Na2Cr2O7 + H2SO4 + H2O is a strong oxidizing reagent used to convert alcohols into higher-oxidation-state products. When sodium dichromate dissolves in aqueous sulfuric acid, it forms chromic acid (H2CrO4), which is the species that actually carries out the oxidation. You’ll encounter this combination frequently in organic chemistry courses as one of the classic ways to oxidize primary alcohols all the way to carboxylic acids and secondary alcohols to ketones.
How Chromic Acid Forms
Sodium dichromate (Na2Cr2O7), potassium dichromate (K2Cr2O7), and chromium trioxide (CrO3) all produce the same active oxidant when mixed with aqueous acid: chromic acid, H2CrO4. The sulfuric acid provides the acidic, aqueous environment needed for this conversion. So when you see any of these chromium sources paired with H2SO4 and water, treat them as interchangeable. The important chemistry comes from chromic acid itself.
A closely related preparation called the Jones reagent uses CrO3 dissolved in aqueous sulfuric acid to achieve the same result. For exam purposes, Na2Cr2O7/H2SO4/H2O and Jones reagent are functionally equivalent.
What It Does to Primary Alcohols
Primary alcohols (R-CH2OH) are oxidized to carboxylic acids (R-COOH). This happens in two stages: the alcohol first becomes an aldehyde, then the aldehyde is oxidized further to the carboxylic acid. Under these reaction conditions, you cannot stop at the aldehyde. The water present in the mixture allows the aldehyde to form a hydrate, which chromic acid readily oxidizes a second time.
In a typical lab setup, the reaction is heated under reflux with excess dichromate to ensure the oxidation goes all the way. Reflux means the flask is fitted with a vertical condenser so any aldehyde that forms stays in the reaction vessel long enough to be pushed to the carboxylic acid stage. This is a key distinction from milder reagents like PCC, which lack water and stop cleanly at the aldehyde.
What It Does to Secondary Alcohols
Secondary alcohols (R2-CHOH) are oxidized to ketones (R2-C=O). Because ketones don’t have a hydrogen on the carbonyl carbon, they resist further oxidation under these conditions. The reaction stops at the ketone stage on its own, so no special precautions are needed.
Why Tertiary Alcohols Don’t React
Tertiary alcohols have no hydrogen atom on the carbon bearing the OH group, and oxidation requires removing that hydrogen. Chromic acid simply has nothing to work with, so tertiary alcohols remain unchanged. This difference in reactivity is actually useful as a diagnostic tool: you can use acidified dichromate to distinguish primary, secondary, and tertiary alcohols based on whether and how the reaction proceeds.
The Color Change
One of the most recognizable features of this reaction is the visible color shift. The chromium starts in the +6 oxidation state, which gives the solution a deep orange color. As it oxidizes the alcohol, chromium is reduced to the +3 state, turning the solution green. This orange-to-green color change is a straightforward visual confirmation that oxidation has occurred, and it’s the basis for the chromic acid test (also called the Jones test) used to identify oxidizable alcohols.
Milder Alternatives
If you need to oxidize a primary alcohol to just the aldehyde (stopping short of the carboxylic acid), you need a milder reagent. PCC (pyridinium chlorochromate) is the most commonly taught option. It works in anhydrous conditions, so no water is present to form the hydrate that would allow over-oxidation. PCC takes primary alcohols to aldehydes and secondary alcohols to ketones.
Other alternatives include the Dess-Martin periodinane and the Swern oxidation, both of which avoid chromium entirely. The toxicity and environmental hazards of hexavalent chromium compounds have pushed modern labs toward these greener methods. Sodium dichromate is classified as a human carcinogen, with strong evidence linking hexavalent chromium exposure to lung cancer. It is also highly toxic to aquatic life. For these reasons, chromium-based oxidations are increasingly replaced in research and industry, though they remain standard material in organic chemistry courses.
Quick Reference by Substrate
- Primary alcohol (R-CH2OH): oxidized to carboxylic acid (R-COOH)
- Secondary alcohol (R2-CHOH): oxidized to ketone (R2-C=O)
- Tertiary alcohol (R3-COH): no reaction
- Aldehyde (R-CHO): oxidized to carboxylic acid (R-COOH)