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Introduction to Alkene Configurational Isomers

Configurational Stereoisomers of Alkenes

The carbon-carbon double bond is formed among two sp2 hybridized carbons, and contains two occupied molecular orbitals, a pi orbital and a sigma orbital. Rotation of the end groups of a double bond that are relative to each other destroys the p-orbital overlap which creates the bond or pi orbital. Due to the pi bond has bond energy of roughly 60 kcal/mole, this resistance to rotation stabilizes the planar configuration of this functional group. The result is, as a pair of configurational stereoisomers certain disubstituted alkenes may exist, often designated Trans and cis. For this stereoisomerism the major requirement is that each carbon of the double bond must have two different substituent groups (one may be hydrogen). This is demonstrated by the general formulas that are defined in the examples. In the first case, the left-hand side double bond carbon has two similar substituents (A) that's why stereoisomerism about the double bond is not possible (reversing substituents on the right-hand carbon gives the same configuration). In the other two cases, each double bond carbon atom has two distinct substituent groups and stereoisomerism exists, in spite of, whether the two substituents on one carbon are similar as those on the other.

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Some illustrations of this configurational stereoisomerism are displayed below in the diagram. Note: cycloalkenes smaller than eight carbons cannot exist in a stable trans configuration because of ring strain. Than the ten carbons a similar restriction that is holds against cycloalkynes is smaller. Because alkynes are linear, with the carbon-carbon triple bond there is no stereoisomerism associated.

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Nomenclature of Alkene Stereoisomers

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Configurational stereoisomers of the type shown above in the diagram require an additional nomenclature prefix added to the IUPAC name, in order to identify the spatial orientations of the groups attached to the double bond. So far, the prefixes cis- and trans- have served to differentiate stereoisomers; though, which isomer should be called trans and which cis it is not all the time clear. For an instance, refer the two compounds that are displayed on the right. Both compound A (1-bromo-1-chloropropene) and compound B (1-cyclobutyl-2-ethyl-3-methyl-1-butene) can be existing as a pair of configurational stereoisomers. Assignment of a trans or cis prefix to any of these isomers can simply done in an arbitrary manner, so a more rigorous technique is required. A totally unambiguous system, that is based on a set of group priority rules, assigns a Z (German, zusammen for together) or E (German, entgegen for opposite) to designate the stereoisomers. In the isomers that explained above, for which cis-trans notation was adequate, Z is equal to cis and E is equivalent to trans.

Sequence Rule for Alkene Configurations

Assign priorities to the double bond substituents by looking at the atoms attached directly to the double bond carbons.

1. Higher the atomic number of the immediate substituent atom, higher the priority.

For instance, H- < C- < N- < O- < Cl- (Priority increases from left to right)

(Different isotopes of similar element are assigned a priority according to their atomic mass.)

2. If two substituents have similar immediate substituent atom, move to next atom (away from the double bond) until a difference is found.

For instance, CH3- <  C2H5-  <  ClCH2-  <  BrCH2-  <  CH3O-.

Once the relative priorities of the two substituents on each one of the double bond carbons has been defined, a trans orientation is termed E and cis orientation of the higher priority pair is designated Z. While applying these rules to the isomers of compounds A and B shown above, we assign configuration of the 1-bromo-1-chloropropene isomer as E (Br has higher priority than the Cl and CH3 a higher priority than H). The configuration of the 1-cyclobutyl-2-ethyl-3-methyl-1-butene isomer is determined to be Z (C4H7 has higher priority than H, and isopropyl group has higher priority than an ethyl group). The subsequent instances explain the priority determination for a more complex case.

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