Question

Draw a structural formula for the product of each \(\mathrm{S}_{\mathrm{N}} 2\) reaction. Where configuratios the starting material is given, show the configuration of the product.

Short answer

Question: Draw the structural formula of the product formed in the SN2 reaction between 2-chlorobutane and sodium cyanide, and determine the configuration of the product. Answer: In an SN2 reaction between 2-chlorobutane and sodium cyanide, we can follow the steps discussed above to determine the product. Step 1: The reaction mechanism is SN2, meaning that a nucleophilic attack will result in inversion of stereochemistry at the electrophilic center. Step 2: The starting material is 2-chlorobutane with chlorine as the leaving group. The carbon attached to the chlorine atom is the electrophilic center. Step 3: The nucleophile is the cyanide anion (\(\mathrm{CN}^{-}\)), provided by the sodium cyanide. Step 4: The cyanide anion will attack the electrophilic center of 2-chlorobutane, and the chlorine will leave, resulting in 2-cyanobutane. Since the reaction is SN2, there will be an inversion of stereochemistry at the electrophilic center. Step 5: Assuming the starting material (2-chlorobutane) has an R configuration at the electrophilic center, the product (2-cyanobutane) will have an S configuration at the electrophilic center due to the inversion caused by the SN2 reaction. Thus, the product of the SN2 reaction between 2-chlorobutane and sodium cyanide is 2-cyanobutane with an S configuration at the electrophilic center.

Step-by-step solution

Understand the SN2 reaction mechanism

An SN2 reaction (Substitution Nucleophilic Bimolecular) involves nucleophilic attack by a nucleophile on an electrophilic center (usually a tetrahedral carbon bonded to a leaving group) resulting in the leaving group's expulsion and the formation of a new bond between the nucleophile and the electrophilic center. One important feature of SN2 reactions is that they result in an inversion of stereochemistry at the electrophilic center.

Identify the starting material and leaving group

To draw the structural formula for the product of each SN2 reaction, start by identifying the starting material and leaving group in the given exercise. The leaving group is usually a halide atom, such as \(\mathrm{Cl}\), \(\mathrm{Br}\), or \(\mathrm{I}\), or any other group with a higher electronegativity that can easily detach from the electrophilic center.

Identify the nucleophile

Identify the nucleophile participating in the reaction. This is typically an anionic species containing a lone pair of electrons, which can donate them to the electrophilic center leading to the formation of a new bond.

Draw the products of the SN2 reaction

Based on the knowledge of the starting material, leaving group, and nucleophile, draw the structural formula of the product formed in SN2 reaction. Keep in mind the stereochemical inversion at the electrophilic center caused by the SN2 reaction. This will result in an inverted configuration at the electrophilic center, relative to the configuration of the starting material.

Show the configuration

If the configuration of the starting material is given, depict the configuration of the product using R/S notation according to the Cahn-Ingold-Prelog rules. Remember, the SN2 reaction leads to an inverted configuration of the electrophilic center. So, if the starting material configuration is R, the product configuration will be S, and vice versa. By following these steps, you will be able to draw the structural formula of an SN2 reaction's product and determine its configuration based on the starting material's given configuration.

Key concepts

Stereochemistry Inversion

One of the most intriguing aspects of an SN2 reaction is the stereochemistry inversion that occurs. This inversion is akin to an umbrella turning inside out in a strong wind. In chemistry terms, the nucleophile attacks the carbon that is attached to the leaving group from the opposite side. This attack from the rear is what causes the inversion of configuration at the carbon atom involved in the reaction.

Imagine a three-dimensional space where the carbon atom is at the center, and the groups attached to it are pointing towards the corners of a tetrahedron. When the nucleophile approaches, it forces the leaving group to depart, and in the process, the spatial arrangement of the molecules changes. This switch is not just a fascinating dance of atoms; it is crucial for understanding the reactivity and properties of the resulting compound - especially in biological systems, where the shape and orientation of molecules can determine the difference between a useful medicine and a harmful toxin.

Nucleophilic Substitution

Nucleophilic substitution is the heart of the SN2 reaction. Let's break down the term: 'nucleophilic' means 'nucleus loving,' indicating that the species doing the attacking (the nucleophile) is in search of a positive charge. 'Substitution' means that one group will take the place of another.

The process begins when a nucleophile, armed with its lone pair of electrons, seeks out a positively charged atom (typically a carbon atom in organic molecules) to form a bond. In an SN2 reaction, the magic happens in a single step - the nucleophile attacks the carbon, ejecting the leaving group and taking its place. This 'single-step' character means that SN2 reactions are quite sensitive to the steric hindrance – the more crowded the carbon atom is, the less likely the nucleophile will be able to approach and successfully attack.

Structural Formula

A structural formula is a plan of attack for visualizing molecules. It doesn’t just tell us what is in the compound; it shows us how atoms in the molecule connect and arrange themselves. In organic chemistry, the structural formula is invaluable, as it conveys both the molecular composition and the organization of the molecule.

When we draw structural formulas for SN2 reactions, we’re not just scribbling lines and letters; we are creating a map that shows how the nucleophile joins the fray and how the leaving group exits. The use of wedges and dashes helps us to represent the three-dimensional arrangement of the atoms, which is crucial for understanding the outcome of stereochemistry-involving reactions like SN2.

R/S Configuration Notation

Staring at molecular structures can feel like being lost in a hall of mirrors, but the R/S configuration notation is our guide out of the labyrinth. This system allows chemists to communicate about a molecule's 3D orientation in a simple and standardized way. The letters 'R' (from the Latin 'rectus', meaning 'right') and 'S' (from 'sinister', meaning 'left') are used to describe the arrangement of atoms around a chiral center — a carbon atom with four different groups attached to it.

In SN2 reactions, the nucleophile's backside attack causes a flip in this configuration: 'R' becomes 'S' and vice versa. This is critical when drawing the product of an SN2 reaction with given starting material configurations; it’s not just about erasing one group and adding another, it’s about capturing this mirror-image transformation.

Cahn-Ingold-Prelog Rules

The Cahn-Ingold-Prelog (CIP) rules are like the rules of a board game that decides the 'priority' of substituents attached to a chiral center. You can think of them as a hierarchy system that gives each group a rank based on its atomic number: the heavier the atom, the higher its priority. When applying CIP rules to R/S configuration, chemists first assign priorities to each group attached to the chiral center. Then, they orient the molecule so that the group with the lowest priority is pointing away. Observing from the side of the remaining three groups, if the order of priority goes clockwise, it’s 'R'; if it’s counterclockwise, it’s 'S'.

Students must wield the CIP rules like a sword to cut through the confusion of complex molecules. These rules are pivotal when deducing the stereochemistry of a molecule before and after an SN2 reaction. When you know a reaction causes an inversion, knowing the original 'R' or 'S' allows you to confidently declare the opposite configuration for the product.