A molecule has a 8-carbon chain with no double bond and a separate 6-carbon chain that includes a C=C. Which chain must serve as the parent chain for IUPAC naming?
AThe 8-carbon chain, because IUPAC rules always select the longest continuous chain
BThe 6-carbon chain, because the parent chain must contain the double bond even if a longer all-single-bond chain exists
CEither chain may be selected, with the double bond named as a substituent on whichever chain is chosen
DThe 8-carbon chain, with the double bond assigned a separate locant as a branch
In alkene nomenclature, the parent chain must include the carbon-carbon double bond — this takes priority over chain length. If the only chain containing the C=C is shorter than another all-single-bond chain, the shorter chain containing the double bond is selected. The suffix changes from -ane to -ene and a locant identifies the lower-numbered carbon of the double bond.
Question 2 Multiple Choice
Consider (Z)-1-chloro-2-fluoroethene. At C1: Cl (Z=17) and H (Z=1), so Cl has higher priority. At C2: F (Z=9) and H (Z=1), so F has higher priority. The higher-priority groups (Cl and F) are on the same side. A student concludes 'Z always means the same side, just like cis.' When does this reasoning break down?
AIt breaks down for terminal alkenes, which are always assigned Z regardless of substituent arrangement
BIt breaks down whenever there are three or four different substituents, because cis/trans becomes ambiguous about which pair of groups to reference — E/Z uses CIP priorities to resolve this unambiguously
CIt breaks down only when one substituent has a heteroatom (O, N, or halogen), which reverses the E/Z designation
DIt never breaks down — Z and cis always describe identical compounds
The cis/trans system specifies 'same side' or 'opposite side,' but for trisubstituted or tetrasubstituted alkenes this is ambiguous: same side as what? The E/Z system eliminates ambiguity by using CIP priority rules to designate one group on each sp² carbon as 'higher priority,' then asks whether those two groups are on the same side (Z) or opposite sides (E). For unsymmetrical disubstituted alkenes, Z can correspond to what a student might call 'trans' based on group size if the smaller group happens to have higher atomic number — the two systems do not always agree.
Question 3 True / False
In IUPAC nomenclature of alkenes, the double bond position must be given the lowest possible locant when numbering the parent chain.
TTrue
FFalse
Answer: True
Correct. The carbon-carbon double bond is the principal characteristic group in alkenes, and numbering runs in the direction that gives it the lowest locant. For example, a double bond between C2 and C3 in a 6-carbon chain is named hex-2-ene (not hex-4-ene). If there is a tie, the same tiebreaker rules as in alkane nomenclature apply (lowest set of substituent locants).
Question 4 True / False
For any alkene, Z typically corresponds to the cis isomer and E generally corresponds to the trans isomer.
TTrue
FFalse
Answer: False
This correspondence holds only for symmetrically disubstituted alkenes (one substituent on each double-bond carbon, and the same two types of substituents repeated). Once three or four different groups are present, 'cis' and 'trans' are ambiguous, and E/Z may not align with size-based intuition. For example, if the smaller of two groups on a carbon has a higher atomic number (e.g., -F vs. -CH₂CH₃), CIP priorities and spatial 'size' order diverge, potentially producing an E designation for what looks 'cis' by inspection.
Question 5 Short Answer
Why is the E/Z system necessary for trisubstituted or tetrasubstituted alkenes, and what would go wrong if we relied on cis/trans labels instead?
Think about your answer, then reveal below.
Model answer: With three or four different groups on the double-bond carbons, 'cis' and 'trans' become ambiguous — they require specifying which pair of substituents is the reference, but no single pair is privileged. The E/Z system resolves this by applying CIP priority rules to each sp² carbon independently, selecting the higher-priority group on each, and asking whether those two groups are on the same side (Z) or opposite sides (E). This procedure is unambiguous regardless of the number of different substituents.
The cis/trans labels work reliably only when each double-bond carbon carries exactly one non-hydrogen substituent (making the choice of reference pair trivial). Any additional substituent creates multiple possible reference pairs with potentially contradictory assignments. The CIP-based E/Z system avoids this by enforcing a unique priority ranking at each carbon, giving one correct designation for any alkene structure.