Question

A set of compounds in which the reactivity of halogen atom is in the ascending order is (1) chlorobenzene, vinyl chloride, chloroethane (2) chloroethane, chlorobenzene, vinyl chloride (3) vinyl chloride, chlorobenzene, chloroethane (4) vinyl chloride, chloroethane, chlorobenzene

Short answer

Option (3): vinyl chloride, chlorobenzene, chloroethane.

Step-by-step solution

Understanding Reactivity

Reactivity of a halogen atom in a compound is influenced by the nature of the carbon-halogen bond and the electronic environment around it.

Evaluate Chlorobenzene

In chlorobenzene, the halogen atom is attached to a benzene ring. The resonance of the benzene ring leads to a partial double bond character, making the C-Cl bond stronger and less reactive.

Evaluate Vinyl Chloride

In vinyl chloride (CH2=CH-Cl), the halogen is attached to a carbon-carbon double bond. The sp2 hybridized carbons hold the electrons more tightly, making the C-Cl bond moderately stable and reactive.

Evaluate Chloroethane

In chloroethane (CH3-CH2-Cl), the halogen is attached to a saturated carbon atom (sp3 hybridized). This makes the C-Cl bond weaker due to the electron-donating nature of the alkyl group, making it more reactive.

Arrange the Compounds

Based on the evaluations: Chlorobenzene (least reactive), Vinyl Chloride (moderately reactive), and Chloroethane (most reactive). Hence, the ascending order of reactivity is: Chlorobenzene < Vinyl Chloride < Chloroethane.

Select the Correct Option

Comparing with the given options, the set that matches the ascending order of reactivity is (3): vinyl chloride, chlorobenzene, chloroethane.

Key concepts

carbon-halogen bond

The carbon-halogen bond is a crucial factor in determining the reactivity of halogenated compounds. The bond strength varies based on several factors such as the type of carbon (sp, sp2, sp3 hybridized), the nature of the halogen, and the overall molecular structure.
The stronger the bond, the less reactive it is. For instance, a carbon-fluorine bond is much stronger due to the high electronegativity of fluorine, leading to lower reactivity compared to other halogens like chlorine or bromine.
In the context of the exercise, the reactivity of chlorobenzene, vinyl chloride, and chloroethane can be explained by analyzing their carbon-halogen bonds. A benzene ring in chlorobenzene leads to a partial double bond character, making the bond stronger and less reactive.
In vinyl chloride, the sp2 hybridized carbon-carbon double bond contributes to moderate bond strength and reactivity, while in chloroethane, the sp3 hybridized carbon forms the weakest bond and is hence the most reactive.

electronic environment

The electronic environment around the carbon-halogen bond significantly impacts its reactivity. An electron-rich environment can share electron density with the halogen, thereby stabilizing the bond. However, electron-withdrawing groups can make the bond weaker and more reactive.
In chlorobenzene, the benzene ring provides a resonant structure that delocalizes the electron density, giving the carbon-halogen bond a partial double bond character. This electron delocalization reduces the reactivity of chlorobenzene.
For vinyl chloride, the sp2 hybridized carbon atoms hold the electrons tighter, creating a more stable but still relatively moderate bond. The presence of a carbon-carbon double bond influences the electronic environment, leading to intermediate reactivity.
Chloroethane, on the other hand, has an sp3 hybridized carbon that is not delocalizing electron density as much. The single-bonded environment allows the electron-donating alkyl group to make the carbon more nucleophilic and the carbon-halogen bond more reactive.

sp2 hybridization

Hybridization plays a vital role in the strength and reactivity of the carbon-halogen bond. In sp2 hybridization, one s orbital mixes with two p orbitals, creating three sp2 hybrid orbitals. This type of hybridization is found in carbon atoms involved in double bonds.
In vinyl chloride (CH2=CH-Cl), the carbon bonded to chlorine is sp2 hybridized. The sp2 orbitals hold electrons more tightly than sp3 hybridized orbitals but less tightly than sp hybridized orbitals. This results in a moderate bond strength and thus intermediate reactivity.
The planar structure due to sp2 hybridization allows for better overlapping of orbitals, which increases bond strength but also introduces some rigidity, thereby making the bond not as flexible or reactive as an sp3 hybridized bond found in chloroethane.

sp3 hybridization

In sp3 hybridization, one s orbital combines with three p orbitals to form four sp3 hybrid orbitals. These are typically seen in single-bonded carbon atoms in alkanes, like in chloroethane (CH3-CH2-Cl).
Sp3 hybridized carbons form bonds that are more flexible and less tightly held compared to sp2 or sp hybridized carbons. This makes the carbon-halogen bond in chloroethane weaker and more reactive.
The electronic environment in sp3 hybridization allows for free rotation around single bonds, contributing to increased reactivity. The alkyl groups attached to the sp3 carbon donate electron density, making the carbon more nucleophilic and enhancing the reactivity of the C-Cl bond.