Question

Complete the following nucleophilic substitution reactions. In each reaction, show all electron pairs on both the nucleophile and the leaving group.

Short answer

Answer: The main difference between the mechanisms of SN1 and SN2 reactions is that SN1 reactions occur in two steps involving a carbocation intermediate, while SN2 reactions occur in a single concerted step where the nucleophilic attack and leaving group departure happen simultaneously.

Step-by-step solution

SN1 Reaction: Step 1 – Formation of the Carbocation

First, identify the substrate, which is the molecule that contains the leaving group. The leaving group is usually a halogen (like Cl, Br, or I). The nucleophile can be any species with a lone pair of electrons that can form a new bond with the substrate. In an SN1 reaction, the leaving group will depart the substrate, making the substrate electron-deficient, specifically creating a carbocation intermediate. Show the first step with electron movements by using arrows to indicate the leaving group disconnecting from the substrate, leaving a positive charge on the carbon. Example: R-Br -> R^(+) + Br^(-)

SN1 Reaction: Step 2 – Nucleophilic Attack

In the second step of an SN1 reaction, the nucleophile (denoted as Nu) will attack the carbocation intermediate formed in the previous step. The lone pair on the nucleophilic atom will use to form a new bond with the electrophilic carbon in the carbocation. Draw the nucleophile approaching and forming a bond with the electrophilic carbon using arrows pointing from the lone pair on the nucleophile to the carbocation carbon. Example: R^(+) + Nu^- -> R-Nu

SN1 Reaction: Complete Mechanism

Now, combine both steps of the SN1 reaction into a single mechanism, with the nucleophile attacking the carbocation intermediate, formed after the leaving group has left the substrate. Example: R-Br -> R^(+) + Br^(-) R^(+) + Nu^- -> R-Nu

SN2 Reaction: Step 1 – Nucleophilic Attack & Leaving Group Departure

Unlike SN1 reactions, SN2 reactions occur in a single concerted step. The nucleophile attacks the electrophilic carbon that is attached to the leaving group from the backside, which is opposite from where the leaving group is connected. At the same time, the bond between the electrophilic carbon and the leaving group breaks, and the leaving group departs. Use arrows to show the nucleophile forming a bond with the electrophilic carbon and the bond breaking between the electrophilic carbon and the leaving group. Example: Nu^- + R-Br -> R-Nu + Br^(-)

SN2 Reaction: Complete Mechanism

Since the SN2 reaction occurs in a single step, simply show the nucleophile attacking the substrate and the leaving group departing in the same mechanism. Example: Nu^- + R-Br -> R-Nu + Br^(-)