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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

Nomenclature of Alkenes

• we'll start our next section with the nomenclature of alkenes. Just to remind you, alkenes are unsaturated hydrocarbons, meaning they contain at least one double bond, and are therefore not fully saturated with hydrogens. For each double bond they have 2 less hydrogens than the corresponding saturated hydrocarbon.
• the names of alkenes have the general ending -ene to indicate the presence of a double bond. If the position of the double bond is not obvious, as in ethene and propene, then you must tell where the double bond is. The number indicates the first carbon of the double bond. The numbering should be chosen to give the smallest number.
• what about branched alkenes? You must pick the longest carbon chain containing the double bond, and number it so that the double bond carbons get the lowest number possible. Substituents are named as usual, using the numbering for the alkenes. An interesting demonstration of this is 2-ethyl-3-methyl-1-butene, there is a 5 carbon chain in the molecule, but because it doesn't contain the double bond it doesn't get the parent name.
• if the double bond is equidistant from both ends, number so that the first branch point gets the lowest number.
• name the substituents in alphabetical order as usual
• if more than one double bond is present indicate the positions with numbers then use the suffixes "-adiene", "-atriene"...
• for cycloalkenes, the carbon atoms of the double bond are numbered 1 and 2 to give the substituent encountered first the lowest number. For example, 3-methylcyclopentene. Note that the numbering is implicit (the double bond is between 1 and 2) and you don't have to include it
• many alkenes have common names that are still used. By common names I mean non-IUPAC names. Unfortunately, they tend to be highly used, so you should become familiar with them. Let's look at a few important ones. The common name for ethene is ethylene, propene goes by propeylene. A vinyl group is an ethenyl, an allyl group is a 2-propenyl group. Another important one that I will come back to next week is isoprene, which is 2-methyl-1,3-butadiene.

2. Cis-Trans and E,Z Naming:

• let's have a look for a minute at a simple alkene, 2-butene. You can make it using the grey sp² pieces for carbon number 2 and 3. As you make it, you will notice that there are two ways that you can put the last methyl group on, so that it's in the same side as the other one, or the other side.
• so clearly we will have to include something in our nomenclature that indicates this. One system is the cis-trans system, much like we used for cycloalkanes. For very simple alkenes, this can be obvious. For the case of 2-butene, the one with the methyl groups on the same side is called cis and the other one is trans.
• for more complicated alkenes the designation of cis and trans becomes ambiguous.
• to get around ambiguities we have the E,Z system. This is the accepted IUPAC way of naming alkenes.
• in this system each of the 4 groups around a given double bond is assigned a priority. Priority is based on atomic number. The higher the atomic number the higher the priority.
• if the two highest priority groups are on the same side, the designation Z is used. If they are on the opposite sides, the designation is E. The letter is given in parentheses, separated from the rest of the name by a hyphen.
• often it's not the first atom out from the double bond carbon that determines priority. If the first atom out is the same, move out until a different is observed, and assign based on higher atomic number.
• there is one fairly nasty part to this, and that is when you run across another double bond as you move out in a group. In this case, the double bond is equivalent to bonded to two carbon atoms when it comes to priority. Same thing applies for carbonyl groups and triple bonds.