There are two compounds with the formula C3H6

There are two compounds with the formula C3H6, one of which does not have a multiple bond. Draw its structure and explain why it is much less stable than the isomer with the double bond

The Correct Answer and Explanation is :

The two compounds with the formula C3H6 are propene (C3H6) and cyclopropane (C3H6).

Structure of Cyclopropane (the compound without a multiple bond):

Cyclopropane is a cyclic compound where the three carbon atoms form a triangle, each carbon bonded to two hydrogen atoms. The structure is as follows:

     H   H
      \ /
       C
     /   \
  H-C     C-H
     \   /
      C
     /   \
    H     H

Structure of Propene (the compound with a double bond):

Propene, an alkene, has a double bond between two carbon atoms, making it a much more stable molecule than cyclopropane.

  H2C=CH-CH3

Explanation of Stability:

1. Strain in Cyclopropane: Cyclopropane is significantly less stable than propene due to the high angle strain (or Baeyer strain) caused by the triangular structure. In a normal carbon-carbon bond, the bond angle is approximately 109.5° (sp3 hybridized), but in cyclopropane, the bond angles between the carbons are forced to be 60°. This greatly distorts the tetrahedral geometry of the carbon atoms, leading to significant strain.
2. Bonding in Cyclopropane: Additionally, the carbon-carbon bonds in cyclopropane are not normal single bonds. Due to the forced angles, these bonds exhibit orbital overlap strain where the bonds are weaker than typical single bonds. This makes them more reactive and prone to breaking.
3. Double Bond in Propene: On the other hand, propene has a double bond between two of its carbon atoms. The double bond is more stable as it allows for better orbital overlap between the atoms involved, creating a strong, stable bonding arrangement. The geometry around the double bond is planar, which leads to a more stable structure compared to the highly strained cyclopropane ring.
4. Reactivity: Because of the strain and weak bonds in cyclopropane, it is much more reactive and tends to undergo reactions that relieve this strain, such as ring-opening reactions. In contrast, propene is less reactive because it does not experience this strain.

In conclusion, cyclopropane is much less stable than propene because of the severe angle strain and weak bonds caused by its three-membered ring structure, while propene’s double bond provides more stability and a lower energy configuration.