Question

The major product of the following reaction is $$ (-)-2 \text {-methylbutan-1-ol }+\operatorname{Conc} \cdot \mathrm{HCl} \stackrel{\Delta}{\longrightarrow} $$ (A) \((-)-2\)-chloro-3-methyl butane (B) (+)-2-chloro-3-methyl butane (C) \((+)-1\)-chloro- 2 -methyl butane (D) 2-methyl but-2-ene

Short answer

The major product of the given reaction is \((-)-2\)-chloro-3-methyl butane (A), as it follows the SN1 reaction mechanism with a secondary carbocation intermediate and retains the same stereochemistry as the reactant.

Step-by-step solution

Identify the reaction type

This is a nucleophilic substitution (SN1) reaction between an alcohol with concentrated HCl under heat. The mechanism involves the formation of a carbocation as an intermediate, and then substitution by the nucleophile (Cl-).

Identify the leaving group and carbocation formation

In this reaction, the hydroxyl group (-OH) in (-)-2-methylbutan-1-ol is the leaving group. When heated with concentrated HCl, it will be protonated to form water, which is a better leaving group. Then, water leaves to form a carbocation. $$ (-)-2\text{-methylbutan-1-ol} + HCl \rightarrow (-)-2\text{-methylbutan-1-yl}^{+} $$

Analyze carbocation stability

The carbocation formed is secondary, and no rearrangement takes place since there is no more stable carbocation available. The carbocation will maintain its stereochemistry.

Nucleophilic attack

Now, the nucleophile, Cl- from HCl, will attack the carbocation and form a new bond. $$ (-)-2\text{-methylbutan-1-yl}^{+} + Cl^{-} \rightarrow (-)-2\text{-chloro-3-methyl butane} $$

Product analysis and answer

The major product formed is (-)-2-chloro-3-methyl butane with the same stereochemistry as the reactant, as it follows the SN1 reaction mechanism. Therefore, the correct answer is (A) \((-)-2\)-chloro-3-methyl butane.

Key concepts

Nucleophilic Substitution

In chemistry, understanding reactions is crucial, and one common type of reaction is the nucleophilic substitution, abbreviated as SN1. This process involves the replacement of a leaving group in a molecule by a nucleophile. In simple terms, a nucleophile is a 'nucleus-loving' entity with a lone pair of electrons ready to form a bond.

During an SN1 reaction, the leaving group, which is often weakly bonded or a poor electron-pair donor, departs from the substrate, typically leading to the formation of an intermediate structure known as a carbocation. This positively charged ion is highly reactive and is quickly attacked by the nucleophile, which results in the substitution of the leaving group.

Example in Practice:In the textbook exercise, a hydroxyl group is initially attached to the substrate, which is -2-methylbutan-1-ol. It is converted into a better leaving group, water, in the presence of concentrated HCl, and leaves the molecule. This departure ushers in the nucleophilic attack by the chloride ion, forming the new product, 2-chloro-3-methylbutane.

Carbocation Stability

The stability of carbocations plays a pivotal role in determining the pathway and outcome of many chemical reactions, including SN1. Carbocations are intermediates with a positive charge on a carbon atom. These positively charged carbon centers are unstable, as carbon prefers to have four bonds rather than three to maintain a neutral charge.

The stability of a carbocation is influenced by several factors:

• The number of alkyl groups attached to the positively charged carbon.
• The presence of electron-donating groups nearby that can help disperse the positive charge.
• Resonance stabilization, if available.

During the SN1 reaction in our exercise, the carbocation formed is secondary, as two R groups are attached to it, increasing its stability compared to a primary carbocation. No more stable carbocation can be formed by rearrangement; hence, the reaction proceeds with this intermediate.

Stereochemistry

Stereochemistry is the study of the three-dimensional structure of molecules and how this affects their chemical behavior. One key aspect of stereochemistry is the concept of chirality, which refers to molecules that have non-superimposable mirror images, much like human hands. These molecules are referred to as enantiomers, and they can have dramatically different biological activities.

During an SN1 reaction sequence, the stereochemistry at the carbon center can be altered due to the carbocation intermediate, which allows the nucleophile to attack from either side. This typically results in a racemic mixture — a 1:1 mixture of enantiomers. However, if the substituting nucleophile attacks in such a way that the configuration of the carbon center remains similar to the starting material, the product's stereochemistry will be retained.

In our exercise, the substitution maintains the stereochemistry of the reactant because the nucleophile attacks from the same side as the leaving group departed. This retention of configuration in the produced 2-chloro-3-methylbutane mirrors the chiral structure of the original alcohol.