Naming alkenes is not as hard as it seems.
Way back when you were just an organic chemistry infant, you learned that alkenes existed and that they could be labeled either as a cis alkene or a trans alkene. It was nice, and you were happy not knowing that there were other, more sinister, alkenes out there that did not want to fall into such neat little categories. One of these diabolical olefins (another name for an alkene) is below:
Further into your organic chemistry odyssey, you are going to worry about how you might synthesize this molecule. Right now, you are merely concerned with what to name it. Is this a cis or trans alkene? Unfortunately, cis and trans do not apply to more complex alkenes, so we need a new system of nomenclature. Enter the Cahn/Ingold/Prelog system for naming double bonds.
The system itself is simple: assign the atom connected to the olefin a priority based on its atomic number then determine whether the two highest priority groups are on the same side of the double bond, or on opposite sides. For example:
In the above example, we compare Cl to F and quickly determine the Cl is of greater priority based on its atomic number. When the same comparison is made for H and CH3, we determine that C is the higher priority. Thus, the alkene above is an “E” alkene.
Further, there is a nice mnemonic used to remember which letter goes with which alkene:
– “Z” alkenes are on the “ZAME side”
– “E” alkenes are far “E-WAY” from each other.
While figuring out which type of alkene is present is easy when there are four different atoms connected to the alkene, we have to ask what would happen if there was a tie because there were two of the same atom on the alkene.
In the above alkene, we are posed with the problem of four carbon atoms bound to the double bond. Here, we can use the AceOrganicChem.com method for determining priority. For each substituent on the alkene, write out the atom attached directly to the alkene and each of the three atoms attached to it, in order of atomic number. For example, -CH3 would be C (H, H, H), and -CH2CH3 would be C (C, H, H). If we were to say what we were writing, an ethyl group of C (C, H, H) would translate to “a carbon with one carbon and two hydrogens attached to it”. Since they are already in the parentheses in order of atomic number, ties are easy to judge. If the first atoms in the parentheses are the same, just go to the second atom to break the tie. Thus, an isopropyl group with C (C, C, H) outranks an ethyl group with C (C, H, H)
Now, our molecule looks like this:
It can now easily be seen after the naming alkenes, this is an “E” alkene. This method can also be applied to other functionalities common found attached to olefins:
The trick to looking at these groups is to recognize that a double bond on the substituent is equal to two of that type of atom. For example, a cyano group would be equal to C (N, N, N). Another example of this would be as follows:
The only other trick to be aware of is a problem that has isotopes of the same atom. For example, deuterium is a higher priority than hydrogen because it is heavier. Therefore, the example below would be a “Z” alkene.
Take Home Message: When determining E/Z configuration of an alkene, use the C (X, X, X) method for determining priority, and remember that “Z” is the “ZAME side” and “E” is far “E-way” from the other group.