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Chromic Acid Test for Aldehydes & Alcohols Mechanism

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  • 0:03 Measuring Alcohol Content
  • 0:53 General Mechanism
  • 1:37 Primary Alcohol Test Mechanism
  • 2:13 Aldehyde Test Mechanism
  • 3:16 Secondary & Tertiary…
  • 3:47 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

The chromic acid test is used to measure alcohols and aldehydes. We'll learn how this reaction occurs and what allows it to indicate the presence of alcohols and aldehydes.

Measuring Alcohol Content

When a police officer pulls over a potential drunk driver, a common test to see if the driver is drunk is the breathalyzer. At one point the breathalyzer contained the Jones reactant for the chromic acid test. Due to carcinogenic concerns it is no longer used in breathalyzers, but it's still used in some laboratory situations to test for aldehydes and alcohols.

The chromic acid test uses the Jones reactant to oxidize aldehydes and alcohols and reduce the chromic acid, resulting in a color change. It is able to identify aldehydes, primary alcohol, and secondary alcohol. However, it can't identify tertiary alcohols.

The Jones reactant is a mixture of chromium trioxide, sulfuric acid, and acetone. It's named after Sir Ewart Jones who discovered this reactant and mechanism.

General Mechanism

So how does it work? The chromic acid test starts out with a source of chromic acid. There are several ways chromic acid can be formed, but it's important to note that it has an oxidation state of 6+.

Chromic acid has several oxygens surrounding the chromium. These oxygens are able to act as a source of oxidation for the aldehyde or alcohol. Since chromium loses an oxygen it's reduced to an oxidation state of 3+.

When chromium has an oxidation state of 6+ it's an orange color, but when it has an oxidation state of 3+ it's a green-blue color. So, it's really easy to see if the reaction has occurred. The intensity of the color can then be measured using a spectrophotometer to know the concentration of aldehydes and alcohols.

Primary Alcohol Test Mechanism

Let's see how the mechanism works for a primary alcohol. With primary alcohols there are two carbon-hydrogen bonds. The chromic acid test eventually oxidizes both of these bonds into carbon-oxygen bonds. It first oxidizes the primary alcohol into an aldehyde, then the aldehyde gets oxidized into a carboxylic acid.

  • Electrons from the alcohol attack the chromium

  • A base removes the hydrogen; these electrons form a double bond between the carbon and oxygen, kicking off the chromic acid

Now an aldehyde is formed that can continue to undergo oxidation in these conditions.

Aldehyde Test Mechanism

Aldehydes are oxidized into carboxylic acids. This includes the aldehydes formed in the first part of the primary alcohol reaction.

It may not make sense, at first, why this mechanism is oxidized by chromic acid, since it's water that gives the additional oxygen to the aldehyde. But this reaction would not be possible without the chromic acid. In a neutral state the water would leave the aldehyde much faster than it would be added on. The chromic acid stabilizes the aldehyde, making it harder for the water to leave.

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