Grignard Reagent Reacts with Ethylene Oxide: The Step-Up Reaction
We know that adding a Grignard Reagent to an aldehyde or ketone is a fantastic way to synthesize alcohols. However, there is a very special reaction using a cyclic ether called Ethylene Oxide (Oxirane) that examiners love to ask about in JEE and NEET.
Why? Because it provides a unique way to create a primary (1°) alcohol while stepping up the carbon chain by exactly two carbon atoms. Let's break down the ring-opening mechanism!
Video Tutorial: The Ring-Opening Mechanism
Watch Abhishek Sengar sir from CHEMCA demonstrate the nucleophilic attack of Ethyl Magnesium Bromide on Ethylene Oxide to find the final organic product.
Step-by-Step Reaction Breakdown
When a Grignard reagent (R-MgX) reacts with Ethylene Oxide, the resulting product is always a primary alcohol with the formula R-CH2-CH2-OH. It perfectly extends the alkyl chain by exactly two -CH2- units.
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Nucleophilic Attack & Ring Opening:
Ethylene Oxide (Oxirane) is a 3-membered ring. Due to extreme angle strain, it is highly reactive. The nucleophilic carbon of the Grignard reagent (Ethyl Magnesium Bromide) attacks one of the CH2 carbons in the ring. This breaks the C-O bond, popping the ring open to relieve the strain.
CH3-CH2-MgBr + ΔO → (Dry Ether) CH3-CH2-CH2-CH2-O-MgBr+(The triangle represents the oxirane ring. The resulting open-chain structure is Product A). -
Acidic Hydrolysis:
We add dilute acid (H3O+) to the alkoxide intermediate. The negatively charged oxygen grabs a proton, turning into a terminal hydroxyl (-OH) group.
CH3-CH2-CH2-CH2-O-MgBr+ → (H+ / H2O) CH3-CH2-CH2-CH2-OH
Fig: Ring-opening mechanism. The Grignard provides 2 carbons, and the ring provides 2 carbons.
Total Carbons in Ethylene Oxide = 2
Final Product = 1-Butanol (4 Carbons)
Practice Questions for JEE & NEET
Test your ability to predict Grignard products by manipulating the carbon chain length.
Question 1: What would be the final product if you reacted Methyl Magnesium Bromide (CH3MgBr) with Ethylene Oxide, followed by hydrolysis?
Answer: Propan-1-ol (1-Propanol)
Reasoning:
- The Grignard reagent provides 1 carbon (CH3-).
- Ethylene Oxide always adds exactly 2 carbons (-CH2-CH2-OH) to the end of the chain.
- Combining them gives: CH3-CH2-CH2-OH.
- This is a 3-carbon primary alcohol: Propan-1-ol.
Question 2: Suppose you want to convert Ethyl Magnesium Bromide into Propan-1-ol (a 3-carbon primary alcohol). Since Ethylene oxide adds two carbons, it would overshoot the target. Which reagent should you react the Grignard with to step up the chain by exactly one carbon to form a primary alcohol?
Answer: Formaldehyde (Methanal, HCHO)
Reasoning:
- To add exactly 1 carbon and ensure the hydroxyl group stays at the terminal end (primary alcohol), we must use the simplest aldehyde: Formaldehyde.
- Reaction: CH3CH2MgBr + HCHO → CH3CH2-CH2-OH.
- (Note: Any other aldehyde would result in a secondary alcohol, and ketones result in tertiary alcohols!)
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