Why is (E)-1,2-diisopropylethene more stable than (Z)-1,2-diisopropylethene? Why are they more stable than 1,1-diisopropylethe?

And doesn't the bond angle between the C=C-C carbons increase with (Z)-1,2-diisopropylethene > (E)-1,2-diisopropylethene > 1,1-diisopropylethe? So why isn't the same as (E) being greater than (Z)?

(E)-1,2-diisopropylethene is more stable than (Z)-1,2-diisopropylethene due to the steric effects and hyperconjugation that are present in the former compound. In (E)-1,2-diisopropylethene, the bulky isopropyl groups are in a trans configuration, which allows for a greater separation between them. This reduces steric hindrance, leading to increased stability.

In contrast, (Z)-1,2-diisopropylethene has the isopropyl groups in a cis configuration, resulting in a smaller separation and increased steric hindrance. This leads to reduced stability compared to (E)-1,2-diisopropylethene.

Regarding 1,1-diisopropylethene, it is less stable than both (E)-1,2-diisopropylethene and (Z)-1,2-diisopropylethene due to the absence of conjugation. In 1,1-diisopropylethene, the double bond is located between the two carbon atoms that are directly bonded to the isopropyl groups. As a result, there is no possibility for delocalization of pi electrons through conjugation. This lack of conjugation decreases stability compared to the other two compounds.

Now, let's address your question about bond angles. The bond angle between the C=C-C carbons does not change between (E)-1,2-diisopropylethene, (Z)-1,2-diisopropylethene, and 1,1-diisopropylethene. In all three compounds, the bond angle is approximately 120 degrees.

The distinction between (E)- and (Z)- is related to the arrangement of substituents around the double bond, not the bond angle. The (E)- configuration denotes that the highest priority groups (in this case, the isopropyl groups) are on opposite sides of the double bond, while the (Z)- configuration indicates that the highest priority groups are on the same side of the double bond.

In summary, the stability order for the given compounds is: (E)-1,2-diisopropylethene > (Z)-1,2-diisopropylethene > 1,1-diisopropylethene. The stability differences are primarily attributed to steric effects, hyperconjugation, and the presence or absence of conjugation. The bond angle between C=C-C carbons remains relatively constant, and the (E)-/(Z)- designation refers to the arrangement of substituents around the double bond, not the bond angle itself.