Draw and name all dienes of molecular formula C5H8. When E and Z isomers are possible, draw and name each stereoisomer.
The Correct Answer and Explanation is:To solve this, we need to draw all possible dienes with the molecular formula C₅H₈, and include E/Z stereoisomers when applicable.
Step 1: General considerations
- C₅H₈ is an unsaturated hydrocarbon.
- The degree of unsaturation is calculated as:
DoU=(2C+2−H)2=(2(5)+2−8)2=42=2\text{DoU} = \frac{(2C + 2 – H)}{2} = \frac{(2(5) + 2 – 8)}{2} = \frac{4}{2} = 2
So, there are two degrees of unsaturation → this can mean two double bonds, or one ring and one double bond.
We are only asked to find dienes (compounds with two double bonds, not rings), so we’ll focus on acyclic dienes with two C=C.
Step 2: Structures and Names
1. 1,2-Pentadiene
Structure: CH₂=C=CH–CH₂–CH₃
- An allene (cumulative diene)
- No E/Z isomers possible
✅ Name: 1,2-pentadiene
2. 1,3-Pentadiene
Structure: CH₂=CH–CH=CH–CH₃
- A conjugated diene
Two stereoisomers possible due to the central double bond:
- (E)-1,3-pentadiene
- (Z)-1,3-pentadiene
✅ Names:
- (E)-1,3-pentadiene
- (Z)-1,3-pentadiene
3. 1,4-Pentadiene
Structure: CH₂=CH–CH₂–CH=CH₂
- An isolated diene
- No E/Z isomers
✅ Name: 1,4-pentadiene
4. 2,3-Pentadiene
Structure: CH₃–CH=C=CH–CH₃
- Symmetrical allene
- Chiral compound with enantiomers (optical isomers), but not E/Z
✅ Name: 2,3-pentadiene
Summary Table of C₅H₈ Dienes
| # | Name | Structure Type | Isomerism |
|---|---|---|---|
| 1 | 1,2-Pentadiene | Cumulative allene | None |
| 2 | (E)-1,3-Pentadiene | Conjugated diene | E/Z isomers |
| 3 | (Z)-1,3-Pentadiene | Conjugated diene | E/Z isomers |
| 4 | 1,4-Pentadiene | Isolated diene | None |
| 5 | 2,3-Pentadiene | Cumulative allene | Enantiomers (chiral) |
Explanation
The molecular formula C₅H₈ represents a class of unsaturated hydrocarbons containing two degrees of unsaturation, which for dienes means two double bonds. Dienes can be categorized into conjugated, isolated, and cumulative depending on the arrangement of the double bonds.
1,3-Pentadiene is a conjugated diene, where the double bonds are separated by a single bond, allowing delocalization of π electrons. This compound exists as geometric (E/Z) isomers due to restricted rotation around the central double bond and the presence of different substituents on each carbon of the double bond.
1,4-Pentadiene is an isolated diene, meaning the double bonds are separated by more than one single bond. Since its double bonds are at terminal positions and not adjacent to each other, it lacks geometric isomerism.
1,2-Pentadiene and 2,3-Pentadiene are examples of cumulative dienes or allenes, where the double bonds share a common carbon. In 1,2-pentadiene, this central carbon is bonded to two double bonds in a linear arrangement, making it achiral. However, in 2,3-pentadiene, the geometry of the allene is such that it has chirality due to the perpendicular p orbitals—leading to enantiomers even though no stereocenters exist.
In summary, there are five unique dienes with formula C₅H₈, including structural and stereoisomers. This analysis is essential in organic chemistry for understanding isomerism, reactivity, and structural diversity of unsaturated hydrocarbons.
