Williamson ether synthesis: practice and mechanism
Ethoxide attacks as a nucleophile via SN2: the carbon that carried the halogen now carries –O–CH₂CH₃, an ether.
Example
The alkoxide (ethoxide) attacks as a nucleophile via SN2 and an ether forms. With primary haloalkanes substitution wins; with tertiary ones elimination would dominate.
Mechanism step by step
- 1
The alkoxide (here ethoxide, CH₃CH₂O⁻) is made beforehand from ethanol and sodium or NaH.
- 2
The negatively charged oxygen attacks the primary haloalkane from the back (SN2).
- 3
The halide leaves and the ether R–O–CH₂CH₃ forms.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Here the ethoxide was attached through carbon. The nucleophile is the negatively charged oxygen, so an ether R–O–Et forms.
- That is the elimination product. With a primary haloalkane substitution wins; with tertiary haloalkanes elimination would be the main product.
Often confused with: SN2 with hydroxide · E2 (Zaitsev product)
Frequently asked questions
Why only with primary haloalkanes?
Alkoxides are strong bases. With secondary and especially tertiary haloalkanes they prefer to remove HX (E2) rather than substitute.
How do you make an ether with a tert-butyl group?
Use tert-butoxide as the nucleophile and the primary haloalkane as the electrophile, not the other way round.