Question

Consider the following compounds: 1\. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHCl}-\mathrm{CH}_{3}\) 2\. \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Cl}\) 3\. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Cl}\) These compounds are dehydrohalogenated by treatment with a strong base under identical conditions. The correct sequence of the increasing order of reactivity of these compounds in the given reaction is (a) \(3,1,2\) (b) \(3,2,1\) (c) \(1,2,3\) (d) \(2,1,3\)

Short answer

The correct order is (d) 2, 1, 3.

Step-by-step solution

Identify the Type of Reaction

Dehydrohalogenation is a reaction where a hydrogen halide (HX) is removed from an alkyl halide to form an alkene. This involves the removal of a halogen atom and a hydrogen atom from adjacent carbon atoms.

Atomic and Bonding Considerations

For dehydrohalogenation, you need a good leaving group (like Cl) and a hydrogen atom on a carbon adjacent to where the halogen is attached. The presence of double bonds in the starting material can stabilize the alkene formed via conjugation.

Analyze Each Compound

1. \\( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHCl}-\mathrm{CH}_{3} \) – Secondary chloride with adjacent hydrogens available.\2. \\( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Cl} \) – Allylic chloride, where the chlorine is adjacent to a double bond, leading to increased stability of the resulting alkene due to conjugation.\3. \\( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Cl} \) – Primary chloride with less stable reaction intermediates.

Determine Reactivity Order

The allylic compound (2) will be most reactive due to resonance stabilization of the intermediate. The secondary chloride (1) comes next due to favorable sterics. The primary chloride (3) is least reactive due to less stable intermediates.

Key concepts

Alkyl Halide

Alkyl halides are organic compounds where one or more halogen atoms (like chlorine, bromine, or iodine) are bonded to an alkyl group. These compounds are quite reactive and can participate in a variety of chemical reactions, such as dehydrohalogenation.
Alkyl halides can be classified based on the number of carbons attached to the carbon that holds the halogen. For example:

• Primary alkyl halides: The halogen-bearing carbon is connected to only one other carbon.
• Secondary alkyl halides: The carbon bonded to the halogen is attached to two other carbons.
• Tertiary alkyl halides: The carbon bonded to the halogen is connected to three other carbons.

In the context of dehydrohalogenation, alkyl halides serve as the starting point for the formation of alkenes, with the type of alkyl halide influencing the reactivity of the reaction.

Reaction Mechanisms

Reaction mechanisms describe the step-by-step process through which reactants transform into products. In dehydrohalogenation, the mechanism typically involves two main steps: the removal of a hydrogen halide.
The reactants in dehydrohalogenation include an alkyl halide and a strong base. The base helps in removing a hydrogen ion from a carbon atom adjacent to the one holding the halogen. This, in turn, facilitates the removal of the halogen atom, leading to the formation of a double bond – resulting in an alkene.

• The leaving group (such as chloride) departs, leaving behind a carboanion intermediate.
• The base abstracts a proton (hydrogen), allowing the electrons to form a new pi bond.

Understanding these mechanisms helps predict and control the outcomes of organic reactions.

Chemical Reactivity

Chemical reactivity involves factors that affect how quickly or slowly a chemical reaction proceeds. In dehydrohalogenation reactions, several elements influence the reactivity of the alkyl halides.
One major factor is the stability of the intermediate during the reaction process. Compounds forming stable intermediates react faster. For instance, in allylic halides, the intermediate formed after the halogen departs is resonance-stabilized. This greatly enhances reactivity.

• Allylic stability: Conjugation and resonance stabilize intermediates.
• Steric effects: Bulky groups can hinder reactions by creating less accessible sites for a base or other reactants.

These influences dictate why some compounds react more readily than others in identical conditions.

Organic Chemistry

Organic chemistry is the branch of chemistry that studies the structure, properties, and reactions of organic compounds, primarily those containing carbon and hydrogen. It is the foundation for understanding complex biochemical processes and designing new materials.
Dehydrohalogenation is a classic reaction in organic chemistry that exemplifies core concepts, such as:

• The formation and breaking of covalent bonds, especially those leading to pi bond formation in alkenes.
• The reactivity trends of different functional groups, such as halides.
• The use of bases to initiate chemical transformations, offering a pathway to develop further reactions beyond just the initial alkene production.

Grasping the principles of organic chemistry is crucial for anyone looking to innovate in fields ranging from pharmaceuticals to new material synthesis.