The Wurtz Reaction

The Wurtz Reaction. One of the best-known methods for the preparation of hydrocarbons is the reaction of organic halides with metallic sodium  

The process is usually called the Wurtz reaction if alkyl halides are employed, the Fittig reaction if aryl halides are used, or the Wurtz-Fittig reaction if it is carried out with a mixture of alkyl and aryl halides. Although all the reactions lead to more than one product when a mixture of two different halides is employed, the Wurtz-Fittig reaction shows a remarkable tendency bo unite two dissimilar fragments, w-Butyl-benzene, for example, can be prepared from bromobenzene and n-butyl bromide in yields of 65 to 70 per cent.6 

There has been much discussion as to whether these reactions proceed by free radical or ionic paths. Certainly a heterogeneous reaction conducted in a nonpolar environment would not be expected to follow an ionic course, and indeed the appearance of benzene, biphenyl, o-phenyl-biphenyl, and triphenylene (I) from the reaction of bromobenzene with sodium does suggest a free radical reaction.6 

It has been pointed out by Morton, however, that although a free radical mechanism has been neither proved nor disproved, it is nevertheless possible to interpret the Wurtz reaction on the basis of the two steps, A and B,7 

7 Morton, Davidson, and Hakan, ibid., 64, 2242 (1942) Morton, Davidson, and Newey, ibid., 2240. 

---

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. 

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. ... 

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. 

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. 

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,... 

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. 

This is, of course, the Wurtz reaction, and support for such a mechanism involving carbanions (radicals may be involved under some conditions, however) is provided by the observation that in some cases it is possible, with optically active halides, to demonstrate inversion of configuration at the carbon atom undergoing nucleophilic attack. The carbanion, e.g. (61), can also act as a base and promote elimination ... 

The Grignard compounds react with varying ease with organic halides in the manner of the Wurtz reaction according to the equation ... 

We might expect that the reaction would lead to the formation of tetraphenylmethane from benzene and carbon tetrachloride in the presence of aluminium chloride, but this is not so. In this case the fourth Cl-atom remains in the reaction product. Triphenylchloromethane (C6H5)3CC1 has acquired extraordinary importance because, when applied in the Wurtz reaction, it made possible the discovery of the first free organic radicle (Gomberg, 1900). Compare p. 352. 

Many reducing agents are capable of severing a carbon-halogen bond. Cathodic cleavage provides perhaps the most versatile method, and has been put to excellent use. The electrochemical variation of the Wurtz reaction constitutes a powerful method for the construction of a variety of rings, particularly strained systems. Dramatic examples are provided by the assembly of bicyclobutane (308) [89], bicyclohexene (310) [90-92], [2.2.2]propellane (312) [93], spiropentane (316) [94], j -lactams 318 [95], and a variety of small-ring heterocycles (320) [96,97]. 

Coupling of Alkyl Halides. The Wurtz Reaction De-halogen-coupling... 

In the Wurtz Reaction, both halides are alkyl. For the Wurtz-Fittig Reaction, there is one alkyl and one aryl group, while in the Fittig Reaction, both coupling partners are aryl halides. 

A senes of cyclopolyarsines is known. They may be prepared by a generally useful reaction that is reminiscent of the Wurtz reaction of orgunomctallic chemistry ... 

---