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.

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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:

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There are three other groups that contain a C=C double bond which have common names:

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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.

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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.

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