Questions: Alkene Structure, Nomenclature, and E/Z Isomerism

Question 1

Why do geometric isomers (E/Z) of alkenes exist as stable, separate compounds at room temperature, unlike conformational isomers of alkanes?

• A. The sp2 carbons are larger than sp3 carbons and cannot rotate
• B. Rotation around C=C would require breaking the pi bond, which has a significant energy barrier (~60 kcal/mol)
• C. Substituents on the double-bond carbons create steric strain that locks the geometry
• D. The sigma bond in alkenes is stronger than in alkanes, preventing rotation

Answer: (B)

The pi bond is formed by sideways overlap of p orbitals perpendicular to the C-C axis. Rotation would twist one p orbital out of alignment with the other, breaking the overlap and effectively breaking the pi bond. The energy required (~60 kcal/mol) far exceeds available thermal energy at room temperature, so the two geometric isomers are permanently locked in their configurations and are distinct compounds with different physical properties.

Question 2

For a trisubstituted alkene, the cis/trans naming system works just as well as E/Z because one can usually identify which groups are 'the same'.

• T. True
• F. False

Answer: False

Cis/trans nomenclature requires that each double-bond carbon bears two different substituents AND that one substituent on each carbon is the same group (to define 'same side' vs 'opposite side'). For a trisubstituted alkene — where one carbon bears two different groups and the other bears one group and one hydrogen — there is no unambiguous 'same' group to reference. The E/Z system using CIP priority rules assigns priority to any two different substituents unambiguously and always gives a definite answer.

Question 3

In CIP priority assignment for E/Z isomers, how do you determine which of two substituents attached to a double-bond carbon has higher priority?

Model answer: Compare the atomic numbers of the atoms directly bonded to the double-bond carbon. The substituent whose first atom has the higher atomic number gets higher priority. If those atoms are the same element, move outward to the next set of attached atoms and compare again, repeating until a difference is found.

For example, -Br beats -Cl because Br (Z=35) > Cl (Z=17). A -CH2Br substituent beats -CH2Cl for the same reason at the second atom. The key mistake is ranking substituents by overall 'size' or molecular weight rather than following the CIP algorithm strictly from the point of attachment outward.