Chemistry » Organic Molecules » Addition, Elimination And Substitution Reactions

Elimination Reactions

Elimination reactions

  1. Dehydrohalogenation

    Fact:

    Reflux is a technique whereby a reaction solution is placed in a container with a small opening at the top. A tube that is constantly being cooled is connected to the opening. The solution is heated to approximately its boiling point, but any gases produced are condensed in the tube and fall back into the container. This way the reaction rate can be increased without the loss of material.

    In dehydrohalogenation a haloalkane is exposed to a base, the base then helps the elimination of the halogen and a hydrogen atom. A double bond is formed (alkane \(\to\) alkene). Dehydrohalogenation is considered the opposite of hydrohalogenation. The elimination of iodine from iodoethane is an example of dehydrohalogenation (see figure below).

    555748b5dcc3971834dc754387961161.png

    The dehydrohalogenation of iodoethane with potassium hydroxide (\(\text{KOH}\)) to form ethene, potassium iodide (\(\text{KI}\)) and water.

    In order for elimination to occur the following reaction conditions must be used:

    • heat under reflux (approximately \(\text{70}\)\(\text{°C}\))
    • a concentrated, strong base (e.g. \(\text{NaOH}\), \(\text{KOH}\))
    • the base must be dissolved in pure ethanol (hot ethanolic base)

    Dihalogenated saturated compounds can also undergo elimination reactions to become unsaturated, losing one halogen and one hydrogen atom. In the example below (see figure below), an atom of hydrogen and chlorine are eliminated from the original compound to form an unsaturated haloalkene and hydrochloric acid.

    989b15ac10e6090a57470539c70298ec.png The elimination of hydrochloric acid (\(\text{HCl}\)) from 1,2-dichloroethane to form chloroethene.

    If more than one product is possible the major product will be the compound where:

    • the hydrogen atom is removed from the \(\color{red}{\textbf{more substituted carbon atom}}\)
    • i.e. the carbon that is bonded to the most number of carbon atoms

    See the figure below.

    4457044dfeb6310f95919e5729e7b44f.png The dehydrohalogenation of 2-bromobutane to form but-2-ene (major product) and but-1-ene (minor product).

  2. Dehydration of an alcohol

    Fact:

    Sulfuric acid (\(\text{H}_{2}\text{SO}_{4}\)) is a dehydrating agent and is often used to prepare dried fruits.

    During the dehydration of an alcohol the hydroxyl (\(-\text{OH}\)) group and a hydrogen atom are eliminated from the reactant. A molecule of water is formed as a product in the reaction, along with an alkene (see figure below). This can be thought of as the reverse of a hydration (addition) reaction.

    3ebe240e1b7f89bd643c732aeddc1dd6.png

    The dehydration of ethanol to form ethene and \(\text{H}_{2}\text{O}\).

    Reaction conditions:

    • an excess of a strong acid catalyst (generally \(\text{H}_{2}\text{SO}_{4}\) or \(\text{H}_{3}\text{PO}_{4}\))
    • high temperature (approximately \(\text{180}\)\(\text{°C}\))

    If more than one elimination product is possible the major product will be the compound where:

    • the hydrogen atom is removed from the carbon atom bonded to the most number of carbon atoms (i.e., the \(\color{blue}{\textbf{more substituted carbon atom}}\))

    This is shown in the figure below.

    98991d8ef0ec8b12dde82c557e229175.png

    The dehydration of butan-2-ol to form but-2-ene (major product) and but-1-ene (minor product).

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