Problem 16 - Basic Principles of Organic Chemistry

Question

Explain the following observations: a. The tertiary chloride, apocamphyl chloride, is unreactive in either \(S_{N} 1\) or \(S_{N} 2\) reactions. For example, no reaction occurs when its solution in aqueous ethanol containing \(30 \%\) potassium hydroxide is refluxed for 20 hours. CC1(C)CCCCC1(C)C chlonide b. Chloromethyl alkyl (or aryl) ethers, \(\mathrm{ROCH}_{2} \mathrm{Cl}\), are very reactive in \(S_{\mathrm{N}} 1\) solvolysis reactions. Compared to chloromethane, the rate of hydrolysis of chloromethyl phenyl ether is about \(10^{14}\). Also, the rate of hydrolysis is retarded significantly by lithium chloride.

Step-by-Step Solution

Answer

a. Apocamphyl chloride is sterically hindered, preventing \(S_{N}1\) and \(S_{N}2\) reactions. b. Chloromethyl alkyl ethers stabilize carbocations, enhancing \(S_{N}1\) reactivity, while lithium chloride retards the reaction by common ion effect.

Step 1: Understanding Apocamphyl Chloride Structure

Apocamphyl chloride is a tertiary chloride, meaning the chlorine atom is attached to a tertiary carbon atom. Its structure is represented by the smiles notation: \( ext{CC1(C)CCCCC1(C)C Cl} \), which indicates a highly branched and sterically hindered molecule.

Step 2: Analyzing Apocamphyl Chloride's Reaction Mechanism

Both \(S_{N}1\) and \(S_{N}2\) reactions are hindered in apocamphyl chloride. For \(S_{N}1\), a stable carbocation is required, but the steric hindrance and cyclic structure of apocamphyl chloride prevent the formation of a stable carbocation. For \(S_{N}2\), the steric hindrance around the reaction site inhibits a backside attack by the nucleophile, making \(S_{N}2\) reactions unfeasible.

Step 3: Understanding the Structure of Chloromethyl Alkyl Ethers

Chloromethyl alkyl ethers have the general structure \( ext{ROCH}_{2} ext{Cl} \), where the ether group can stabilize positive charges through resonance. This stabilization is significant in reaction mechanisms involving carbocation intermediates.

Step 4: Analyzing Chloromethyl Alkyl Ethers' Enhanced Reactivity

The resonance stabilization provided by the ether group in \( ext{ROCH}_{2} ext{Cl} \) significantly stabilizes the carbocation formed during \(S_{N}1\) reactions, making these compounds very reactive. Compared to chloromethane, this enhanced stabilization increases reactivity by a factor of \(10^{14}\) due to ease of carbocation formation.

Step 5: Evaluating the Impact of Lithium Chloride on Solvolysis

Lithium chloride adds an additional common ion (Cl-) to the reaction mixture, which can either shift equilibria or stabilize intermediates less effectively. This results in the significant retardation of the solvolysis rate, as the mechanism becomes less energetically favorable due to decreased carbocation stability.

Key Concepts

Tertiary Chloride Reactivity

Tertiary chlorides, like apocamphyl chloride, are characterized by a chlorine atom bonded to a tertiary carbon. This specific configuration greatly affects its reactivity in chemical reactions, especially in nucleophilic substitution reactions. Apocamphyl chloride is highly branched, which increases steric hindrance. This structural feature makes it resistant to both \(S_{N}1\) and \(S_{N}2\) mechanisms. In \(S_{N}1\) reactions, the tertiary carbon would typically form a carbocation intermediate, but in the case of apocamphyl chloride, steric hindrance impedes this process. For \(S_{N}2\) reactions, the crowded environment around the chlorine atom prevents nucleophiles from easily attacking the carbon atom. Thus, the unique tertiary structure of apocamphyl chloride renders it unreactive in these types of reactions.

SN1 and SN2 Reactions

Understanding the differences between \(S_{N}1\) and \(S_{N}2\) reactions is crucial for predicting how a compound will behave in a reaction. In \(S_{N}1\) reactions, the first step involves the formation of a carbocation intermediate when the leaving group departs. This reaction type is unimolecular and dependent largely on the stability of the carbocation that forms. \(S_{N}1\) reactions tend to favor more stable carbocations, typically tertiary carbocations.

On the other hand, \(S_{N}2\) reactions involve a bimolecular mechanism where a nucleophile directly attacks the carbon containing the leaving group. This type of reaction occurs in a single step, where the attacking nucleophile pushes the leaving group out in what is termed a 'backside attack.' \(S_{N}2\) reactions are sensitive to steric hindrance; the more crowded the reactive center, the slower or even non-reactive the molecule will be.

Carbocation Stability

Carbocation stability plays a pivotal role in guiding \(S_{N}1\) reactions. A carbocation is a positively charged carbon atom, with three bonds and a significant electron deficiency. The stability of carbocations follows a predictable order: tertiary > secondary > primary. This is due to the hyperconjugation and electron-donating effects of the surrounding alkyl groups.

For instance, in apocamphyl chloride, despite being a tertiary chloride, the surrounding bulky groups inhibit the formation of a carbocation, making a successful \(S_{N}1\) reaction unlikely. In contrast, structures that facilitate resonance, like aryl methyl ethers, can greatly stabilize carbocations, enhancing their aptitude for \(S_{N}1\) reactions.

Resonance Stabilization

Resonance stabilization is a key concept in understanding why some compounds exhibit enhanced reactivity in \(S_{N}1\) reactions. This phenomenon occurs when a positive charge, such as in a carbocation, can be delocalized over two or more atoms through overlapping \(\pi\)-bonds or lone pair-p-\(\pi\) conjugation.

An excellent example is chloromethyl alkyl ethers (\(\mathrm{ROCH}_{2}\mathrm{Cl}\)), where the ether group significantly stabilizes the carbocation. This stabilization occurs via resonance, where the lone pairs on oxygen can help delocalize the positive charge. In practical terms, this means chloromethyl alkyl ethers are much more reactive than chloromethane in \(S_{N}1\) reactions, as they can more easily form a stable carbocation intermediate.

Steric Hindrance

Steric hindrance refers to the physical obstruction of reactive sites by bulky groups within a molecule. In the context of nucleophilic substitution reactions, it is a major factor influencing the rate and feasibility of \(S_{N}1\) and \(S_{N}2\) reactions.

Tertiary chlorides such as apocamphyl chloride illustrate this effect dramatically. The high degree of branching around the reactive center creates a crowded environment, which inhibits nucleophiles from approaching and reacting with the carbon. This makes \(S_{N}2\) reactions particularly difficult or impossible since these reactions require a direct path for the nucleophile to attack. Similarly, the steric environment can also block the formation of carbocation intermediates in \(S_{N}1\) reactions, due to the need for adequate space to accommodate the carbocation structure.