Question

Among \(\mathrm{PCl}_{3}, \mathrm{PCl}_{5}\) and \(\mathrm{SOCl}_{2}, \mathrm{SOCl}_{2}\) is prefered to prepare \(\mathrm{R}-\mathrm{Cl}\) because (A) It gives \(S_{\mathrm{N}} 1\) product (B) Other two products are escapable gases (C) Retention of configuration is occuring (D) Diastereomeric intermediates are formed

Short answer

The correct answer is (C) Retention of configuration is occurring, as SOCl2 reacts with alcohols in a way that retains the stereochemistry at the carbon atom, making it the preferred choice for preparing R-Cl compounds when the stereochemistry of the product is crucial.

Step-by-step solution

Understanding the Compounds

The first step is to understand what these compounds do. PCl3, PCl5, and SOCl2 are all compounds that can be used to convert alcohols into alkyl chlorides (R-Cl). Each of these compounds performs its own mechanism to accomplish this.

Understanding Retention and Inversion of Configuration

Retention of configuration in a reaction means the stereochemistry (three-dimensional structure) of the molecule is preserved after the reaction. Inversion of configuration means the stereochemistry is reversed. In the case of changing -OH to -Cl, using PCl3 or PCl5 will cause an inversion of configuration.

Understanding Retention of Configuration with SOCl2

The reaction with SOCl2 happens in a way that does not invert the configuration. Instead, the configuration is retained. This is ideal in cases where the stereochemistry of the product is important.

Identifying the Right Answer

With this information, we can now identify the correct answer. The other options mention SN1 product, escapable gases, and diastereomeric intermediates. However, none of these are relevant to why SOCl2 is preferred for the preparation of R-Cl. The correct answer is (C) Retention of configuration is occurring. In conclusion, understanding the way each compound reacts, as well as the meaning and importance of retention and inversion of configuration in reactions, helps us to understand why SOCl2 is preferred for the preparation of R-Cl.

Key concepts

Stereochemistry

Stereochemistry is like the 3D puzzle of chemistry. It helps us understand the exact arrangement of atoms in space. When we talk about stereochemistry, we focus on how different atoms are oriented around a central carbon. Imagine a carbon atom like a small planet with four bonds pointing in different directions. In some reactions, the direction of these bonds can change, altering the molecule's shape. This is crucial because if the structure changes, it can vastly affect how a molecule behaves. For example, if you have a molecule that's part of a medicine, changing its shape might mean that it won't work as intended anymore. Chemical reactions often lead to either retention or inversion of this stereochemical setup, implying that it's either kept the same or flipped.

SOCl2 Reaction Mechanism

Thionyl chloride (\( \mathrm{SOCl}_2 \)) is popular for converting alcohols into alkyl chlorides smoothly and predictably. This occurs through a mechanism that doesn't just happen in a single leap but through several steps.

• First, the \( \mathrm{OH} \) group of the alcohol attacks the sulfur atom in \( \mathrm{SOCl}_2 \), kicking off the reaction chain.
• This attachment helps create a good leaving group, which means the \( \mathrm{OH} \) part can depart with ease, paving the way for chloride ions to take its place.
• The reaction involves the formation of a chlorosulfite intermediate, a temporary structure during which all this transition occurs.

In the presence of a base like pyridine, the reaction proceeds with better control, minimizing side reactions and ensuring the conversion is as clean as possible. This multistep process is unique compared to other methods like using \( \mathrm{PCl}_3 \) or \( \mathrm{PCl}_5 \), which rely on different mechanisms.

Retention of Configuration

A remarkable feature of using \( \mathrm{SOCl}_2 \) in these transformations is its capacity for retention of configuration. This is important because it maintains the original stereochemistry of the alcohol.Retention means that if you start with a molecule in a certain three-dimensional arrangement, you'll end with one in the same arrangement. This is vital in the synthesis of compounds where specific stereochemistry is required to achieve the desired function. Comparatively, other methods might cause inversion, which flips the spatial arrangement and can lead to a different outcome. Inverting the arrangement is like accidentally wearing your shirt inside-out. It appears somewhat similar, but there's a significant difference as you closely inspect it. This ability to retain the exact layout greatly impacts industries focused on pharmaceuticals where the correct spatial setup is critical to a drug's effectiveness. Choosing \( \mathrm{SOCl}_2 \) ensures you keep the intended configuration intact, avoiding any unintended flipping that could alter the product's characteristics.