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Aldehyde & Ketone Synthesis by Oxidation of Alcohols

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

This lesson will explore the oxidation of alcohols to produce aldehydes and ketones. We'll explain what type of alcohols form which molecules and the applications of these reactions.

What Are Aldehydes and Ketones?

What do explosives, lacquers, sugars, and perfumes all have in common? These seemingly diverse compounds all contain aldehydes and ketones.

Aldehydes and ketones are any molecule with a carbonyl functional group, which is a carbon atom connected to an oxygen atom with a double bond. So, how do you tell aldehydes and ketones apart? Aldehydes have a carbonyl carbon connected to a hydrogen atom, and ketones have the carbonyl carbon connected to another carbon. The location of the carbonyl carbon gives aldehydes and ketones different properties and allows them to be synthesized into other different compounds depending on the carbonyl location.

Structure of aldehydes and ketones
aldehydes and ketones

Aldehydes and ketones are used to make many substances, such as industrial solvents used in perfumes, lacquers, textile dyes, varnishes, and even pharmaceuticals. One method of making aldehydes and ketones is to oxidize alcohols.

Oxidation of Alcohols

Alcohols are organic molecules with a carbon attached to a hydroxyl (OH) group. Alcohols can be classified based on the location of their functional group.

  • Primary alcohols have the alcohol group attached to a carbon at the end of the molecule, which is also attached to two hydrogen atoms.
  • Secondary alcohols have an alcohol group attached to a carbon, which is attached to only one hydrogen atom and another carbon.
  • Tertiary alcohols have an alcohol group attached to a carbon with no hydrogen atoms, and is instead attached to three other carbons.

Primary and secondary alcohols can be oxidized to make aldehydes and ketones. But, what is oxidation? Oxidation reactions are always paired with reduction reactions. The method of these reactions can be remembered by using the acronym 'Leo says Ger', which stands for 'lose electrons oxidize' and 'gain electrons reduce'. Thus, oxidation reactions are a type of reaction where one molecule gains electrons. The other molecule in the reaction is reduced, since it loses electrons.

During oxidation of alcohols to aldehydes and ketones, the oxidizing agent is usually sodium or potassium dichromate (VI) mixed with dilute sulfuric acid. Potassium dichromate (VI) is initially orange and changes to green upon reduction to chromium (III).

Starting color of potassium dichromate VI prior to reduction
potassium dichromate

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