Question

Give the structures of both 1,2 - and 1,4 -adducts resulting from reaction of 1 equivalent of \(\mathrm{HCl}\) with penta- 1,3 -diene.

Short answer

1,2-adduct: \( \text{CH}_2\text{Cl}-\text{CH}_2-\text{CH}=\text{CH}-\text{CH}_3 \); 1,4-adduct: \( \text{CH}_3-\text{CH}=\text{CH}-\text{CHCl}-\text{CH}_3 \).

Step-by-step solution

Identify the Diene

Penta-1,3-diene is a conjugated diene which means there are two double bonds in the molecule. In this molecule, there is a double bond at the first and third carbon atoms: \( \text{CH}_2=\text{CH}-\text{CH}=\text{CH}-\text{CH}_3 \).

Understand the Reaction Mechanism: Electrophilic Addition

The reaction with HCl involves an electrophilic addition. The hydrogen ion (\( \text{H}^+ \)) from HCl attacks one of the double bonds to form a carbocation.

Form the 1,2-Adduct

The hydrogen atom adds to carbon 1, forming a carbocation at carbon 2. The chloride ion (\( \text{Cl}^- \)) then attaches to carbon 2. This results in the 1,2-adduct: \( \text{CH}_2\text{Cl}-\text{CH}_2-\text{CH}=\text{CH}-\text{CH}_3 \).

Form the 1,4-Adduct

The hydrogen adds to carbon 1, forming a carbocation at carbon 2, which then shifts to carbon 4 due to resonance. The chloride ion then adds to carbon 4, resulting in the 1,4-adduct: \( \text{CH}_3-\text{CH}=\text{CH}-\text{CHCl}-\text{CH}_3 \).

Draw Structures

1,2-adduct structure is \( \text{CH}_2\text{Cl}-\text{CH}_2-\text{CH}=\text{CH}-\text{CH}_3 \); 1,4-adduct structure is \( \text{CH}_3-\text{CH}=\text{CH}-\text{CHCl}-\text{CH}_3 \).

Key concepts

Conjugated Dienes

A conjugated diene is a type of organic molecule that contains two alternating double bonds separated by a single bond. This unique arrangement allows for certain special reactions to occur. In the case of penta-1,3-diene, the structure can be written as \( \text{CH}_2=\text{CH}-\text{CH}=\text{CH}-\text{CH}_3 \).
This pattern allows the molecule to participate in reactions not typical of isolated double bonds.

When undergoing reactions, conjugated dienes can form various products such as 1,2-addition and 1,4-addition products. These are named based on where the new bonds form in the molecule. Understanding these patterns is crucial for predicting reaction outcomes.

• 1,2-addition involves adding atoms to the first and second carbon atoms.
• 1,4-addition involves adding atoms to the first and fourth carbon atoms.

These reactions are foundational in organic chemistry, making conjugated dienes a key concept for students to grasp.

Carbocation Intermediate

During electrophilic addition reactions with conjugated dienes, a carbocation intermediate is formed. This is a significant step occurring when a molecule temporarily gains a positive charge. For instance, when HCl reacts with penta-1,3-diene, \( \text{H}^+ \) from the acid adds to a carbon atom.

The addition typically occurs at the more stable position, forming a carbocation. In penta-1,3-diene, the initial carbocation forms at carbon 2 after \( \text{H}^+ \) adds to carbon 1. This intermediate is crucial because its placement determines the structure of the final product.

• In 1,2-adduct formation, the carbocation quickly reacts with \( \text{Cl}^- \) from HCl, stabilizing at carbon 2.
• In 1,4-adduct formation, resonance allows the positive charge to shift to carbon 4 before \( \text{Cl}^- \) attaches, creating a different product.

Understanding carbocation intermediates helps predict how molecules rearrange and stabilize during reactions.

Resonance in Organic Molecules

Resonance is a fundamental concept that explains the delocalization of electrons within a molecule. This allows electrons to be distributed across multiple structures, providing additional stability. In the case of penta-1,3-diene, resonance plays a crucial role in enabling the formation of different products during electrophilic addition.

When the initial carbocation forms, resonance can shift the positive charge across the conjugated system. This shift allows the formation of the 1,4-adduct by moving the positive charge from the second to the fourth carbon.

• Resonance structures do not exist in isolation; they are a representation of possible arrangements.
• The actual molecule is a hybrid of all potential resonance structures.

This concept is vital for understanding the dynamic nature of reactions and predicting the most likely products.