Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds possesses the \(\mathrm{C}-\mathrm{H}\) bond with the lowest bond dissociation energy? (a) n-pentane (b) toluene (c) benzene (d) 2,2 -dimethylpropane

Step-by-Step Solution

Answer

Toluene has the \(\mathrm{C}-\mathrm{H}\) bond with the lowest bond dissociation energy.

Step 1: Understanding Bond Dissociation Energy

Bond dissociation energy (BDE) is the energy required to break a specific bond in a molecule. For \(\mathrm{C}-\mathrm{H}\) bonds, lower bond dissociation energy means the bond is weaker and easier to break.

Step 2: Analyzing n-pentane

n-Pentane is an aliphatic hydrocarbon with uniform single \(\mathrm{C}-\mathrm{H}\) bonds along its chain. These bonds typically have moderate bond dissociation energy.

Step 3: Analyzing Toluene

Toluene has a \(\mathrm{C}-\mathrm{H}\) bond on a methyl group attached to a benzene ring. The \(\mathrm{C}-\mathrm{H}\) bond here is slightly weaker due to the resonance stabilization of the benzene ring.

Step 4: Analyzing Benzene

Benzene consists of \(\mathrm{C}-\mathrm{H}\) bonds directly attached to a stable aromatic ring. These bonds benefit from the stabilization energy of the ring, resulting in a relatively strong bond.

Step 5: Analyzing 2,2-dimethylpropane

2,2-Dimethylpropane, also known as neopentane, is a highly branched alkane. The \(\mathrm{C}-\mathrm{H}\) bonds here are in primary carbon environments, typical for simple alkanes but potentially less stable due to steric hindrance.

Step 6: Comparing and Identifying the Weakest Bond

Toluene's methyl group attached to the benzene ring provides some resonance stabilization, making its \(\mathrm{C}-\mathrm{H}\) bond lower in bond dissociation energy compared to the other structures, as the resonance effect weakens this bond.

Key Concepts

C-H Bond

The carbon-hydrogen (C-H) bond is a fundamental component in organic chemistry. It's the bond between a carbon atom and a hydrogen atom within a molecule. The strength of the C-H bond, known as bond dissociation energy (BDE), reflects how much energy is required to break it. This aspect is pivotal when analyzing chemical reactions.

• In alkanes, these bonds are generally robust due to the nature of the single covalent bond that exists in their saturated hydrocarbons.
• Bonds in aromatic compounds, such as benzene, typically have different characteristics due to resonance stabilization effects, which can alter their overall stability and energy requirements for breaking.

Understanding these differences helps in grasping why certain C-H bonds are easier or harder to break during chemical reactions.

Resonance Stabilization

Resonance stabilization is an essential concept when analyzing molecules with delocalized electrons, especially in compounds featuring conjugated systems. This phenomenon occurs when electrons are shared across multiple atoms, rather than being localized, enhancing the molecule's stability.

• Toluene, for example, exhibits resonance stabilization due to the presence of a benzene ring. This resonance spreads out the energy across the structure, affecting the adjacent C-H bonds.
• Consequently, these C-H bonds appear weaker because the energy required to break them is less, owing to the delocalized electrons that are partially bonding the site.

This insight can explain why some C-H bonds in aromatic compounds have lower bond dissociation energies compared to their aliphatic counterparts.

Aromatic Compounds

Aromatic compounds are a special class of molecules characterized by stable ring structures, often exemplified by benzene. They are notably recognized for their resonance and stabilization energies.

• Each carbon in an aromatic ring like benzene forms bonds that contribute to a delocalized system. This distribution of electrons drastically impacts the overall stability of the molecule.
• The electron delocalization leads to unique properties, such as certain C-H bonds experiencing reduced bond dissociation energy due to the stabilizing effect of the ring.

Aromatic compounds thus play a fascinating role in understanding chemical stability and reactions, distinguishing them from typical aliphatic systems.

Aliphatic Hydrocarbons

Aliphatic hydrocarbons are composed of carbon and hydrogen arranged in straight or branched chains, and they lack the ring structures seen in aromatic compounds. They mainly include alkanes, alkenes, and alkynes, with varying levels of saturation.

• In alkanes, which are fully saturated hydrocarbons, C-H bonds are generally strong due to the absence of any electron delocalization or additional stabilization.
• This class of hydrocarbons does not exhibit resonance stabilization, resulting in comparatively higher bond dissociation energies for C-H bonds than those found in aromatic hydrocarbons.

Aliphatic hydrocarbons represent simplicity in their structures, offering a contrast to the complex, stabilized forms found in aromatic molecules.