Question

There are two ways of making 2-ethoxyoctane from octan-2-ol using the Williamson ether synthesis. When pure (-) -octan-2-ol of specific rotation -8.24⁰is treated with sodium metal and then ethyl iodide, the product is 2-ethoxyoctane with a specific rotation of -15.6⁰. When pure (-) -octan-2-ol is treated with tosyl chloride and pyridine and then with sodium ethoxide, the product is also 2-ethoxyoctane. Predict the rotation of the 2-ethoxyoctane made using the tosylation/sodium ethoxide procedure, and propose a detailed mechanism to support your prediction.

Short answer

The most reliable and versatile method for synthesis of ethers is given by Williamson method. In this method,SN² attack of an alkoxide ion takes place on an unhindered primary alkyl halide or tosylate. The yield is poor if secondary alkyl halide is used. The alkoxide is generally made by adding Na, K,or NaH to the alcohol.

general form of Williamson ether synthesis

Step-by-step solution

Williamson ether synthesis

The most reliable and versatile method for synthesis of ethers is given by Williamson method. In this method,SN² attack of an alkoxide ion takes place on an unhindered primary alkyl halide or tosylate. The yield is poor if secondary alkyl halide is used. The alkoxide is generally made by adding Na, K,or NaH to the alcohol.

general form of Williamson ether synthesis

Mechanism

When pure (-)-octan-2-ol of specific rotation -8.24⁰ undergoes treatment with sodium metal and then ethyl iodide forms 2-ethoxyoctane with a specific rotation of -15.6⁰.

formation of (R)-2-ethoxyoctane from (-)-octan-2-ol

Again, when pure (-)-octan-2-ol undergoes treatment with tosyl chloride and pyridine and then with sodium ethoxide also forms 2-ethoxyoctane.

formation of (S)-2-ethoxyoctane from (-)-octan-2-ol

The product formed from the second reaction is (S)-2-ethoxyoctane. (S)-2-ethoxyoctane has the same value of specific rotation as its enantiomer but with opposite sign, (+15.6⁰).

SN²reaction showing inversion in configuration

This is a SN² reaction where inversion of configuration takes place. Nucleophile attacks from the opposite side of the leaving group to form (S)-2-ethoxyoctane.