray analysis of the products obtained. As the cis alkene gives only cis epoxide and trans alkene gives trans epoxide, the reaction must be concerted, i.e., the one step mechanism retains the stereochemistry of the starting alkenes. 

Using this method a variety of alkenes could be epoxidized. The epoxidation of electron-deficient olefins can be achieved with in presence 

Caron-carbon bond formation from the treatment of alkyl halides and sodium or magnesium metals. 

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This reaction is precisely parallel to the Wurtz Reaction in the aliphatic series, by which, for instance, n-butane can be obtained by the action of sodium on ethyl bromide. 

All hydrocarbons prepared by th -. Wurtz reaction contain small quantities of unsaturated hydrocarbons. These may be removed by shaking repeatedly with 10 pier cent, of the volume of concentrated sulphuric acid until the acid is no longer coloured (or is at most extremely pale yellow) each shaking should be of about 5 minutes duration. The hydrocarbon is washed with water, 10 pier cent sodium carbonate solution, water (twice), and dried with anhydrous magnesium or calcium sulphate. It is then distilled from sodium two distillations are usually necessary... 

Aliphatic hydrocarbons can be prepared by the reduction of the readily accessible ketones with amalgamated zinc and concentrated hydrochloric acid (Clemmensen method of reduction). This procedure is particularly valuable for the prep>aration of hydrocarbons wdth an odd number of carbon atoms where the Wurtz reaction cannot be applied with the higher hydrocarbons some secondary alcohol is produced, which must be removed by repeated distillation from sodium. 

Two mechanisms have been proposed for the Wurtz reaction (compare Section III,7) and for the Wurtz-Fittig reaction. According to one, sodium reacts with the alkyl halide to produce a sodium halide and a free radical, which subsequently undergoes coupling, disproportionation, etc. ... 

Williamson synthesis Wohl-Ziegler reaction Wolff rearrangement Wolff Kishner reduction Wurtz reaction Wurtz-Fittig reaction... 

As stated above, intermolecular coupling reactions between carbon atoms are of limited use. In the classical Wurtz reaction two identical primary alkyl iodide molecules are reduced by sodium. /i-Hectane for example, has been made by this method in 60%... 

The Wurtz reaction, which reties on in situ formation of an active organosodium species, is also usefiil for preparing tetraorganotin compounds and is practiced commercially. Yields are usually only fair and a variety of by-products, including ditins, also form ... 

A variant of the Wurtz reaction is the preparation of tetrabutyltin from activated magnesium chips, butyl chloride, and stannic chloride in a hydrocarbon mixture. Only a small amount of tetrahydrofuran is required for the reaction to proceed in high yield (86). 

This reaction, based on the Wurtz reaction, tends to go to completion and the yield of technically useful chlorosilane is low. 

Dehydration to 2-vinylthiophene is better achieved from 2-(2-thienyl) ethanol with powdered potassium hydroxide in the presence of copper than from 1-(2-thienyl) ethanol. a-Chloro-2-thienylpro-pane undergoes a Wurtz reaction with active iron to give 3,4-di-(2-thienyl) hexane in low yield, which has also been obtained through coupling with n-butyllithium. ... 

The coupling of alkyl halides 1 upon treatment with a metal, e.g. elemental sodium, to yield symmetrical alkanes 2, is called the Wurtz reaction. Aryl alkanes can be prepared by the Wurtz-Fittig reaction, i.e. the coupling of aryl halides with alkyl halides. 

In the case of an intramolecular Wurtz reaction less side-reactions are observed this variant is especially useful for the construction of strained carbon skeletons. For example bicyclobutane 5 has been prepared from l-bromo-3-chlorocyclobutane 4 in a yield of > 90% ... 

Cyclopropanes undergo a ready reaction with bromine to give 1,3-dibromopropane, and can in turn be formed from 1,3-dibromopropane by the Wurtz reaction. A variation... 

Somewhat similar observations have been made in the reaction of alkyl halides with sodium mirrors (the Wurtz reaction) in which alkyl coupling occurs. Thus, ethane formed on treatment of methyl iodide with sodium in a field of 20 G shows n.m.r. emission (Garst and Cox, 1970). The phase is consistent with polarization via T j-S mixing,... 

One type of Wurtz reaction that is quite useful is the closing of small rings,... 

It seems likely that the mechanism of the Wurtz reaction consists of two basic steps. The first is halogen-metal exchange to give an organometallic compound (RX -(- M —+ RM), which in many cases can be isolated (12-36). Following this, the organometallic compound reacts with a second molecule of alkyl halide (RX + RM —> RR). This reaction and its mechanism are considered in the next section (10-94). 

Next to the formation of Grignard reagents, the most important application of this reaction is the conversion of alkyl and aryl halides to organolithium compounds, but it has also been carried out with many other metals, (e.g., Na, Be, Zn, Hg, As, Sb, and Sn). With sodium, the Wurtz reaction (10-93) is an important side reaction. In some cases, where the reaction between a halide and a metal is too slow, an alloy of the metal with potassium or sodium can be used instead. The most important example is the preparation of tetraethyl lead from ethyl bromide and a Pb—Na alloy. 

The mechanism is not known with certainty. It seems likely that it is basically a two-step process, similar to that of the Wurtz reaction (10-93), which can be represented schematically by... 

The Wurtz reaction between bromobenzene and 1-bromobutane, sodium being the metal, needs to be carried out between 15 and 30°C. It is difficult to start the reaction below 15°C starting compounds will accumulate and the delayed start in a halogen medium, which is too concentrated, will lead to a detonation. Above 30°C the reaction is too violent and cannot be controlled. 

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