Carbonyl Compounds

Carbohydrates
 Elmhurst College
Synthesis - Aldehydes/Acids Reduction  Synthesis - Acetals  Chemistry Department
Synthesis - Ketones Synthesis - Hemiacetals  Virtual ChemBook

Synthesis of Acetals

Hemiacetal, Hemiketal, and Acetal Functional Groups:

In the simplest form, the hemiacetal is really the combination of two functional groups. A hemiacetal is an alcohol and ether ATTACHED TO THE SAME CARBON. The fourth bonding position is occupied by a hydrogen. A hemiacetal is derived from an aldehyde.

A hemiketal is an alcohol and ether attached to the same carbon, along with two other carbons. A hemiketal is derived from a ketone.

An acetal is two ether groups ATTACHED TO THE SAME CARBON. The acetal is derived from a hemiacetal and an alcohol making the second ether group.

Acetal Formation:

A hemiacetal plus an alcohol yields an acetal. At the simplest level this is really the reaction of an alcohol plus alcohol reaction to form an ether.

General Equation for Acetal Formation:

Hemiacetal + Alcohol <===> Acetal + HOH

The acetal group can be recognized by finding a carbon with TWO ether functional group attached to it.

Synthesis of an Acetal:

 

The process for carrying out this reaction is as follows:

a. The -H is removed from the hemiacetal and combined with an -OH from another alcohol to form water.

b. The two molecules are joined by an ether linkage. This is the link of the blue methyl group with the oxygen on the hemiacetal.


Acetal Functional Group:

Carbon # 1 (red on left) is called the anomeric carbon and is the center of an acetal functional group. A carbon that has two ether oxygens attached is an acetal.

The Alpha position is defined as the ether oxygen being on the opposite side of the ring as the C # 6. In the chair structure this results in a downward projection. This is the same definition as the -OH in a hemiacetal.