Question

Complete the following nucleophilic substitution reactions. In each reaction, show all electron pairs on both the nucleophile and the leaving group. (a) (b) CCC(I)CSCSC (c) CC(C)CCBr

Short answer

Answer: In a nucleophilic substitution reaction, the nucleophile is usually a negatively charged species or a molecule with lone pair electrons, while the leaving group is typically a halide ion or a similar electron-withdrawing group attached to the carbon atom. Showing the movement of electron pairs using curved arrow notation illustrates the mechanism of the reaction, highlighting the attack of the nucleophile on the electrophilic carbon and the departure of the leaving group. This helps in understanding the process and predicting the products of the reaction.

Step-by-step solution

Identify the nucleophile and leaving group

Identify the nucleophile (Nu) and the leaving group (X) in the reaction. In a general SN2 reaction, the nucleophile can be any negatively charged species or a molecule with lone pair electrons, and the leaving group is usually a halide ion or a similar electron-withdrawing group attached to the carbon atom. For example, consider a reaction between alkyl bromide (CH3-Br) and hydroxide ion (OH-). Here, OH- is the nucleophile, and Br- is the leaving group.

Show the movement of electron pairs

In an SN2 reaction, the nucleophile attacks the carbon atom to which the leaving group is attached, from the opposite side of the leaving group. Show the movement of electron pairs by using curved arrow notation: 1. Draw a curved arrow from the lone pair of electrons on the nucleophile (Nu) to the electrophilic carbon atom (C) attached to the leaving group (X). 2. Draw another curved arrow from the bond between the carbon atom (C) and the leaving group (X) to the leaving group (X), indicating the breaking of the bond and the leaving group acquiring a negative charge. For the example reaction (CH3-Br + OH-), the curved arrow notation would look like this: OH:¯ ---> CH3-Br ---> Br:¯

Draw the products of the reaction

After showing the movement of electron pairs, draw the products of the nucleophilic substitution reaction. In an SN2 reaction, the nucleophile forms a bond with the electrophilic carbon, and the leaving group departs with a negative charge. For the example reaction (CH3-Br + OH-), the final product would be: CH3-OH + Br:¯ Now you should be able to show the steps of a nucleophilic substitution reaction, including all electron pairs on both the nucleophile and the leaving group. Always remember to identify the nucleophile and leaving group, show the movement of electron pairs using curved arrow notation, and then draw the products of the reaction.

Key concepts

SN2 Mechanism

Nucleophilic substitution reactions often occur through an SN2 mechanism, which stands for Substitution Nucleophilic Bimolecular reaction. The SN2 process involves a single step, where the nucleophile gets involved directly in the rate-determining step of the reaction. This type of reaction is characterized by its concerted nature, meaning that bond-breaking and bond-forming actions happen simultaneously.

In an SN2 reaction, the nucleophile attacks the electrophilic carbon carrying the leaving group from the side opposite to the leaving group. This leads to the inversion of configuration at the carbon, much like flipping an umbrella inside out. The stereochemistry plays a crucial role, especially if you're dealing with chiral centers. This reaction mechanism is often seen in reactions involving primary or methyl halides because they offer less steric hindrance for the nucleophile's backside attack.

• The rate of the reaction depends on both the concentration of the nucleophile and the substrate, making it a second-order reaction.
• The SN2 process is generally faster when the leaving groups are weak bases, such as halide ions like chloride or bromide.
• Polar aprotic solvents, like acetone or dimethyl sulfoxide (DMSO), can help increase the reactivity of the nucleophile by not solvating it too strongly.

Nucleophile and Leaving Group

Understanding the nucleophile and the leaving group is vital in mastering nucleophilic substitution reactions.

A **nucleophile** is a chemical species that donates a pair of electrons to an electrophile in order to form a chemical bond. Nucleophiles are often negatively charged or neutral molecules with lone pairs of electrons. Common examples include hydroxide ions (OH⁻), chloride ions (Cl⁻), and molecules like ammonia (NH₃). A good nucleophile tends to have a strong affinity for a positively polarized or electron-deficient carbon.

On the other hand, a **leaving group** is an atom or group of atoms that leaves the substrate molecule, taking a pair of electrons with it, in the reaction. The quality of a leaving group is often judged by its ability to stabilize its negative charge. Halides like bromide and iodide are good leaving groups, while hydroxide is not. The better the leaving group, the more favorable the nucleophilic substitution reaction.

• **Nucleophile Examples:** OH⁻, CN⁻, HS⁻, NH₃
• **Good Leaving Groups:** Cl⁻, Br⁻, I⁻, H₂O

Curved Arrow Notation

One of the most effective ways to represent the movement of electrons in chemical reactions is through curved arrow notation. This technique is especially important for visualizing the SN2 mechanism and helps students track how electron pairs migrate during the reaction.

Curved arrow notation uses arrows with tails and heads to indicate electron flow. The tail of an arrow starts from an electron pair source, such as a lone pair on a nucleophile, and points towards the electron-deficient target area, like an electrophilic carbon atom.

Steps to Show SN2 Reaction Using Curved Arrows

• Draw a curved arrow from the nucleophile's lone pair to the electrophilic carbon atom. This illustrates the nucleophile's electron pair forming a new sigma bond with the carbon.
• Show a second curved arrow starting from the bond between the carbon atom and the leaving group, pointing towards the leaving group. This arrow represents the electrons in the sigma bond moving to the leaving group as it breaks away.

Visual tools like this help in understanding the dynamic nature of the reaction and ensure that all electron pairs are accounted for. This is crucial when you need to track changes in charge and identify the new configuration of atoms post-reaction.