Problem 108 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following compounds would have the highest heat of hydrogenation? (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2}\)

Step-by-Step Solution

Answer

Compound (d) \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2}\) has the highest heat of hydrogenation.

Step 1: Understand Heat of Hydrogenation

The heat of hydrogenation refers to the amount of heat released when an alkene is hydrogenated. It is a measure of the stability of the double bond; less stable double bonds release more heat when hydrogenated.

Step 2: Analyze Compound (a)

The compound \(\left(\mathrm{CH}_{3}\right)_{2}\mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) is a tetra-substituted alkene, meaning all four positions around the double bond are occupied by carbon atoms. This makes it very stable, so it should have a relatively low heat of hydrogenation.

Step 3: Analyze Compound (b)

The compound \(\mathrm{H}_{3}\mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}_{2}\) is a monosubstituted alkene. This type of alkene is less stable due to fewer alkyl groups to donate electron density to the \(\pi\)-bond, resulting in a higher heat of hydrogenation compared to more substituted alkenes.

Step 4: Analyze Compound (c)

The compound \(\mathrm{H}_{3}\mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}\) is a disubstituted alkene. It is more stable than a monosubstituted alkene but less stable than tri- and tetra-substituted alkenes, giving it an intermediate heat of hydrogenation.

Step 5: Analyze Compound (d)

The compound \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2}\) is an unsubstituted alkene (ethylene). Unsubstituted alkenes are least stable and have the highest heat of hydrogenation due to maximum release of strain and unsaturation upon hydrogenation.

Step 6: Determine the Compound with Highest Heat of Hydrogenation

Among the options, compound (d) \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2}\) has no substituents and is the least stable. Hence, it will have the highest heat of hydrogenation due to the greater release of energy when it is hydrogenated.

Key Concepts

Alkene Stability

Alkenes are hydrocarbons containing carbon-carbon double bonds, and their stability can vary significantly. The stability of alkenes depends on several factors, including the number and types of substituents attached to the carbon atoms involved in the double bond.
A more stable alkene will have a lower heat of hydrogenation, as there is less strain in the molecule to release when the double bond is converted to a single bond during hydrogenation.
An important consideration in alkene stability is the number of alkyl groups attached to the double bond:

• Tetra-substituted alkenes have the highest stability because they have four carbon-containing groups attached, which provide electron donation to stabilize the double bond.
• Tri-substituted and di-substituted alkenes offer moderate stability, with di-substituted being less stable than tri-substituted ones.
• Mono-substituted alkenes are less stable than their more substituted counterparts, having only one carbon-containing substituent.
• Unsubstituted alkenes, like ethylene, are the least stable due to the lack of substituents contributing to the stabilization of the double bond.

Understanding alkene stability is crucial when analyzing and predicting the outcomes of reactions involving these compounds.

Substituted Alkenes

Substituted alkenes play a pivotal role in organic chemistry, particularly in determining the properties and reactions of alkenes. "Substituted" refers to the presence of additional atoms or groups of atoms, such as alkyl groups, that replace hydrogen atoms around the double bond.
The substitution pattern significantly influences the physical and chemical properties of the alkene:

• Electron Donation: Substituents often donate electrons to the c-bond, enhancing the overall stability of the alkene.
• Hyperconjugation: This is an interaction where electrons in sigma bonds (single bonds) are delocalized into an adjacent empty or partially filled c-orbital, offsetting instability.
• Steric Effects: More substituents can increase steric crowding, affecting how the alkene can interact with other molecules and its physical state.

By analyzing these factors, chemists can predict how different alkenes might behave in reactions, such as determining the likelihood of substitution or addition reactions.

Hydrogenation Process

The hydrogenation process is a chemical reaction where hydrogen is added to an alkene, converting the carbon-carbon double bond into a single bond. This reaction not only changes the unsaturated alkene into a saturated alkane but also releases energy, known as the heat of hydrogenation.
This process is pivotal in industries, being widely used in the production of margarine from vegetable oils and in refining petrochemical products.
Key aspects of the hydrogenation process include:

• Energy Release: The amount of energy released during hydrogenation indicates the initial stability of the alkene; less stable alkenes release more energy.
• Catalysis: The reaction often requires a catalyst, such as palladium, platinum, or nickel, to proceed at a desirable rate.
• Product Formation: The end product is typically a saturated alkane, which is more stable and less reactive than the original alkene.

Understanding these aspects of hydrogenation can help in designing reactions and processes in both laboratory and industrial settings.