What is E-Z isomerism?

1 Answer
Jun 11, 2015

Answer:

#E-Z# isomerism is a type of stereoisomerism that exists because of restricted rotation about double bonds.

Explanation:

In stereoisomers, the atoms are joined in the same order, but they have a different spatial arrangement.

In #E-Z# isomers you must have:

  • restricted rotation, often involving a #"C=C"# double bond
  • two different groups on one end of the bond and two different groups on the other end.

An alkene such as but-2-ene has two different groups on each alkene carbon.

It can exist as #E-Z# isomers that differ in the positions of the substituents on the double-bonded atoms.

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The substituents can be given "priorities", with atoms with higher atomic numbers given higher priorities (the Cahn-Ingold-Prelog rules).

If the highest priority groups for each carbon are on the same side of the molecule, we have the #Z# isomer.

If the highest priority groups for each carbon are on opposite sides of the molecule, we have the #E# isomer.

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One way to remember the designations is to think of #Z# as Zame Zide (same side).

Since #"CH"_3# has higher priority than #"H"#, it is pretty easy to determine that the butene isomers above are #E# and #Z#, respectively.

But what about the isomers of 2-methylbut-2-enoic acid (below)?

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#"CH"_3# has higher priority than #"H"#, and #"COOH"# has higher priority than #"CH"_3#.

Angelic acid has the higher priority groups on the same side, so it is the #Z# isomer.

Tiglic acid has the higher priority groups on opposite sides, so it is the #E# isomer.