Chapter 16.2, Problem 4CC

(a)

Interpretation Introduction

Interpretation:

From the given isomeric dienes, the compound liberating least heat and the compound liberating most heat upon hydrogenations with 2 moles of hydrogen gases are needed to be identified.

Concept introduction:

• The heat liberation in a reaction is depends upon the stability of the reactant compounds. If the reactant compound is more stable, then the heat liberation will be less.

Based on the proximity of $\text{C}\text{=}\text{C}$ bonds in dienes, they are classified into three.

• Cumulated dienes: the $\text{C}\text{=}\text{C}$ bonds are located adjacent carbons.
• Conjugated dienes: the $\text{C}\text{=}\text{C}$ bonds or $\text{C}\text{=}\text{C}$ and $\text{C}\text{=}\text{O}$ are separated by one σ-bond.
• Isolated dienes: the $\text{C}\text{=}\text{C}$ bonds are separated by two or more σ-bonds.
• The conjugated dienes are more stable as compare to isolated dienes because the delocalization of π bonds is taking place, so this forms some stabilization energy in conjugated system.
• The more substituted alkenes are more stable as compared to less substituted alkenes. Because the more alkyl group attachment to a double bond make it more stable due to the ⁺I effect of alkyl groups. It means the alkyl groups will push electrons towards the double bonded carbon atoms, so the double bond becomes strengthened and highly stabilized. 

To identify: the compound liberating least heat and the compound liberating most heat upon hydrogenations with 2 moles of hydrogen gases from the given isomeric dienes.

(b)

Interpretation Introduction

Interpretation:

From the given isomeric dienes, the compound liberating least heat and the compound liberating most heat upon hydrogenations with 2 moles of hydrogen gases are needed to be identified.

Concept introduction:

• The heat liberation in a reaction is depends upon the stability of the reactant compounds. If the reactant compound is more stable, then the heat liberation will be less.

Based on the proximity of $\text{C}\text{=}\text{C}$ bonds in dienes, they are classified into three.

• Cumulated dienes: the $\text{C}\text{=}\text{C}$ bonds are located adjacent carbons.
• Conjugated dienes: the $\text{C}\text{=}\text{C}$ bonds or $\text{C}\text{=}\text{C}$ and $\text{C}\text{=}\text{O}$ are separated by one σ-bond.
• Isolated dienes: the $\text{C}\text{=}\text{C}$ bonds are separated by two or more σ-bonds.
• The conjugated dienes are more stable as compare to isolated dienes because the delocalization of π bonds is taking place, so this forms some stabilization energy in conjugated system.
• The more substituted alkenes are more stable as compared to less substituted alkenes. Because the more alkyl group attachment to a double bond make it more stable due to the ⁺I effect of alkyl groups. It means the alkyl groups will push electrons towards the double bonded carbon atoms, so the double bond becomes strengthened and highly stabilized.