Skip to main content
Chemistry LibreTexts

18.11: Nucelophilic Addition of Alcohols (Acetal Formation)

  • Page ID
    183100
  • Introduction

    It has been demonstrated that water adds rapidly to the carbonyl function of aldehydes and ketones to form geminal-diol. In a similar reaction alcohols add reversibly to aldehydes and ketones to form hemiacetals (hemi, Greek, half). This reaction can continue by adding another alcohol to form an acetal. Hemiacetals and acetals are important functional groups because they appear in sugars.

    ch 19 sect 11 example 1.png

                                                                                 hemiacetal                         acetal

    To achieve effective hemiacetal or acetal formation, two additional features must be implemented. First, an acid catalyst must be used because alcohol is a weak nucleophile; and second, the water produced with the acetal must be removed from the reaction by a process such as a molecular sieves or a Dean-Stark trap. The latter is important, since acetal formation is reversible. Indeed, once pure hemiacetal or acetals are obtained they may be hydrolyzed back to their starting components by treatment with aqueous acid and an excess of water.

    ch 19 sect 11 dean stark.png

    Figure:  Dean-Stark Trap for Isolating Hemiacetals and Ketals

    Formation of Hemiacetals

    1.jpg

    Example: Formation of Hemiacetals

    2.jpg

    Example: Hemiacetal Reversibility

    3.jpg

    Formation of Acetals

    Acetals are geminal-diether derivatives of aldehydes or ketones, formed by reaction with two equivalents (or an excess amount) of an alcohol and elimination of water. Ketone derivatives of this kind were once called ketals, but modern usage has dropped that term. It is important to note that a hemiacetal is formed as an intermediate during the formation of an acetal.

    4.jpg

    Example: Formation of Acetals

    5.jpg

    Example: Acetal Reversibility

    6.jpg

    Mechanism for Hemiacetal and Acetal Formation

    The mechanism shown here applies to both acetal and hemiacetal formation

    1) Protonation of the carbonyl

    7.jpg

    2) Nucleophilic additional reaction by the alcohol

    8.jpg

    3) Deprotonation to form a hemiacetal

    4) Protonation of the alcohol

    10.jpg

    5) Removal of water

    11.jpg

    6) Nucleophilic addition reaction by the alcohol

    12.jpg

    7) Deprotonation by water

    Exercise

    18. Draw the products for the following reactions.

    ch 19 sect 11 exercise question.png

    Answer

    18.

    ch 19 sect 11 exercise solution.png

    Contributors