Which of the following alkenes exhibits geometrical (cis–trans) isomerism?
- Pent-$1$-ene
- But-$2$-ene
- Propene
- $2$-methylpropene
Show answer and solution
Correct answer: B. But-$2$-ene
**Test.** For cis–trans isomerism each sp²-carbon of the C=C bond must carry two *different* groups. If either carbon has two of the same substituent, the two configurations are identical.
- **A. 2-methylpropene** $(CH_3)_2C=CH_2$ — left carbon has two CH₃ groups → no.
- **B. Propene** $CH_3CH=CH_2$ — the CH₂ end has two H → no.
- **C. But-2-ene** $CH_3CH=CHCH_3$ — each sp²-C carries one CH₃ and one H
→ **yes**, gives cis (Z) and trans (E).
- **D. Pent-1-ene** $CH_2=CHCH_2CH_2CH_3$ — terminal C has two H → no.
The rule of thumb: terminal alkenes never show cis–trans
internal alkenes do *only if* the two substituents on each end differ.