SN1 Reaction (Energy Diagram and Mechanism) PDF

SN1 Reaction (Energy Diagram and Mechanism) CHEMISTRY 3 LG 2.10  Objective 1 Summarize the characteristics of unimolecular nucleophilic substitution SN1.  Nucleophilic Substitutions In nucleophilic substitution, one group is replaced by a nu...

SN1 Reaction (Energy Diagram and Mechanism) CHEMISTRY 3 LG 2.10  Objective 1 Summarize the characteristics of unimolecular nucleophilic substitution SN1.  Nucleophilic Substitutions In nucleophilic substitution, one group is replaced by a nucleophile (a nucleophile is an electron-rich species). The group that leaves is called a leaving group. In the example above, the leaving group is Br-. There are two main types of nucleophilic substitution: SN1 and SN2. SN1 – Substitution, 1st step is for the bond between the leaving group and the α-carbon to break and the leaving group Nucleophilic, to depart. This forms a carbocation intermediate which is Unimolecular then attacked by the nucleophile in the 2nd step.  The 1st step is the rate determining step (RDS), and SN1 – Substitution, as the nucleophile isn’t involved in this step, its concentration and strength is irrelevant to the Nucleophilic, overall rate of the reaction. Unimolecular. The RDS step only involves one species (the substrate) and is, therefore, unimolecular – hence the 1 in SN1.  SN1 Reaction Mechanism The mechanism of an SN1 reaction would be drawn as follows: Note the curved arrow formalism that is used to show the flow of electrons. 6 Key features of the SN1 mechanism are that it has two steps, and carbocations are formed as reactive intermediates.  SN1 Kinetics SN1 reactions exhibit 1st order kinetics. The reaction is unimolecular – involving only the alkyl halide. The identity and concentration of the nucleophile have no effect on the reaction rate. Therefore, the nucleophile does not appear in the rate equation.  Stereochemistry of SN1 Reactions In Step , attack of the nucleophile can occur on either side to afford two products which are a pair of enantiomers. Loss of the leaving group in An equal amount of the two Step generates a planar enantiomers is formed—a racemic carbocation that is achiral. mixture. This process is called racemization. Stereochemistry of SN1 Reactions  Sample Problem Draw the products of each SN1 reaction and indicate the stereochemistry of any stereogenic centers. 1. 2. Summary: SN1 Mechanism  When can SN1 occur? In the first step of an SN1 reaction, the bond between the α-carbon and leaving group breaks, and a carbocation is formed. This step has the highest barrier to reaction and therefore, is the rate determining step. If we enhance this step, we will enable SN1 to occur. There are two ways we can do this:  When can SN1 occur? 1. Stable carbocation intermediate. The reaction intermediate is a carbocation. If we stabilize this carbocation, we will bring down the barrier to reaction for the first step.  When can SN1 occur? 2. Good leaving group. A good leaving group will lower the amount of energy we have to put in to remove it and lower the barrier to reaction. A good leaving group is essential for SN1. Without it, SN1 does not occur.  The Leaving Group In a nucleophilic substitution reaction of R-X, the C-X bond is heterolytically cleaved, and the leaving group departs with the electron pair in that bond, forming X:¯ The more stable the leaving group X:¯, the better able it is to accept an electron pair. For example, H2O is a better leaving group than HO¯ because H2O is a weaker base. Trends in Leaving Group Ability The weaker the base, the better the leaving group. Trends in Leaving Group Ability The weaker the base, the better the leaving group. Good Leaving Groups  Poor Leaving Groups Conjugate bases of weaker acids are poorer leaving groups.  Sample Problem Which alkyl halide in each pair reacts faster in an SN1 reaction? 1. or 2. or  Nucleophiles and Bases Nucleophiles and bases are structurally similar: both have a lone pair or a  bond. They differ in what they attack. Bases attack protons. Nucleophiles attack other electron-deficient atoms (usually carbons).  Solvation by Polar Protic Solvents Polar protic solvents solvate both cations and anions well. If the salt NaBr is used as a source of the nucleophile Br¯ in H2O: The Na+ cations are solvated by ion-dipole interactions with H2O molecules. The Br¯ anions are solvated by strong hydrogen bonding interactions. Nucleophilicity in Polar Protic Solvents Smaller, more electronegative anions are solvated more strongly, effectively shielding them from reaction. In polar protic solvents, nucleophilicity increases down a column of the periodic table as the size of the anion increases. This is the opposite of basicity.  What are the Polar Aprotic Solvents? Polar aprotic solvents also exhibit dipole-dipole interactions, but they have no O-H or N-H bonds. They are incapable of hydrogen bonding.