What is 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 industrial preparation of alkanes. The method is used to prepare symmetrical alkanes, it is not used for asymmetrical alkanes. The reaction involves exchange of halogen and metal with the involvement of radical species R to form carbon-carbon bond arising in 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.
The basic Wurtz equation is R-X + 2Na + X-R→ R–R + 2NaX
in which X = halogen such as (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 the joining two compound of alkyl halide and eradicate the halogen atoms in the form of sodium halide.
In other example, if two different type 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 mixture of propane, butane and ethane will be formed, although it’s difficult to separate the alkanes from the mixture. Reactions which 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 odd number of carbon atoms, as it provides a combination of non-separable alkanes. And, it is very difficult to separate them in two individual compounds. The reaction shows productive results with primary alkyl iodides. The yields can be poor and with alkene contamination. Depending on the condition, two types of mechanism have been suggested for performing Wurtz reaction.
Mechanism One: By a formation of free radical as an intermediate.
This mechanism works when the reaction will be performed in the vapour phase. Vapour phase is consider as a suitable phase for free radicals. The steps taken by in the reaction are as follow:
Step 1: A transfer of one electron from sodium atom makes a free radical of alkyl
R–X + Na → R• + Na+X−
Step 2: In second step, second sodium atom release one more electron to the free radical and provide a carbonium ion.
R• + Na → R−Na+
Step 3: A halide ion is displaced by alkyl anion from another molecule of alkyl halide. This reaction is considered as a 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 electron.
Mechanism Two: This mechanism used organo-metallic compound as 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 also used for the production of ethane and ethylene.
Wurtz Reaction Examples
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 reacts strongly in the presence of water.
Na + H2O —› NaOH + H2
This reaction produce NaOH that reacts with alkyl halide to produce alcohol
R-X + OH¯ —› R-OH
CH3-Cl + Na —› CH3-CH3 + NaCl
Methane (CH4) is not prepared by using Wurtz reaction because the number of carbon atoms is every time double 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 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 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 very low difference in their boiling points and needs 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 fails when tertiary alkyl halides are used because tertiary alkyl halides include elimination reactions as side reactions. It is happened due to steric-hindrance.
In case of 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 alkyl free radical in the reaction; this free radical is a highly basic and can eliminate proton 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 Wurtz reaction is known as Wurtz-Fittig reaction. It is a reaction that involves alkyl and aryl halides.
Example of Wurtz-fittig reaction -
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