Question

Given below are the four compound which are stabilized either by inductive, mesomeric or hyperconjugation effects.. (a) CC(=O)C1=C(C)CCCC1 (b) BrC1=CCCCC1 (c) CC1=CCCCC1C (d) CC=CCl

Short answer

Compound (a) is stabilized by inductive and hyperconjugation effects, (b) by inductive and mesomeric effects, (c) by hyperconjugation, and (d) primarily by inductive effect.

Step-by-step solution

Analyze Compound (a)

The compound (a) is a methyl ketone \[ \text{CC(=O)C}_1\text{=C(C)CCCC}_1 \] which includes a ketone group (C=O) attached to a cycloalkene. The carbonyl group exhibits a minus inductive effect (-I), due to the electronegativity of the oxygen atom, which stabilizes the compound. Additionally, there is hyperconjugation from the methyl groups attached to the double-bonded carbon in the ring.

Analyze Compound (b)

The compound (b) is a bromo-benzene \[ \text{BrC}_1\text{=CCCCC}_1 \] that has a bromine atom attached to a benzene ring. Bromine is electronegative and exhibits a minus inductive effect (-I) stabilizing the compound. Moreover, bromine can participate in resonance (mesomeric effect) due to its lone pairs, which delocalize the electrons into the benzene ring, further stabilizing it.

Analyze Compound (c)

The compound (c) is an alkyl benzene \[ \text{CC}_1\text{=CCCCC}_1\text{C} \] with an ethyl group attached. Here, the methyl groups on either side of the benzene ring can donate electrons via hyperconjugation (due to the presence of the C-H bonds that are adjacent to the C=C bond), providing stability to the compound.

Analyze Compound (d)

The compound (d) is an alkene with a chlorine substituent \[ \text{CC=CCl} \]. The chlorine atom exerts a minus inductive effect (-I) due to its electronegativity, which stabilizes the compound by attracting electron density toward itself. There are no conjugated systems available for mesomeric effects or hyperconjugation to play a significant role in this small alkene.

Key concepts

Inductive Effect

The inductive effect is a fundamental chemical concept that stems from the electronegativity of atoms within a molecule. It involves the permanent shifting of electron density along the carbon chain of an organic compound. This shift is due to the presence of an electronegative atom, like halogens or a functional group, which pulls electron density towards itself.

In our examples, compound (a) and compound (d) perfectly illustrate the inductive effect. In compound (a), the oxygen in the carbonyl group (C=O) and chlorine in compound (d) exert what is called a minus inductive effect (-I). This means they withdraw electron density resulting in stabilization.

The inductive effect can be classified into:

• **Plus Inductive Effect (+I)**: If an atom or group donates electron density.
• **Minus Inductive Effect (-I)**: If an atom or group withdraws electron density, like the oxygen and chlorine atoms mentioned.

The greater the electronegativity of the atom, the stronger the effect. This helps stabilize the structure due to the redistribution of charge within the molecule.

Mesomeric Effect

The mesomeric effect, also known as resonance effect, occurs in molecules where lone pairs and double bonds allow for delocalization or shifting of electrons. It is an important stabilizing factor, especially in aromatic compounds.

This effect is well-demonstrated in compound (b), the bromo-benzene, where the bromine atom, despite being electronegative, can still engage in resonance. This happens because bromine has lone pairs of electrons that can overlap with the pi-electron cloud of the benzene ring. This electron delocalization enhances the stability of bromo-benzene.

Mesomeric effect can also be defined in two ways:

• **Positive Mesomeric Effect (+M)**: When a group donates electrons to the pi-system.
• **Negative Mesomeric Effect (-M)**: When a group withdraws electrons from the pi-system.

This resonance stabilizes the compound by spreading out electron density over a larger part of the molecule, thus lowering the energy and increasing the stability.

Hyperconjugation

Hyperconjugation is a stabilizing interaction that involves the delocalization of electrons in sigma bonds (usually C-H or C-C) into adjacent empty or partially filled pi or p orbitals.
Compounds (a) and (c) showcase the principle of hyperconjugation. In compound (a), the presence of methyl groups adjacent to the double-bonded carbon allows for electrons from the C-H bonds to overlap with the pi-bond of the double bond. This overlap stabilizes the molecule further. Similarly, compound (c) benefits from hyperconjugation through its ethyl group that has C-H bonds near the benzene ring, creating more stability through similar interactions.
Hyperconjugation can be thought of as an "extended resonance," where instead of pi-bonds alone being involved, sigma bonds play a critical role. This broader electron delocalization reduces electron density at some points, spreading it over the framework of a molecule and therefore stabilizing it.