Problem 89 - A Complete Resource Book in Chemistry for JEE Main

Question

An alkyl chloride produces a single alkene on reaction with sodium ethoxide and ethanol. The alkene further undergoes hydrogenation to yield 2 -methylbutane. Identify the alkyl chloride from amongst the following. (a) \(\mathrm{ClCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{ClCH}_{2} \mathrm{C}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{C}(\mathrm{Cl})\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The alkyl chloride is (d) \(\text{CH}_3 \text{C(Cl)(CH}_3\text{)CH}_2 \text{CH}_3\).

Step 1: Understand the Reaction

When an alkyl chloride reacts with sodium ethoxide, a base-induced elimination (E2) reaction occurs, producing an alkene by removing a hydrogen and the chlorine atom, resulting in a double bond formation.

Step 2: Determine the Product of the Elimination Reaction

The problem states that the alkene undergoes hydrogenation to yield 2-methylbutane. This implies that the alkene must be 2-methylbut-2-ene, as its hydrogenation would result in 2-methylbutane.

Step 3: Analyze Each Alkyl Chloride for Possible Alkene Formation

We need an alkyl chloride that can form 2-methylbut-2-ene during the elimination process:- (a) \(\text{ClCH}_2 \text{CH(CH}_3\text{)CH}_2\text{CH}_3\) can dehydrohalogenate leading to but-2-ene.- (b) \(\text{ClCH}_2\text{C(CH}_3\text{)CH}_3\) has no hydrogens on the adjacent carbon to remove.- (c) \(\text{ClCH}_2 \text{CH}_2 \text{CH}_2 \text{CH}_3\) can only form 1-butene or but-2-ene, neither is suitable.- (d) \(\text{CH}_3 \text{C(Cl)(CH}_3\text{)CH}_2 \text{CH}_3\) will form 2-methylbut-2-ene as it can eliminate the chlorine and hydrogen from a secondary carbon.

Step 4: Identify the Correct Alkyl Chloride

Based on the potential products, (d) \(\text{CH}_3 \text{C(Cl)(CH}_3\text{)CH}_2 \text{CH}_3\) is the only compound that can form the necessary alkene, 2-methylbut-2-ene, which after hydrogenation gives 2-methylbutane.

Key Concepts

Alkyl Chlorides

Alkyl chlorides are organic compounds that contain a chlorine atom bonded to an alkyl group. These molecules are often reactive due to the presence of the polar C-Cl bond. The carbon atom connected to the chlorine has a partial positive charge, which makes it susceptible to nucleophilic attacks and elimination reactions.
Alkyl chlorides can undergo substitution reactions where the chlorine atom is replaced by another atom or group. Alternatively, they can undergo elimination reactions to form alkenes by removing both the chlorine and a neighboring hydrogen atom.

• In elimination reactions, the chlorine's departure helps form a double bond, changing the alkyl chloride into an alkene.
• This process is often facilitated by a base, like sodium ethoxide, that abstracts a proton (H⁺) and destabilizes the C-Cl bond.

Understanding the behavior of alkyl chlorides is crucial in predicting the outcome of reactions leading to unsaturated hydrocarbon formation, like alkenes.

Alkenes

Alkenes are hydrocarbons characterized by having at least one carbon-carbon double bond. This unsaturation gives them unique chemical properties compared to alkanes: they are more reactive due to the electron-rich double bond which can participate in a variety of chemical reactions.
The conversion of an alkyl chloride to an alkene involves the removal of hydrogen and chlorine, forming the double bond. This results in a structure that has distinct stereochemistry and reactivity.

• Alkenes can participate in addition reactions, where elements add across the double bond, reducing its unsaturation.
• The type of alkene formed in an elimination reaction depends significantly on the structure of the starting alkyl chloride and the reaction conditions.

Alkenes are the building blocks for further transformations, such as hydrogenation, leading to a saturated molecule.

Hydrogenation

Hydrogenation is a chemical reaction where hydrogen ( H₂) is added to an unsaturated compound, often in the presence of a catalyst, to saturate it. This process typically involves converting alkenes into alkanes by breaking the double bond.
In the context of the given exercise, the alkene formed is 2-methylbut-2-ene, which upon hydrogenation, becomes 2-methylbutane, a saturated alkane.

• The addition of hydrogen adds stability to the molecule as the reactive double bond is converted into a single bond.
• Catalysts such as palladium or platinum are often used to facilitate the reaction under controlled conditions.

The simplicity and utility of hydrogenation find applications in organic synthesis and industrial processes, such as converting oils into fats (margarine production).

Base-induced Elimination

Base-induced elimination is a reaction where a strong base removes a proton from a molecule, leading to the formation of a double bond, and is represented as a type of E2 reaction. This results in the conversion of an alkyl halide to an alkene.
The E2 reaction is a one-step mechanism that requires a good leaving group, like the chlorine in alkyl chlorides, and a strong base, such as sodium ethoxide.

• The concerted action involves the base abstracting a hydrogen while the halogen leaves, forming a C=C double bond.
• Reaction conditions and molecular structure significantly influence the position and geometry of the formed alkene.

Understanding base-induced elimination is fundamental in organic chemistry for predicting and controlling the synthesis of alkenes.

E2 Reaction

The E2 reaction mechanism is a bimolecular elimination that takes place in a single concerted step. It is characterized by the simultaneous removal of a proton and a leaving group, often leading to the formation of an alkene from an alkyl halide.
For an E2 reaction to occur, a strong base is necessary, and the reaction follows second-order kinetics—depending on both substrate and base concentrations.

• Unlike its counterpart, the E1 reaction, the E2 mechanism does not involve the formation of a carbocation intermediate.
• The reaction is favored under high base concentration and can lead to regioselective and stereoselective outcomes based on the substrate's structure.

Mastery of E2 reactions enables chemists to design routes for the regioselective synthesis of alkenes, as seen in the exercise where an alkyl chloride generates a single alkene type.