Wurtz Reaction

Wurtz reaction is a coupling reaction which is developed by Charles-Adolphe and used in organic chemistry and organometallic chemistry. Wurtz reaction aids in the industrial preparation of alkanes. The method is used to prepare symmetrical alkanes, it is not used for asymmetrical alkanes.

The reaction involves the exchange of halogen and metal with the involvement of radical species R to form a carbon-carbon bond arising in a nucleophilic substitution reaction. Wurtz reaction is one of the first name reactions in organic chemistry. It is a method to synthesize higher alkanes by a reaction between alkyl halides and metallic sodium in the presence of dry ether. 

Wurtz Reaction Equation

The basic Wurtz equation is R-X + 2Na + X-R→ R–R + 2NaX

in which X = halogen (Cl, Br, I)

As an example, we can obtain ethane by reacting methyl bromide with sodium in the presence of anhydrous ether or tetrahydrofuran. 

The reaction is:

2CH₃Br + 2Na → C₂H₆ +2NaBr 

Here, a large alkane molecule is developed by joining two compounds of alkyl halide and eradicating the halogen atoms in the form of sodium halide. 

In other examples, if two different types of alkyl halides are used in the reaction then a combination of three alkanes will be formed. In such a case, if methyl and ethyl iodides are used to react with sodium then a mixture of propane, butane and ethane will be formed, although it’s difficult to separate the alkanes from the mixture. Reactions that took place can be written as follows- 

2CH₃I + 2Na → C₂H₆ + 2NaI

CH₃I + 2Na + C₂H₅I → C₃H₈ + 2NaI

2 C₂H₅I +2Na → C₄H₁₀ + 2NaI

So Wurtz reaction is not considered for the synthesis of alkanes with the odd number of carbon atoms, as it provides a combination of non-separable alkanes. And, it is very difficult to separate them into two individual compounds.  The reaction shows productive results with primary alkyl iodides. The yields can be poor and with alkene contamination. 

Wurtz Reaction Mechanism

Depending on the condition, two types of mechanisms have been suggested for performing the Wurtz reaction.

• First Mechanism: By a formation of free radicals as an intermediate.

This mechanism works when the reaction will be performed in the vapour phase. The Vapour phase is considered a suitable phase for free radicals. The steps taken in the reaction are as follow:

Step 1: A transfer of one electron from a sodium atom makes a free radical of alkyl

R–X + Na → R• + Na⁺X^-

Step 2: In the second step, the second sodium atom releases one more electron to the free radical and provides a carbonium ion. 

R• + Na → R−Na⁺

Step 3: A halide ion is displaced by an alkyl anion from another molecule of alkyl halide.  This reaction is considered an SN2 reaction.

R−Na+ + R–X → R–R + Na⁺X^-

R• is a free radical which is highly reactive in nature because of the presence of unpaired electrons.

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• Second Mechanism: This mechanism uses an organometallic compound as an intermediate and the reaction is performed in a solution. 

CH₃-CH₂-I + 2Na• →CH₃-CH₂^-Na⁽⁺⁾ + NaI

CH₃-CH₂^-Na⁽⁺⁾ + CH₃-CH₂-I →C₂H₅- C₂H₅ +NaI

The mechanism is also used for the production of ethane and ethylene.

Wurtz Reaction Examples

The examples of the Wurtz reaction is given below:

• Alkyl halide in presence of dry ether medium when treated with sodium metal yields di-alkane.

R-X + 2 Na + X-R →R-R + 2 Na-X (Basic reaction)

R = alkyl group

X = halogen (F, Cl, Br, I)

• Dry ether is used to provide anhydrous condition as moisture and sodium metal react strongly in the presence of water.

Na + H₂O →NaOH + H₂

This reaction produces NaOH that reacts with an alkyl halide to produce alcohol 

R-X + OH^- →R-OH

E.g.: CH₃-Cl + Na →CH₃-CH₃ + NaCl

Limitations of Wurtz Reaction

Some limitations of the Wurtz reaction are given below.

• Methane (CH₄) is not prepared by using the Wurtz reaction because the number of carbon atoms is increasing every time in production. The minimum number of carbon atoms for the reaction should be two which does not apply in the case of methane. That means the lowest alkane developed through the Wurtz reaction is ethane. 

• Oxygen and moisture should not be allowed in the reaction medium, else sodium will be burnt by reacting with water and oxygen. Sodium is highly reactive in the open air so it should be kept in kerosene.

Na + H₂O →NaOH + H₂ (gas)

4Na + O₂ 2Na₂O

• The Wurtz reaction is restricted to the symmetric alkane’s synthesis. It is not applicable for the synthesis of two dissimilar alkyl halides as the product of these could be a combination of alkanes that are not easy to separate. If the reaction is between R-X and R'-X, it gives R-R and R'-R'- it will be difficult to separate the two products. They are not easily separable as they have a very low difference in their boiling points and need a close watch to be distinguished quickly.  

• Due to steric repulsion in alkyl groups, 3R-X does not provide a worthy yield of R-R. The Wurtz coupling method would generally fail when tertiary alkyl halides are used because tertiary alkyl halides include elimination reactions as side reactions. It happened due to steric-hindrance. 

• In the case of the Wurtz reaction, there exists a side reaction using which an alkene product is formed. In case the alkyl halides turn out to be bulky in nature, especially at the halogen end, there is a greater amount of alkene formed.

• The Wurtz reaction strictly needs anhydrous conditions as it forms an alkyl free radical in the reaction; this free radical is highly basic and can eliminate protons from water. Tetrahydrofuran is considered as a solvent in the place of ether when aryl and alkyl fluorides, and aryl chlorides are used. 

Wurtz-Fittig reaction

A modification in the Wurtz reaction is known as the Wurtz-Fittig reaction. It is a reaction that involves alkyl and aryl halides.  This reaction is performed with aryl halides and alkyl halides and Na metal in the presence of dry ether to give substituted aromatic compounds.

Example of Wurtz-Fittig reaction - 

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Hence, the Wurtz reaction is an important reaction that is used for the preparation of symmetrical alkanes. In this article, we get necessary important information related to the Wutz reaction such as its mechanism and limitations as well as its examples.