6.1: Cahn-Ingold-Prelog Convention
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Alkenes are hydrocarbons that contain a C=C double bond. We name them similarly to alkanes, but use the suffix –ene instead of –ane. We normally indicate the position of the double bond with a number. For example, 2-butene (or but-2-ene) and 1-butene (or but-1-ene) have different structures.
When substituents are placed along the chain, the priority rules much be followed. Alkenes are higher priority than halides, but lower priority than alcohols, for example. So:
There are three other groups that contain a C=C double bond which have common names:
E/Z nomenclature of alkenes
The Cahn-Ingold-Prelog convention dictates that we assign priority to the various groups attached to the alkene in order to distinguish between isomers. We use the prefixes E- (for entgegen, German for “opposite”) to indicate that the two highest priority groups lie on opposite sides of the alkene, or Z- (for zusammen, German for “same”) to indicate that the two highest priority groups lie on the same side. This E/Z system of nomenclature assigns priority based on atomic number:
- atoms that are directly attached to the alkene are assigned priority based on decreasing atomic number (highest atomic number = highest priority)
- if two groups have the same atomic number, consider atomic mass (higher atomic mass = higher priority)
- if two groups have the same atomic mass AND number, move along the chain to a different atom until there is a difference in priority
- if two groups have the same atomic number AND atomic mass AND lie in the same position along the chain (say, -CH2OH vs. –CH=O), but one of them is doubly bonded (such as C=O), then count that atom twice (as if it were attached to two oxygen atoms. But beware that this is not additive!)
- if two highest priority groups are on the same side = Z, opposite sides = E. Note that we do not use the terms cis and trans when naming alkenes, only when indicating the relative position of groups.
We call the Z and E isomers of the same compound geometric isomers, which is a class of stereoisomers known as diastereomers. These isomers do not have the same physical properties because they have different energies. In fact, E-but-2-ene is 0.7 kcal/mol more stable that Z-but-2-ene. There is a van der Waals interaction when two groups come into close contact, such as the cis methyl groups. This is steric strain that drives up the energy of the cis isomer.