Cross Wurtz Reaction: How Many Alkanes are Formed?
The Wurtz Reaction is a classic method in the Hydrocarbons chapter used to prepare symmetric alkanes containing an even number of carbon atoms.
But what happens if you try to use the Wurtz Reaction to prepare an alkane with an odd number of carbons? To do this, you must mix two different alkyl halides. This is known as a Cross Wurtz Reaction, and it comes with a major catch that JEE and NEET examiners love to test!
Video Tutorial: The Three Pathways
Watch Abhishek Sengar sir from CHEMCA break down the reaction between Bromomethane and Bromoethane, mapping out all three possible products.
Step-by-Step Problem Breakdown
Problem Statement:
How many alkane products are possible if a mixture of Bromomethane (CH3Br) and Bromoethane (C2H5Br) is treated with Sodium metal in dry ether?
In a beaker, you don't just have one molecule of each reactant; you have millions. The Sodium metal creates free radicals (or carbanions) of both alkyl groups. These highly reactive groups will collide and combine randomly.
Therefore, they can either couple with a copy of themselves (Self-Coupling) or couple with each other (Cross-Coupling).
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Pathway 1: Self-Coupling of Bromomethane
Two Methyl groups (-CH3) collide with each other.
CH3• + •CH3 → CH3-CH3 (Ethane) -
Pathway 2: Self-Coupling of Bromoethane
Two Ethyl groups (-C2H5) collide with each other.
C2H5• + •C2H5 → C4H10 (Butane) -
Pathway 3: Cross-Coupling (The target reaction)
One Methyl group collides with one Ethyl group.
CH3• + •C2H5 → CH3-CH2-CH3 (Propane)
Fig: The three parallel collision pathways occurring in the reaction vessel.
Practice Questions for JEE & NEET
Examiners love to ask "Assertion-Reason" questions about the practical utility of this reaction. Test your knowledge below!
Question 1: Why is the Wurtz Reaction considered an unsuitable method for preparing alkanes with an odd number of carbon atoms (like Propane or Pentane) in a laboratory setting?
Answer: Because it yields a mixture of alkanes that are very difficult to separate.
Reasoning:
To produce an odd-carbon alkane, you must use a mixture of two different alkyl halides (a Cross Wurtz Reaction). As proven above, this will always produce a mixture of 3 different alkanes (e.g., Ethane, Propane, and Butane).
These three alkanes are all non-polar homologues with very similar molecular masses. Because their boiling points are very close to one another, it is exceedingly difficult to separate them using fractional distillation. Therefore, the yield of the target odd-carbon alkane is very poor, making the method useless for practical synthesis.
Question 2: You attempt a Wurtz reaction using a Tertiary Alkyl Halide, such as 2-bromo-2-methylpropane (Tert-butyl bromide), with Sodium in dry ether. What will be the major product?
Answer: An Alkene (via Elimination).
Reasoning:
This is a major limitation of the Wurtz reaction! The reaction works beautifully for Primary (1°) alkyl halides. However, Tertiary (3°) alkyl halides are incredibly bulky.
Sodium metal acts not just as an electron donor, but also generates strong basic conditions. Because of the steric hindrance blocking the coupling process, the tertiary alkyl halide will undergo Dehydrohalogenation (β-elimination) instead of coupling. The major product will be an alkene (e.g., Isobutylene) rather than a higher alkane.
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