Synthesis of 4-Methoxybenzaldehyde (Anisaldehyde)

EXPERIMENT 3

3. OXIDATION OF ALCOHOLS USING PYRIDINIUM CHLORO CHROMATE/SILICA GEL

Synthesis of 4-methoxybenzaldehyde (anisaldehyde)

3.1 EQUATION

3.2 INTRODUCTION

Pyridinium chloro chromate, together with the Jones’ and Collins’ reagents, are all derivatives of Cr(VI)trioxide and widely used for alcohol oxidation reactions. PCC is also known as the Corey-Suggs reagent and the structure is depicted below:

Some of the advantages of using PCC rather than the Jones’ and Collins reagents is: It is milder and less acidic than the Jones reagent, and promotes fewer side reactions due to overoxidation (selective oxidation of a primary alcohol to the aldehyde). PCC is easier and safer to prepare, shelf-stable, and more efficient than the Collins reagent (a big excess of Collins reagent may be required to lead to completion of the reaction).

In order to improve the tempo of the oxidation reaction using PCC, an accelerant is added. Most commonly used are molecular sieves, size 3Ǻ. Addition of acetic anhydride, sonication in an ultrasound bath and addition of acids, also organic acids like p-toluene sulphonic acid, acetic acid, ammonium acetate, trichloro acetic acid etc., are also used for this purpose. During the PCC reaction (like in all Cr(VI) reactions), a dark precipitate is formed. The dark matter consists of reduced chromium salts. Addition of solid material like silica, alumina, Celite®, magnesium sulfate, etc. allow the chromium residues to adsorb onto these materials and can then easily be filtered off.

Mechanism:

3.3 EXPERIMENTAL PROCEDURE

A mixture of pyridinium chlorochromate (x g, 12.8 mmol) and silica gel (2.77 g) was ground to a fine powder. The light orange mixture was dissolved in dichloromethane (30 mL). While swirling the orange suspension, 4-OMe-benzyl alcohol (Y mL, x g, 6.4 mmol) was added in one portion. Swirling was continued and the solution darkened after 5 min. On completion of the reaction [TLC (H:EtOAc; 5:5], the...