Question 26.9: Assignment of Configurations of Alkenes. Name each of the fo......

Analyze
To assign the configuration of the alkene, we must first assign the priorities to the substituents attached to each carbon in the double bond. Once this is done, we can assign the E or Z designation.

Solve
(a) One of the sp² carbon atoms is bonded to a Cl atom and a C atom; thus, the chlorine atom has highest priority. The other sp² carbon is bonded to an ethyl group and an isopropyl group. The carbon of the isopropyl group is bonded to C, C, and H, and the carbon of the ethyl group to C, H, and H. A carbon can be canceled in each group. Of the remaining atoms, the carbon has the highest priority. Thus, the isopropyl group takes precedence. The groups of highest priority are on the same side of the double bond. The complete name of the molecule is (Z)-1,2-dichloro-3-ethyl-4-methylpent-2-ene.
(b) One of the sp² carbon atoms is bonded to a fluoromethyl group and an isopropyl group. The carbon of the fluoromethyl group is bonded to F, H, and H; the carbon of the isopropyl group to C, C, and H. Of these six atoms, the fluorine has the highest priority. Thus, the fluoromethyl group takes precedence.
The other sp² carbon atom is bonded to an ethyl group and a chloroethyl group. Because the first point of difference on these substituent chains is a Cl atom, the chloroethyl group takes precedence. The groups of highest priority are on opposite sides of the double bond. The complete name of the molecule is (E)-2-chloro-3-ethyl-4-fluoromethyl-5-methylhex-3-ene.

Assess
For (b), the carbon atom bonded to chlorine is chiral and so, the molecule exists as either the R or S enantiomer.
It takes quite a bit of practice to master organic nomenclature and, admittedly, it is not the most exciting of topics. However, it is an important part of organic chemistry. There could very well be someone else who shares your name, but there is only one (Z)-1,2-dichloro-3-ethyl-4-methylpent-2-ene.