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Dehydrating Alcohols to Make Alkenes

  • Page ID
    3901
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    This page discusses the dehydration of alcohols in the lab to make alkenes—for example, dehydrating ethanol to produce ethene.

    The dehydration of ethanol to give ethene

    This is a simple method of making gaseous alkenes such as ethene. If ethanol vapor is passed over heated aluminum oxide powder, the ethanol is essentially cracked to yield ethene and water vapor.

    OrganicCore_Alcohols13.png

    To produce a few test tubes of ethene, the following apparatus can be used:

    etheneprep.gif

    This system can be scaled up by boiling ethanol in a flask and passing the vapor over aluminum oxide that is heated in a long tube.

    Dehydration of alcohols using an acid catalyst

    The acid catalysts normally used in alcohol dehydration are either concentrated sulfuric acid or concentrated phosphoric(V) acid, H3PO4. Concentrated sulfuric acid produces messy results. Because sulfuric acid is also a strong oxidizing agent, it oxidizes some of the alcohol to carbon dioxide and is simultaneously reduced itself to sulfur dioxide. Both of these gases must be removed from the alkene. Sulfuric acid also reacts with the alcohol to produce a mass of carbon. There are other side reactions as well (not discussed here).

    The dehydration of ethanol to yield ethene

    In this process, ethanol is heated with an excess of concentrated sulfuric acid at a temperature of 170°C. The gases produced are passed through a sodium hydroxide solution to remove the carbon dioxide and sulfur dioxide produced from side reactions. The ethene is collected over water.

    OrganicCore_Alcohols14.png

    The concentrated sulfuric acid is a catalyst. Therefore, it is written over the reaction arrow rather than in the equation.

    The dehydration of cyclohexanol to yield cyclohexene

    This is a preparation commonly used to illustrate the formation and purification of a liquid product. The fact that the carbon atoms are joined in a ring has no bearing on the chemistry of the reaction. Cyclohexanol is heated with concentrated phosphoric(V) acid, and the liquid cyclohexene distils off and can be collected and purified. Phosphoric(V) acid tends to be used instead of sulfuric acid because it is safer and facilitates a less complex reaction.

    OrganicCore_Alcohols15.png

    The dehydration of more complicated alcohols

    With more complicated alcohols, the formation of more than one alkene is possible. Butan-2-ol is a good example of this, with three different alkenes formed when it is dehydrated.

    When an alcohol is dehydrated, the -OH group and a hydrogen atom from the next carbon atom in the chain are removed. With molecules like butan-2-ol, there are two possibilities for this.


    OrganicCore_Alcohols16.png

    The following products are formed:

    OrganicCore_Alcohols17.png

    The products are but-1-ene, CH2=CHCH2CH3, and but-2-ene, CH3CH=CHCH3.

    This situation is further complicated by the fact that but-2-ene exhibits geometric isomerism; thus, a mixture of two isomers is formed: cis-but-2-ene and trans-but-2-ene.

    OrganicCore_Alcohols18.png

    The compoundcis-but-2-ene is also known as (Z)-but-2-ene; trans-but-2-ene is also known as (E)-but-2-ene. Which isomer is formed is a matter of chance.

    Hence, the dehydration of butan-2-ol leads to a mixture containing the following compounds:

    • but-1-ene
    • cis-but-2-ene (also known as (Z)-but-2-ene)
    • trans-but-2-ene (also known as (E)-but-2-ene)

    Contributors

    Jim Clark (Chemguide.co.uk)


    This page titled Dehydrating Alcohols to Make Alkenes is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Jim Clark.

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