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

Cis-trans (E/Z) isomerism is a type of geometrical isomerism seen in alkenes. It occurs only when:
1. There is a C=C double bond, and
2. Each of the double bonded carbon atoms is attached to two different groups (i.e., different priority substituents).
Let’s evaluate the options one by one based on this rule:
(a) 1-butene: Structure: CH\textsubscript{2}=CH–CH\textsubscript{2}–CH\textsubscript{3}
Here, one of the double-bonded carbon atoms (CH\textsubscript{2}) is attached to two identical hydrogen atoms.
Hence, cis-trans isomerism is not possible in 1-butene.
(b) 2-butene-1-ol: Structure: HO–CH\textsubscript{2}–CH=CH–CH\textsubscript{3}
The double bond is between C2 and C3. C2 is attached to H and CH\textsubscript{2}OH; C3 is attached to H and CH\textsubscript{3}.
Since both double bonded carbons have two different groups, cis-trans (E/Z) isomerism is possible.
(c) 2-chloro-3-hexene: Structure: CH\textsubscript{3}–CH(Cl)–CH=CH–CH\textsubscript{2}–CH\textsubscript{3}
C3 (CH=) is bonded to C2 (which bears a Cl) and to C4; C4 is attached to two different groups (CH\textsubscript{2}CH\textsubscript{3} and H), and similarly for C3.
Hence, E/Z isomerism is possible.
(d) 4-chloro-2-pentene: Structure: CH\textsubscript{3}–CH=CH–CH(Cl)–CH\textsubscript{3}
The double bond is between C2 and C3. Check the groups attached:
– C2 is attached to CH\textsubscript{3} and H (fine), but
– C3 is attached to CH(Cl) and H. Here, CH(Cl) and CH\textsubscript{3} may seem different, but the priority order fails due to symmetry on further expansion, making it ambiguous.
However, due to the similar environment and branching, geometrical isomerism is restricted or not well-defined.
Hence, E/Z isomerism is not reliably possible here.

Answer 2

Cis-trans, or E/Z, isomerism appears in an alkene only when each of the two double bond carbons carries two different groups. If either double bond carbon has two identical groups attached, that alkene cannot show cis-trans isomerism. Checking each option against this rule identifies the one alkene that is the exception.

1. 1-butene: The structure is CH2=CH-CH2-CH3. The first double bond carbon carries two hydrogen atoms, which are identical to each other. Since one of the double bond carbons does not have two different groups, this alkene cannot show cis-trans isomerism.
2. 2-butene-1-ol: The structure is HOCH2-CH=CH-CH3. One double bond carbon carries hydrogen and a CH2OH group, and the other carries hydrogen and a methyl group. Both double bond carbons carry two different groups, so this molecule does show cis-trans isomerism.
3. 2-chloro-3-hexene: The structure is CH3-CHCl-CH=CH-CH2-CH3. One double bond carbon carries hydrogen and a CHClCH3 group, and the other carries hydrogen and an ethyl group. Both carbons carry two different groups, so cis-trans isomerism is possible here too.
4. 4-chloro-2-pentene: The structure is CH3-CH=CH-CHClCH3. One double bond carbon carries hydrogen and a methyl group, and the other carries hydrogen and a CHClCH3 group. Both carbons again carry two different groups, so this alkene also shows cis-trans isomerism.

Out of the four alkenes, only 1-butene has a double bond carbon holding two identical hydrogen atoms, which rules out cis-trans isomerism for that molecule, while the other three all satisfy the two-different-groups rule on both carbons.

So the correct answer is 1-butene.