Hybridization of the Electrophilic Carbon Nucleophilic substitution occurs at sp³-hybridized carbons. This type of reaction only rarely occurs at sp²- hybridized carbons and only under special circumstances. The SN2 mechanism is unlikely to occur because the nucleophile would be repelled by electrons in the pi cloud. The SN1 mechanism would not occur because a vinylic carbocation is much less stable than even a primary carbocation. The carbocation center would be on a sp-hybridized carbon with little inductive stabilization. The pi-cloud cannot participate in stabilization because it is orthogonal to the carbocation center Effect of Solvent in Nucleophilic Substitution Mechanisms: Solvent plays an important role in nucleophilic reactions. To begin with, solvents can be polar or nonpolar. In nucleophilic substitution reactions, polar solvents are necessary because the nucleophile, leaving group, transition states and intermediates are all polar. (Remember the rule "like dissolves like".) The next consideration is that polar solvents can be protic or aprotic. Protic solvents are those that undergo hydrogen bonding (e.g., H₂O, ROH, NH3). Aprotic solvents do not undergo hydrogen bonding (e.g., CH2Cl2, CH3CN, acetone). Experiments suggest that polar protic solvents hinder the SN2 process because they strongly solvate the nucleophile, inhibiting its ability to react. Polar aprotic solvents do not solvate the nucleophile as strongly, allowing it more freedom to react with the substrate. Reactions occurring by the SN1 mechanism are favorably affected by polar protic solvents. The transition state and intermediates are more polar than in SN2. Plus, it is likely that the solvent assists in carbocation formation by pulling the leaving group away from carbon. In this experiment, you will study the effect of solvents on two different nucleophilic substitution reactions. In the first reaction, you will observe the reaction of alkyl bromides with silver nitrate dissolved in ethanol (EtOH). A balanced equation for the process is as follows: R-Br EtOH + AgNO3 → R-OEt + AgBr↓ The visual indicator is the formation of AgBr precipitate. The rate of precipitation is the indicator of how fast the reaction occurs. Note, ethanol serves as both the solvent and the nucleophile for this reaction. AgNO3 does not participate directly in the substitution part of the reaction. It is added because it is soluble in ethanol, but the product, AgBr, is not. In the second experiment, you will observe the reaction of alkyl bromides with sodium iodide dissolved in acetone. R-Br Nal R-I + NaBr↓ The visual indicator here is the formation of NaBr precipitate. NaBr is insoluble in acetone, whereas Nal is soluble. B. Nucleophilic Substitution Nucleophilic substitution involves replacing a leaving group (typically a halogen) on an sp³-hybridized carbon atom with a nucleophile. R-X Nuc: > R-Nuc +X: It has been well established that nucleophilic substitution occurs by two different mechanisms: SN1 and SN2. SN2 Mechanism: The SN2 mechanism is a concerted process where the nucleophile replaces the leaving group in a single step, with no reaction intermediate. Nuc: 8- 18+ 5 Nuc C. X : Nuc-C Reactivity of the substrate goes as follows: methyl > 1° > 2° >3°, where 3° substrates rarely react by SN2. This order is driven by transition state stability. In the transition state, there are 5 groups surrounding carbon resulting in significant steric hindrance. With smaller groups, less crowding occurs, resulting in a faster reaction. SN1 Mechanism The SN1 mechanism is a two-step process and involves a carbocation intermediate. -X Ꮎ Nuc -Nuc (RDS) Looking at the mechanism, you will see that the first step is rate-limiting. In this step a carbocation is formed, and its stability affects reaction rate. Carbocations are fundamentally electron deficient, and alkyl groups inductively donate electron density, increasing their stability. The more stable the carbocation, the faster it forms, and the faster the rate of the reaction. As a result, reactivity of the substrates follows the pattern: 3° > 2° > 1° > methyl. Effect of Leaving Groups Regardless of whether the reaction occurs by SN1 or SN2, the nature of the leaving group will affect the reaction rate. This is because loss of the leaving group occurs in the rate-limiting step of both mechanisms. The best leaving groups are the weakest bases (most stable anions). Amongst the halides, leaving groups abilities are as follows: I (best) > Br > CI >>> F (worst). The quicker the leaving group leaves, the faster the reaction.