Advices For Studying Organic Chemistry

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3) The overall result of second step is the reduction of HCrO 4 – to HCrO 3 – , a two
electron (2 e–) change in the oxidation state of chromium, from Cr(VI) to Cr(IV).
4) At the same time the alcohol undergoes a 2 e– oxidation to the ketone.
5) The remaining steps of the mechanism are complicated which involve further
oxidations (and disproportionations), ultimately, converting Cr(IV) compounds
to Cr3+ ions.

2. The aldehydes initially formed are easily oxidized to carboxylic acids in aqueous
   solutions.

1. The aldehyde initially formed from a 1° alcohol reacts with water to form an
   aldehyde hydrate.


2. The aldehyde hydrate can then react with HCrO 4 – (and H+) to form a chromate
   ester, and this can then be oxidized to the carboxylic acid.


3. In the absence of water (i.e., using PCC in CH 2 Cl 2 ), the aldehyde hydrate does
   not form ⇒ further oxidation does not happen.

Aldehyde hydrate

Carboxylic acid

H

C

R

O O

H

H H

H

O

C

R O−

+ H

HCrO 4 −

OH

O H

C

H

R

O Cr

O H

C

H

R

O

O

OH

O

H

H

O

C

R

O

H

+ HCrO 3 − + H 3 O+

, H+

3. The chromate ester from 3° alcohols does not bear a hydrogen that can be
   eliminated, and therefore no oxidation takes place.