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: Understanding Heat of Hydrogenation

The heat of hydrogenation is the heat released when an unsaturated compound is hydrogenated into a saturated compound. It provides an indication of the stability of the alkene; less stable alkenes release more heat upon hydrogenation.

Step 2: Evaluating Stability of Alkenes

In general, the stability of alkenes increases with more substituted double bonds. Tetrasubstituted alkenes are more stable than trisubstituted alkenes, which are more stable than disubstituted alkenes, followed by monosubstituted alkenes.

Step 3: Analyzing Each Compound

(a) \( (\mathrm{CH}_3)_2 \mathrm{C} = \mathrm{C}(\mathrm{CH}_3)_2 \) is a tetrasubstituted alkene. (b) \( \mathrm{H}_3 \mathrm{C}-\mathrm{CH}_2-\mathrm{CH}=\mathrm{CH}_2 \) is a monosubstituted alkene. (c) \( \mathrm{H}_3 \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_3 \) is a disubstituted alkene. (d) \( \mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2 \) is an unsubstituted alkene.

Step 4: Determining the Least Stable Alkene

The least substituted alkene is the least stable and will have the highest heat of hydrogenation. (d) \( \mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2 \) is an unsubstituted alkene, making it the least stable and thus expected to have the highest heat of hydrogenation.

Key Concepts

Understanding Alkene Stability

Alkenes are hydrocarbons that contain a carbon-carbon double bond (C=C). The stability of these compounds plays a crucial role in predicting the heat of hydrogenation, which is the heat released during the hydrogenation process. Generally, the stability of an alkene is determined by how substituted the double-bonded carbons are. Here's how substitution affects stability:

• More Substituted Alkenes: These alkenes, where more carbon substituents are attached to the double-bonded carbons, have greater stability. This is because alkyl groups are electron-donating, stabilizing the double bond.
• Less Substituted Alkenes: With fewer substituent groups, these alkenes are less stable. As a result, they release more heat when hydrogenated.

To identify the most stable alkene, count the number of alkyl groups attached to the carbon atoms in the double bond. The more groups, the more stable the alkene.

The Role of Substituted Alkenes

Substituted alkenes play a significant role in organic chemistry due to their varied reactivity and stability. They are categorized based on the number of carbon substituents attached to the double bond:

• Tetrasubstituted Alkenes: These have four carbon substituents, making them the most stable in terms of alkene reactivity.
• Trisubstituted Alkenes: These include three substituents and are slightly less stable than tetrasubstituted alkenes.
• Disubstituted Alkenes: With two substituents, they are less stable than the trisubstituted alkenes but more stable than monosubstituted ones.
• Monosubstituted Alkenes: Only one substituent is attached, making them less stable.

The stability order becomes critical when predicting outcomes in reactions like hydrogenation.

Exploring Unsaturated Compounds

Unsaturated compounds are molecules with at least one double or triple bond between carbon atoms, such as alkenes. This unsaturation means that they have fewer hydrogen atoms than their saturated counterparts (alkanes) but more potential energy. During the process of hydrogenation, these compounds are converted into saturated compounds by adding hydrogen across the unsaturated bonds.
The amount of energy released during this conversion, known as the heat of hydrogenation, is a direct indicator of how much less stable the original unsaturated compound was. In practice, higher energy release suggests a lower initial stability. Understanding these principles allows chemists to manipulate the reactions of unsaturated compounds effectively, greatly influencing industrial applications and research in material sciences.