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Consider the following general substitution reaction:

Substitution Reactions. Substitution reactions are reactions in which a nucleophile displaces an atom or group of atoms (the leaving group) from a tetrahedral carbon atom. Consider the following general substitution reaction:. How might this reaction proceed?. Substitution Reactions.

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Consider the following general substitution reaction:

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  1. Substitution Reactions Substitution reactions are reactions in which a nucleophile displaces an atom or group of atoms (the leaving group) from a tetrahedral carbon atom. Consider the following general substitution reaction: How might this reaction proceed?

  2. Substitution Reactions Substitution reactions are reactions in which a nucleophile displaces an atom or group of atoms (the leaving group) from a tetrahedral carbon atom. Consider the following general substitution reaction: How might this reaction proceed?

  3. The two mechanisms that are operative in substitution reactions are the SN1 and SN2 reactions. • Reactions in which the leaving group leaves before the attack of the nucleophile are referred to as SN1 reactions. • Reactions in which the nucleophile attacks at the same time as the leaving group leaves are referred to as SN2 reaction. S stands for substitution N stands for nucleophilic The number refers to the number of reacting molecules in the rate determining step. In substitution reactions the RDS is the step where the leaving groups leaves. Consider the two different mechanisms for a substitution reaction. What factors would favour one pathway over the other?

  4. All of these reactions: • involve displacement of a heteroatom from carbon -- the "leaving group" • the leaving group is always more EN than carbon making the carbon electrophilic • the carbon is always tetrahedral • there is always a nucleophile present, either -ve or  - Remember: Good leaving groups are weak bases!

  5. The SN2 Reaction. The SN2 reaction is favoured for 1° and methyl substrates. Sketch a reaction profile diagram for the above reaction.

  6. The SN2 Reaction. Now sketch a reaction profile diagram for the following reaction. Only the first reaction proceeds via SN2. How do your reaction profile diagrams account for this?

  7. The SN2 Reaction - Kinetics. Recall that the ‘2’ in SN2 refers to the number of reacting molecules in the rate determining step. Because there are two reactant molecules in the RDS in an SN2 reaction, we say that it is second order. The rate of a reaction is generally measured as the change in the concentration of one reactant over a given unit of time. Alternatively, the rate can also be measured as the change in concentration of product over a given unit of time. These are expressed mathematically as: or

  8. The SN2 Reaction - Kinetics. Consider: • What happens to the rate of production of CH3I if the concentration of CH3Br is held constant and the concentration of I- is increased? • What happens to the rate of production of CH3I if the concentration of I- is held constant and the concentration of CH3Br is increased?

  9. The SN2 Reaction - Kinetics. We can also write a rate law for this reaction. A rate law is a mathematical equation that relates the concentrations of each reactant (in the RDS) to the overall rate of reaction. The rate law for the above reaction is: • k is the rate constant and must be determined experimentally.

  10. The SN2 Reaction - Kinetics. Considering what happens to the concentrations of reactants, what would a graph or reaction rate vs. time look? This type of graph is not that as useful since we cannot predict the rate of reaction at some future time. In order to do so we would need to convert the graph into a linear function which is outside the scope of this course.

  11. The SN2 Reaction So far we have only considered an SN2 reaction with one elementary step. Consider the following two-step SN2: • For this reaction: • draw a reasonable mechanism • identify the rate determining step • sketch a reaction profile diagram

  12. The Stereochemistry of the SN2 reaction The reaction geometry is such that the collision must occur with the nucleophile attacking the back side of the C—LG bond. When attack occurs at an asymmetric carbon, the reaction occurs exclusively with inversion of the absolute stereochemistry.

  13. The Stereochemistry of the SN2 reaction Why backside attack? Consider the MOs that are involved in this reaction.

  14. SN2 reactions - Reactivity Substrate dependance:

  15. SN2 reactions - Reactivity When evaluating SN2 reactions, you have to consider everything involved in the reaction. This includes the substrate, the nucleophile, the leaving group and the solvent. For each set of reactions, draw the SN2 products. Then indicate which reaction should proceed faster and why. A B

  16. SN2 reactions - Reactivity For each set of reactions, draw the SN2 products. Then indicate which reaction should proceed faster and why. C

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