conversion of alcohol to halide

Question

How do you convert a chiral, optically active secondary alcohol into an optically active halide that illustrates Zaitsevs rule? Also, how do you convert the halide in I1 into an ethyl ether?

Answer 1

Zaitsev's rule governs eliminations reactions to form alkenes. If you make an alkene from the alcohol, you lose the optical activity. You don't regain that optical activity when you convert the alkene to a halide, you get a racemate instead. There are advanced methods that use asymmetric catalysis to take alkenes to optically active halides and/or alcohols, but I will assume you are not asking about those. Let me know if I am wrong.

 

Going directly from the optically active alcohol to the optically active halide IS possible, by avoiding alkene formation. You can do this by using SOCl₂ (thionyl chloride), which will convert the alcohol directly to the chloride while inverting the stereocenter (SN₂ reaction, goes from R to S, or vice versa).

 

As for your second question, you can make the ethyl ether from the halide via SN₁ reaction in ethanol, otherwise known as solvolysis. This will give a racemate though, so you will lose optical activity again. If you try for direct substitution (using sodium ethoxide, for example), you will get the elimination product, an alkene, since the ethoxide is highly basic.

 

If you need clarification, please ask.