3-(2-Methoxyethoxy)prop-1-ene
The Williamson Ether synthesis continues to be an important reaction in organic synthesis. The procedure is simple and usually produces the desired compound in high overall yield. It is therefore frequently employed to introduce ether protective groups and ether functional groups in natural products.
The synthesis of 3-(2-methoxyethoxy)prop-1-ene from 2-methoxyethanol and allyl bromide is an excellent example of the procedure. In this case the electrophilic organobromide species is allyl bromide. Allyl bromide is much more reactive than a simple primary alkyl halide because the incipient positive charge that develops as the carbon undergoes nucleophilic attack can be delocalized by the π-orbital of the carbon-carbon double bond. Benzyl bromide also enjoys an increased reaction rate because the positive charge can be stabilized by the π-system of the aromatic ring. The Williamson Ether synthesis can proceed through either the SN1 or SN2 mechanism with these reagents, whereas simple primary alkyl halides such as ethyl bromide generally can only proceed through the SN2 mechanism.
Procedure: 3-(2-methoxyethoxy)prop-1-ene
A solution of potassium hydroxide (25.5 g, 0.45 mol) in dry 2-methoxyethanol (32.5 mL, 31.15 g, 0.41 mol) is cooled to 0 °C. The reaction temperature is maintained below 10 °C as allyl bromide (36 mL, 25.0 g, 0.41 mol) is added dropwise with rapid stirring over several minutes. The reaction is then allowed to warm slowly to room temperature and is stirred for 10 hours at ambient temperature. The resulting mixture is then filtered and the solids are washed with 150 mL pentane in small portions. The combined organic fractions are dried over magnesium sulfate. Fractional distillation over calcium hydride provides the title compound in fractions collected between approximately 115-129 °C.