SN2 reaction of alkoxide ions with alkyl halides to give ethers (Williamson synthesis)

by James

Description: Alkyl halides (or tosylates) will react with alkoxy ions to form ethers. This reaction is called the Williamson ether synthesis.

Notes: X here is a halide (Cl, Br, I) or sulfonate (OTs, OMs). The counter-ion on the alkoxy ion can be any alkali metal (e.g. Li, Na, K)

Examples: 

Notes: Note that since this is an SN2 reaction and proceeds via backside attack, there will be inversion of configuration at the carbon (note the last two examples). Note that in the second example that this ether would be difficult to make the opposite way (CH3O- attacking a tertiary alkyl bromide) since SN2 reactions don’t work on tertiary centers.

Mechanism: In the SN2 reaction the nucleophile (RO-) attacks the carbon with the good leaving group, forming a C–O bond and breaking the C–Br bond (Step 1, arrows A and B).

Notes: Again, the Na(+) is not crucial here, it’s just a spectator ion.

 

{ 5 comments… read them below or add one }

kadam vinayak January 30, 2012 at 11:24 am

u dnt explain why the reaction is taking place in sn2 manner.

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james February 3, 2012 at 4:05 am

True, although that’s a rather large subject. Here, note that it works best for primary (and to some extent secondary) alkyl halides.

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David March 6, 2012 at 7:33 pm

He doesn’t need to explain why it’s SN2 :)
When the halide group leaves, if the alkoxy ion doesn’t atake at the same time, you would be left with a primary carbo-cation. These are very unstable because you only have a carbon giving charge to the primary. If you had two more carbons linked to the carbon with the halide, you could have a SN1 reaction, since the carbons around the tertiary carbo-cation would be estabilizing the positive charge.

I hope I made myself clear. I’m Portuguese :)

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OrgoStudent December 4, 2013 at 6:09 pm

If you’re starting with an ether and trying to determine which starting materials are required, does it matter which halide you use for the leaving group?

Reply

James February 2, 2014 at 3:58 pm

It absolutely matters. Since the reaction is SN2, you want to set the reaction up so that your alkyl halide is (preferably) primary. You certainly wouldn’t want to try to get an SN2 to work on a tertiary alkyl bromide.

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