Introduction

 
 
                               
  ALDEHYDE                                        KETONE

Aldehydes and ketones react with Alcohol to give adducts called hemiacetals (hemi, Greek, half). These hemiacetals are intermediates on the way to acetals. Both acids and bases catalyze this process. Noteworthy is the fact that only acids transform hemiacetals to acetals by replacement of the hydroxy group with an alkoxy substituent. Bases fail to move beyond the hemiacetal stage.

HEMIACETAL                                        ACETAL

Hemiacetal Formation

ALDEHYDE                  AlCOHOL                              HEMIACETAL

 KETONE                    ALCOHOL                            HEMIACETAL

The addition reactions above are governed by equilibria that usually favor the starting materials. Therefore, Hemiacetals are usually not isolable. However, the exceptions are those formed from reactive carbonyl compounds such as formaldehyde or 2,2,2-trichloroacetaldehyde.

Furthermore, hemiacetals are also isolable if cyclization occurs in hydroxy aldehydes and ketones to form relatively strain-free five- and six-membered rings. This is also known as intramolecular hemiacetal formation.

Intramolecular Hemiacetal Formation

5-Hydroxypentanal                                    A cyclic, hemiacteal, stable

Intramolecular Hemiacetal formation is common in sugar chemistry. For example, the common sugar glucose exists in the cylcic manner more than 99% of the time in a mixture of aqueous solution.

Acids-Catalyzed Acetal Formation

In the presence of excess alcohol, the acid-catlayzed reaction of the carbonyl group in aldehydes and ketones proceeds beyond the hemiacetal stage. Under these circumstances, the hydroxy function of the initial adduct is replaced by another alkoxy unit derived from the alcohol. As mentioned and depicted before, the resulting compounds are called acetals. Note: Ketal is an older term for an acetal derived from a ketone.

Acetal Synthesis

KETONE                        2 moles of Alcohol                    Acetal                H2O

The overall/net change is the replacement of the carbonyl oxygen by two alkoxy groups. The formation of an equivalent of water is noteworthy as well.

THE MECHANISM

STEP 1. Hemiacetal Generation

    STEP 2. Acetal Generation

Every step in the Mechanism of Acetal Formation is reversible. The entire sequence is an equilibreum process. In the equilibreum, the entire result can be either the acetal, which is done by using excess alcohol, or it can be the carbonyl group, which is done by adding excess water; a process called acetal hydrolysis. However, in contrast with hydrates and hemiacetals, acetals may be isolated as pure substances by neutralizing the acid catalyst used in their formation.

References

1. Vollhardt, K. Peter C., and Neil E. Schore. Organic Chemistry: Structure and Function. New York: W.H. Freeman and Company, 2007 
2. Carey, Francis. Advanced Organic Chemistry. 5^th ed. Springer, 2007.

Outside Links

• http://en.wikipedia.org/wiki/Acetal
• http://en.wikipedia.org/wiki/Hemiacetal

Problems

1.      (via Basic Condiitions)

2.     

3.  Can an Acetal be isolated?

4. Can Hemiacetals be isolated?

5. Can Hydrates be isolated?

Answers

1.     
2.    
3.  Yes. They can be isolated if one removes the acid. If an acid is present however, the reverse reaction can take place
4. It depends. Hydration usually favors the carbonyl compund. The exceptions to this rule are carbonyl compounds with strong electron withdrawing groups. Strong electron withdrawing groups destabalize the carbo cation, thus making the reaction very exothermic, ultimately favoring the hemiacetal.
5. It depends. For the same reasons listed above.

Contributors

• Ekram Alexander and Ahmed Rahim (UCD)

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