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),  ext{H}_{2} ext{C}= ext{CH}_{2}, has the highest heat of hydrogenation.

Step 1: Understand Heat of Hydrogenation

The heat of hydrogenation refers to the energy released when an alkene is converted to an alkane by the addition of hydrogen. The more stable the alkene, the less energy it releases upon hydrogenation, meaning a lower heat of hydrogenation.

Step 2: Determine Alkene Stability

Alkene stability is influenced by the number of alkyl groups attached to the sp2 hybridized carbon atoms (degree of substitution). More substituted alkenes are more stable. Analyze each compound: - (a) is a tetrasubstituted alkene, thus very stable. - (b) is a monosubstituted alkene, so it is less stable. - (c) is a disubstituted alkene, thus moderately stable. - (d) is an unsubstituted alkene (ethylene), making it the least stable.

Step 3: Compare Stability and Predict Heat Release

The least stable alkene will release the most energy because it has the most potential energy to convert to more stable products. Based on Step 2, compound (d), an unsubstituted alkene, is the least stable, so it will have the highest heat of hydrogenation.

Key Concepts

Alkene Stability

Alkene stability is an essential concept in organic chemistry that determines how easily an alkene can exist without reacting. The stability of alkenes largely depends on their structure, specifically the degree of substitution around the double bond. Substitution refers to the number of alkyl groups attached to the carbon atoms involved in the double bond.

Alkenes are generally more stable when they have more alkyl groups bonded to the carbon atoms of the double bond because alkyl groups can donate electron density through hyperconjugation. This donation of electrons helps to delocalize the charge and reduce strain around the double bond.

• Tetrasubstituted alkenes have four alkyl groups attached and are the most stable.
• Trisubstituted alkenes have three alkyl groups and are quite stable.
• Disubstituted alkenes have two alkyl groups, offering moderate stability.
• Monosubstituted alkenes have one alkyl group and are less stable.
• Unsubstituted alkenes (ethylene) have no alkyl groups attached and are the least stable.

This stability affects how much energy an alkene will release during chemical reactions like hydrogenation.

Degree of Substitution

The degree of substitution is a critical factor in determining the stability of an alkene. It describes how many carbon-containing groups (alkyl groups) are attached to the carbon atoms that participate in the double bond of an alkene.

Increased substitution often leads to greater stability of the alkene due to a few reasons:

• More alkyl groups can stabilize the double bond through hyperconjugation. This means electron density can be shared better across the structure, thus reducing the energy.
• Alkyl groups are bulkier, making the alkene molecule more shielded and less likely to react spontaneously with other species.
• Stabilization occurs because of the dispersal of electron density over more atoms, leading to less overall energy in the molecule.

Thus, knowing the degree of substitution helps predict how stable an alkene is and how it might behave in a reaction.

Energy Release in Chemical Reactions

Energy release in chemical reactions is a fundamental concept that helps understand why some substances react more vigorously than others. When we consider reactions involving alkenes, the heat of hydrogenation illustrates the concept beautifully. During hydrogenation, an alkene converts into an alkane, releasing energy in the process.

The amount of energy released depends on the alkene's stability. The less stable the alkene, the more energy it has stored and therefore, the more energy it releases upon hydrogenation. Chemically, this is because the reactive potential of the molecule is higher.

• Stable alkenes are lower in energy and release less upon hydrogenation.
• Unstable alkenes hold more potential energy and release more when reaching a more stable alkane state.

Understanding energy release in these reactions is crucial for predicting reaction behaviors and energies, especially in practical applications like synthesis and industry.