Carbonyl Compounds

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

Synthesis of Hemiacetals

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.

Hemiacetal Formation:

An aldehyde or ketone plus an alcohol may exist in equilibrium with a hemiacetal.

General Equation for Hemiacetal Formation:

Aldehyde + Alcohol <===> Hemiacetal

The hemiacetal group can be recognized by finding a carbon with BOTH an alcohol AND an ether functional group attached to it.

Synthesis of an Hemiacetal:

 

The process for carrying out this reaction is as follows:

The aldehyde and alcohol approach each other as follows because of the attraction of opposite charges. (C=O is partially positive and negative respectively. The O-H is partially negative and positive respectively. The reaction is written as an equilibrium because the hemiacetal is unstable and reverts back to the original aldehyde and alcohol.

1. The alcohol oxygen becomes bonded to the carbonyl carbon to form the
ether.
2. The carbon double bond oxygen electrons are used to bond the alcohol
hydrogen to the carbonyl oxygen.
3. The alcohol oxygen-hydrogen bond is broken.
The whole process can be remembered by saying: The aldehyde or ketone
oxygen becomes an alcohol. The alcohol oxygen becomes an ether.


Click for larger image 

Ring Structure for Glucose:

In reality, an aqueous sugar solution contains only 0.02% of the glucose in the chain form, the majority of the structure is in the cyclic chair form.

Since carbohydrates contain both alcohol and aldehyde or ketone functional groups, the straight-chain form is easily converted into he chair form - hemiacetal ring structure. Due to the tetrahedral geometry of carbons that ultimately make a 6 membered stable ring , the -OH on carbon #5 is converted into the ether linkage to close the ring with carbon #1. This makes a 6 member ring - five carbons and one oxygen.

Steps in the ring closure (hemiacetal synthesis):
1. The electrons on the alcohol oxygen are used to bond the carbon #1 to make an ether (red oxygen atom).
2. The hydrogen (green) is transferred to the carbonyl oxygen (green) to make a new alcohol group (green).
The chair structures are always written with the orientation depicted on the left to avoid confusion.
Hemiacetal Functional Group:
Carbon # 1 is now called the anomeric carbon and is the center of a hemiacetal functional group. A carbon that has both an ether oxygen and an alcohol group is a hemiacetal.

 

Open graphic of hemiacetal in a new window


Click for larger image 

Ring Structure for Fructose:

The chair form of fructose follows a similar pattern as that for glucose with a few exceptions. Since fructose has a ketone functional group, the ring closure occurs at carbon # 2. See the graphic on the left.

In the case of fructose a five membered ring is formed. The -OH on carbon #5 is converted into the ether linkage to close the ring with carbon #2. This makes a 5 member ring - four carbons and one oxygen.

Steps in the ring closure (hemiketal synthesis):
1. The electrons on the alcohol oxygen are used to bond the carbon #2 to make an ether (red oxygen atom).
2. The hydrogen (green) is transferred to the carbonyl oxygen (green) to make a new alcohol group (green).

The ring structure is written with the orientation depicted on the left for the monosaccharide and is consistent with the way the glucose is depicted.

Hemiketal Functional Group:

The anomeric carbon is the center of a hemiketal functional group. A carbon that has both an ether oxygen and an alcohol group (and is attached to two other carbons is a hemiketal.

Open graphic of hemiacetal/hemiketal in a new window