Problem 164 - A Complete Resource Book in Chemistry for JEE Main

Question

Hyper conjugation is possible in (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{\ominus}\) (b) Cc1ccccc1 (c) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) (d) CCC(C)(C)C

Step-by-Step Solution

Answer

Hyper conjugation is possible in (b) methyl benzene (toluene).

Step 1: Understanding Hyperconjugation

Hyperconjugation is a phenomenon where electrons in a \(\sigma\) bond are delocalized into an adjacent empty or partially filled \(p\) orbital, \(\pi\) orbital, or antibonding \(\pi^{*}\) orbital. This effect is common in alkyl-substituted alkenes and carbocations where there are no lone pairs involved.

Step 2: Analysis of Option (a)

The species \(6mathrm{CH}_{3} 6mathrm{CH}_{2}^{6ominus}\) is an anion with a negative charge on the ethyl group. Anions typically do not participate in hyperconjugation because there is no stable orbital to accept \(\sigma\) electrons.

Step 3: Analysis of Option (b)

The chemical structure \(6mathrm{Cc1ccccc1}\), representing toluene or methyl benzene, has an aromatic ring with a methyl group. The \(\sigma\) electrons in the C-H bonds of the methyl group can be delocalized into the \(\pi\) system of the benzene ring, making hyperconjugation possible.

Step 4: Analysis of Option (c)

For \(6mathrm{CH}_{2}=6mathrm{CH}_{2}\), which is ethylene, there are no adjacent hydrogen atoms attached to a carbon with another carbon bonded to it. Hyperconjugation requires such hydrogen atoms, which are not present in this molecule.

Step 5: Analysis of Option (d)

The chemical structure \(6mathrm{CCC(C)(C)C}\) represents 3,3-dimethylbutane. There are multiple C-H bonds adjacent to a \(6mathrm{CH}\) group tied to a branched alkyl group. While this can stabilize a carbocation through hyperconjugation, the fully saturated compound itself does not inherently support hyperconjugation without forming a carbocation.

Key Concepts

Sigma Bond

A sigma bond is the strongest type of covalent bond formed by the direct overlap of atomic orbitals along the axis connecting two atomic nuclei. This overlap leads to the sharing of electrons between atoms. Sigma bonds can involve overlaps between:

• Two s orbitals
• One s orbital and one p orbital
• Two p orbitals

These bonds provide a stable connection in molecules and are the primary structure in single covalent bonds. In hyperconjugation, the \(\sigma\) electrons of these bonds are essential because they can be delocalized. This delocalization does not occur freely but involves stretching into a nearby empty or partially filled orbital, facilitating electron flow, stability, and energy optimization in the molecule.

Delocalization

Delocalization in chemistry refers to the spread of electron density over several atoms rather than being localized between two atoms. This is a key feature in molecules where hyperconjugation can occur. Delocalization is often compared to resonance, but they are not identical:

• In resonance, electrons are spread over different structures (resonance forms), often making compounds more stable.
• In hyperconjugation, \(\sigma\) electrons are shifted, allowing them to be shared with empty or partially filled nearby orbitals.

This transfer of electrons plays a crucial role in stabilizing carbocations and enhancing the stability of alkyl-substituted alkenes. Thus, structures with extensive delocalization of electrons often exhibit greater stability.

Alkyl-Substituted Alkenes

Alkyl-substituted alkenes are alkenes that have alkyl groups attached to the carbon atoms involved in the double bond. This substitution can significantly influence the stability and reactivity of the alkene. When alkyl groups are attached to the double-bonded carbon atoms, they impact:

• The **stability** of the alkene, usually by electronic effects due to the alkyl group's ability to donate electron density.
• The **reactivity** of the compound, with alkyl groups providing steric and electronic effects that can alter reaction pathways.

The presence of alkyl groups can lead to hyperconjugation, where the \(\sigma\) bonds in the hydrogen atoms attached to the alkyl groups can interact with the \(\pi\) bonds of the alkene. This interaction allows for the stabilization of the compound, as seen in structures with branched or multiple alkyl groups.

Carbocations

Carbocations are positively charged ions with the positive charge localized on a carbon atom. These ions arise during certain reactions, such as rearrangements or as intermediates in addition reactions. They are characterized by:

• Awarding three substituents to the carbon atom, making it highly reactive due to the trivalent position and an electron-deficient state.
• Exposing the charge to stabilization by hyperconjugation or resonance.

In hyperconjugation, the positive charge on carbocations is often stabilized by adjacent \(\sigma\) bonds.The electron density from these bonds shifts towards the electron-deficient carbocation, dispersing the positive charge and stabilizing the ion. This shift aids in reducing instability and gives carbocations a more permanent intermediate structure during chemical reactions, enhancing reactivity control and stability.