Question

Cis-trans (E/Z) Isomers, EXCEPT

• A. 1-butene
• B. 2-butene-1-ol
• C. 2-chloro-3-hexene
• D. 4-chloro-2-pentene

Hint:

For a compound to show cis-trans (E/Z) isomerism, both double bonded carbon atoms must have two different groups attached. If either carbon has two identical groups, cis-trans isomerism is not possible.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Cis-trans, or E/Z, isomerism in an alkene needs both carbons of the carbon to carbon double bond to carry two different groups each. If even one of the two double bond carbons carries two identical groups, the molecule has no cis-trans isomers, because swapping the identical groups gives back the same structure.

Step 2: Key Formula or Approach:
The approach is to draw out each double bond carbon of every option and list the two groups attached to each side of that double bond, then check if either side repeats the same group twice.

Step 3: Detailed Explanation:
1-butene is CH2=CH-CH2-CH3. The left double bond carbon carries two hydrogen atoms, which are the same group repeated, so this molecule cannot have cis-trans isomers.
2-butene-1-ol is HOCH2-CH=CH-CH3. One double bond carbon holds hydrogen and CH2OH, the other holds hydrogen and CH3, and both sides have two different groups, so cis-trans isomerism is possible.
2-chloro-3-hexene is CH3-CHCl-CH=CH-CH2-CH3. One double bond carbon holds hydrogen and a chlorine bearing side chain, the other holds hydrogen and an ethyl group, so both sides again have two different groups and cis-trans isomerism is possible.
4-chloro-2-pentene is CH3-CH=CH-CHClCH3. One double bond carbon holds hydrogen and methyl, the other holds hydrogen and a chlorine bearing side chain, so both sides have two different groups and this alkene also shows cis-trans isomerism.

Step 4: Final Answer:
Only 1-butene fails the two different groups test, since one of its double bond carbons carries two identical hydrogen atoms, so 1-butene is the alkene that does not show cis-trans isomerism.