E-Z notation
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E-Z notation, or the E-Z convention, is the IUPAC preferred method of describing the stereochemistry of double bonds in organic chemistry. It is an extension of cis/trans notation that can be used to describe double bonds having three or four substituents.
Following a set of defined rules (Cahn-Ingold-Prelog priority rules), each substituent on a double bond is assigned a priority.
If the two groups of higher priority are on opposite sides of the double bond, the bond is assigned the configuration E (from entgegen, German: [ɛntˈɡeːɡən], the German word for "opposite").
If the two groups of higher priority are on the same side of the double bond, the bond is assigned the configuration Z (from zusammen, German: [tsuˈzamən], the German word for "together"). | | | (E)-But-2-ene | | (Z)-But-2-ene |
The letters E and Z are conventionally printed in italic type, within parentheses, and separated from the rest of the name with a hyphen. They are always printed as full capitals (not lower case or small capitals), but do not constitute the first letter of the name for English capitalization rules (as in the example above).
!14.2 to 14.6 Stereoisomerism-Stereochemistry - Definition and Introduction
Stereoisomerism occurs when two or more molecules have identical molecular formula AND the same structural formula (i.e. the atoms are arranged in the same order), BUT differ in their 2D or 3D spatial arrangements of their bonds - which means different spatial arrangement of the atoms - even though they are bonded in the same order!
There are two main sub-divisions of stereoisomerism, (a) E/Z (cis/trans geometrical) isomerism and (b) R/S optical isomerism. A 3rd section (c), looks at other stereochemical examples which do not readily fit into (a) or (b). 14.2(a) E/Z