Using Reduction to Create Primary & Secondary 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 cover how reduction of carbonyl compounds can be carried out to form primary and secondary alcohols. In addition to reviewing the process, we'll also look at examples.

What Are Alcohols?

Many of us are familiar with the alcohol known as ethanol. It's the main ingredient in alcoholic beverages like beer and wine. It's also used in pharmaceuticals, fuels, and perfumes. But, there are other alcohols too. Alcohols are important parts of solvents, fuels, antifreeze, and cleaning solutions. Synthesis of alcohols is an important process in manufacturing too.

All alcohols have a hydroxyl group attached to a carbon that is part of an organic molecule. A hydroxyl group is a hydrogen attached to an oxygen molecule (OH). Alcohols have different classifications depending on where the hydroxyl group is located.

  • Primary alcohols are alcohols that have a hydroxyl group on a carbon that is also attached to two hydrogen atoms and another carbon.
  • Secondary alcohols are alcohols that have a hydroxyl group attached to a carbon that is attached to two other carbons and one hydrogen.
  • Tertiary alcohols have a hydroxyl group attached to a carbon that is bound to three other carbons.

Reduction of Carbonyl Compounds

Carbonyl compounds are molecules that have a carbon attached to an oxygen with a double bond. Carbonyl compounds have a polar covalent bonds, meaning that the electrons are shared unequally between two atoms. The oxygen atom is much more electronegative compared to the carbon atom, so it pulls the electrons closer, creating a partial positive charge on the carbonyl carbon and a partial negative charge on the oxygen atom. This creates a place for the nucleophile to attack during the reduction.

Three types of molecules containing carbonyl groups can be reduced to alcohols, carboxylic acids, esters, and ketones. Let's look at the reduction of each in detail next.

Carboxylic Acids

Carboxylic acids are carbonyl compounds where the carbonyl carbon is attached to a hydroxyl group and an organic molecule. Carboxylic acids can be reduced to primary alcohols using the reagent, lithium aluminum hydride in the presence of water and acid. First, the hydroxyl group is deprotonated by the aluminum. Then, the carbonyl carbon can undergo nucleophilic attack, which causes the oxygen of the hydroxyl group to leave. Then, the lithium can once again attack the carbonyl carbon, causing the oxygen atom to accumulate a negative charge. Lastly, the oxygen atom is protonated into an alcohol group.

carboxylic acid reduction

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