19) Now, build 1-butene using the molecular models, draw it in your notebook and explain why it does not make any sense to specify cis- and trans-1-butene while you must specify cis- and trans-2-butene in order to draw the correct molecule. For tri- and tetra-substituted alkenes, the E,Z-nomenclature system is used to designate specific stereoisomers. This nomenclature system uses the Cahn-Ingold-Prelog rules to designate the higher priority group on each of the sp2-carbons of the C=C. If the two higher priority groups are on opposite sides, then the prefix E- is used, and if the two higher priority groups are on the same side, then the prefix Z- is used. The sequence rules for specification of E,Z-configuration are:
a) Rank each atom attached to each sp2-carbon in order of decreasing atomic number; (for isotopes, > atomic mass = higher priority)
b) If the priority cannot be decided by rule 1, then the priorities are assigned at the first point of difference, by comparison of the next atoms out from the first carbon;
c) Multiply bonded atoms are considered to be equivalent to the number of singly bonded atoms.