Q49E - Organic Chemistry (Mcmurry)

Question

Question : Trans-2-Butene is more stable than cis-2-butene by only 4 kJ/mol, but trans-2,2,5,5-tetramethyl-3-hexene is more stable than its cis isomer by 39 kJ/mol. Explain.

Step-by-Step Solution

Answer

Answer

The steric hindrance present in the cis form of $\text{2,2,5,5 - tetramethyl - 3 - hexene}$  is very large as compared to the hindrance in the cis form of 2- butane. That is why the difference in energy is much higher $\text{2,2,5,5 - tetramethyl - 3 - hexene}$ for than 2-butene.

Step 1: Concept of stability of alkene

Trans alkene are more stable as compared to cis alkene due to steric hinderance present in cis form because the steric repulsion is very high in the cis form.

Step 2: Trans-2-Butene is more stable than cis-2-butene

As we can observe from the structure in cis form there is steric hindrance present but in trans form no steric hindrance present.

The energy difference between cis and trans form of 2-butene is not that much because the repulsion is between two methyl groups

Step 3: Stability of trans-2,2,5,5-tetramethyl-3-hexene

In the cis form form of $\text{2,2,5,5 - tetramethyl - 3 - hexene}$, the repulsion between the six bulky methyl groups is very high. In the trans form, the methyl groups are at a distance from each other and hence the steric repulsions are relaxed. This explains the very high energy difference between the cis and trans form of $\text{2,2,5,5 - tetramethyl - 3 - hexene}$

 .