Which of the following molecules does not show stereo isomerism?

Step 1: Understanding stereoisomerism in alkenes

Stereoisomerism in alkenes arises when there is restricted rotation around the double bond, and the two carbon atoms involved in the double bond are attached to two different groups each. For cis-trans (or E/Z) isomerism to exist, each double-bonded carbon must have two different substituents.

Step 2: Analyzing the options

1. 4-Methylhex-1-ene:
   • The double bond is at the first carbon, so one of the double-bonded carbons (C1) is attached to two identical hydrogen atoms.
   • Result: Does not show stereoisomerism.
2. 3-Ethylhex-3-ene:
   • The double bond is at the third carbon. The substituents on C3 and C4 are identical (both are methyl groups), so there is no possibility for cis-trans isomerism.
   • Result: Does not show stereoisomerism.
3. 3,4-Dimethylhex-3-ene:
   • The double bond is at the third carbon, and both C3 and C4 have different substituents. This allows for the possibility of stereoisomerism (cis-trans isomers).
   • Result: Shows stereoisomerism.
4. 3-Methylhex-1-ene:
   • The double bond is at the first carbon, and one of the double-bonded carbons (C1) has two identical substituents (hydrogen atoms).
   • Result: Does not show stereoisomerism.

Step 3: Determining the correct answer

Among the options, 3-Ethylhex-3-ene (b) does not exhibit stereoisomerism because the substituents on the double-bonded carbons are identical.

Final Solution:

The correct answer is b) 3-Ethylhex-3-ene.