Introduction to Reactions and Mechanisms

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How2: Draw the mechanism for an electrophilic addition reaction

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Key Questions

  • The common things, if we look without considering properties, are the #sigma# and #pi# bonds. But the number of each, as well as the bond distance and properties are entirely different.

    Alkenes utilize #sp^2# hybridization (five #sigma# bonds and one #pi# bond on and around the #"C"="C"# bond), whereas alkynes utilize #sp# hybridization (three #sigma# bonds and two #pi# bonds on and around the #"C"-="C"# bond).

  • Uses of alkenes
    1.They are used as starting materials in the syntheses of alcohols, plastics, laquers, detergents, and fuels. The most important alkenes for the chemical industry are ethene ,
    propene and 1,3-butadiene.

    2.They can be used to make ethanol - alcohol - and polymers - plastics - two crucial products in today's world

    3.Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called

    Uses of Alkynes
    1. Acetylene is used in oxy-acetylene torch. When acetylene is burnt in sufficient oxygen we can get the temperature of around 3000oC which can be used to melt and join the metal parts.
    2. Chlorinated acetylene is used as solvent. Similarly acetylene is a starting material in the synthesis of polyvinyl chloride and polyacrylo nitrile which are used as polymers.
    3. Alkyne carbanions are very good nucleophiles and can be used to attack carbonyl compounds, alkyl halides to give addition product and thus they are important material in the chain expansion of organic compound.

  • Let's consider a comparison between the two transition states (alkene vs. alkyne) of a typical electrophilic addition reaction. When you do these, one way to catalyze them is with an acid, so let's look at the first few steps of the acid-catalyzed hydration of an alkene vs. an alkyne:

    (form of the transition state from Organic Chemistry, Paula Yurkanis Bruice)

    You can see that for the transition state of the alkyne, the hydrogen is not entirely bonded; it is "complexing" with the double bond, forming a #\mathbfpi# complex; "idle", until something breaks the interaction (the nucleophilic attack of the water) to get the molecule out of its unstable state.

    The complex is shaped like a cyclopropane analog, which is highly strained. Also, the high electron density in the double bond makes for some immensely disruptive repulsions that destabilize the transition state.

    This combination of a highly-strained ring structure and high electron density in the intermediate (transition state) makes alkynes less reactive than alkenes in electrophilic addition reactions. Pictorially, the energy of the transition state is higher on the reaction coordinate diagram.