Advices For Studying Organic Chemistry

2. Various oxidizing agents based on chromium(VI) have been used to oxidize 2°
   alcohols to ketones.

1. The most commonly used reagent is chromic acid (H 2 CrO 4 ).


2. Chromic acid is usually prepared by adding chromium(VI) oxide (CrO 3 ) or
   sodium dichromate (Na 2 Cr 2 O 7 ) to aqueous sulfuric acid.


3. Oxidation of 2° alcohols are generally carried out in acetone or acetic acid
   solutions.

CHOH CO+

R'

R

(^33)
R
R'
+H^22 CrO 4 + 6 H+ 2 Cr3+ +8H 2 O
4) As chromic acid oxidizes the alcohol to ketone, chromium is reduced from the
+6 oxidation state (H 2 CrO 4 ) to the +3 oxidation state (Cr3+).

3. Chromic acid oxidations of 2° alcohols generally give ketones in excellent yields
   if the temperature is controlled.
   OH H 2 CrO 4
   Acetone

O

35 oC
Cyclooctanol Cyclooctanone

4. The use of CrO 3 in aqueous acetone is usually called the Jones oxidation.
   
   
   1. Jones oxidation rarely affects double bonds present in the molecule.
   
   
   
   

12.4D MECHANISM OF CHROMATE OXIDATION

1. The mechanism of chromic acid oxidations of alcohols:
   
   
   1. The first step is the formation of a chromate ester of the alcohol.
   
   
   2. The chromate ester is unstable and is not isolated. It transfers a proton to a base
      (usually water) and simultaneously eliminates an HCrO 3 – ion.