CHE 230

University of Kentucky

Draw the elimination product

Draw in the H atoms on the electrophilic C atom and on all C atoms attached to it (Grossman's rule).

• If only one C atom adjacent to the electrophilic C atom bears a H, then follow these steps:

  • Erase the bond between the electrophilic C atom and the leaving group, erase the bond between the adjacent H-bearing C atom and its H atom, and create a new π bond between the two aforementioned C atoms.

  • Does the alkene that you have drawn have the possibility of E/Z stereoisomerism? If so, then then draw the lower-energy stereoisomer of the product unless all of these statements are true:
    • the conditions are basic (E2 mechanism);
    • the electrophilic C atom in the starting material is stereogenic; and,
    • the adjacent H-bearing C atom is also stereogenic.

    If all of these statements are true, determine whether the compound can achieve a conformation in which the C–X bond (X = leaving group) and the adjacent C–H bond are anti.

    • If so, draw the compound in this conformation. Groups that are gauche in this conformation become cis in the product.

    • If not, draw the compound in the conformation in which the C–X bond (X = leaving group) and the adjacent C–H bond are syn (eclipsed). Groups that are eclipsed in this conformation become cis in the product.

• If more than one C atom adjacent to the electrophilic C atom bears a H:

  1. Choose which adjacent C atom will lose a H atom.

     • The most substituted product is usually obtained predominantly (Zaitsev's rule). Loss of H from the C atom with the fewest H atoms already attached will give the most substituted product.

     • If the leaving group is particularly poor (Me₃N⁺, OH, F, even Cl) and the base is strong and particularly hindered (t-BuOK, LDA), then the least substituted product may be obtained predominantly (Hofmann's rule). If so, then the adjacent C atom with the most H atoms already attached loses the H atom.

     • If an adjacent C atom is particularly acidic (for example, because it is adjacent to a carbonyl group), that C atom will lose the H atom.

  2. If any of these statements is false:
     • the conditions are basic (E2 mechanism);
     • the electrophilic C atom in the starting material is stereogenic; or,
     • the adjacent H-bearing C atom is also stereogenic;

     then erase the bond between the electrophilic C atom and the leaving group, erase the bond between the adjacent H-bearing C atom that you have chosen and its H atom, and create a new π bond between the two aforementioned C atoms. If there is a possibility of E/Z isomerism in the product, draw the lower-energy stereoisomer of the product.

     If, on the other hand, all of these statements are true, determine whether the compound can achieve a conformation in which the C–X bond (X = leaving group) and the adjacent C–H bond that you have identified are anti.

     • If so, draw the compound in this conformation. Groups that are gauche in this conformation become cis in the product, and groups that are anti become trans.
     • If the compound cannot achieve such a conformation, then identify the C atom adjacent to the electrophilic C that has the next fewest H atoms attached. If it is also stereogenic, go back to the three statements above.

It is customary not to draw the product that contains the leaving group. If you do choose to draw it, the atom of the leaving group that was attached to the electrophilic C decreases its formal charge by one. However. if the conditions are acidic, and the leaving group is uncharged before it leaves, it may pick up an H atom before it leaves; if so, it is uncharged after it leaves.

The lower-energy stereoisomer is the one that has the sterically larger group on one C atom of the π bond trans to the sterically larger group on the other.

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