Download
1 / 25
290 likes | 685 Vues
Nucleophilic Substitution. is the nucleophile. is the leaving group. Nucleophile Product. But where do alkyl bromides come from?. But what about a primary bromide?. Nucleophilic Substitution works both ways!. Mechanisms of Nucleophilic Substitution Reaction.
E N D
Nucleophilic Substitution is the nucleophile is the leaving group
But where do alkyl bromides come from? But what about a primary bromide? Nucleophilic Substitution works both ways!
Mechanisms of Nucleophilic Substitution Reaction There is more than one. The mechanisms are determined by studying the rate of the reaction. A rate of a reaction refers to the change in concentration of the reactants or products versus time.
First Case: - d [CH3Br] d t Rate = = k [CH3Br] [Nu] The rate that the CH3Br disappears is proportional to the concentration of the CH3Br and the concentration of the nucleophile.
First Case: - d [CH3Br] d t Rate = = k [CH3Br] [Nu] The rate depends on the concentration of two components so it is a bimolecular reaction.
First Case: The reaction is called an: S for substitution N for nucleophilic 2 for bimolecular SN2 reaction
Transition State
Transition State. Exists only for a very short time.
The key step in the reaction is a collision between the two reactants. This means that an increase in the concentration of either reactant will result in a direct increase in the rate. - d [CH3Br] d t Rate = = k [CH3Br] [OH-]
Second Case: Kind of Strange No substitution with HO-, but reacts with H2O
No substitution with HO-, but reacts with H2O HO- is much more basic than H2O. It should be a better nucleophile. Most interestingly the rate does not depend upon how much H2O is present.
- d [(CH3)3CBr] d t Rate = = k [(CH3)3CBr] The rate does not depend upon concentration of H2O The key step in the reaction can not be bimolecular. It must be unimolecular.
It turns out to be a multi step reaction: Step one is ionization to give the t-butyl carbocation and bromide.
The slow step determines the rate of the reaction. It is unimolecular. - d [(CH3)3CBr] d t = k [(CH3)3CBr] Rate =
The reaction is called an: S for substitution N for nucleophilic 1 for unimolecular SN1 reaction
Why does t-butylbromide react via an SN1 reaction while methylbromide reacts via an SN2 reaction?
Why does t-butylbromide react via an SN1 reaction while methylbromide reacts via an SN2 reaction? 1. Steric Hindrance 2. More stable carbocation
Transition State Intermediate Transition State
Transition States occur at a maximum of a Energy Profile. T.S. T.S. Int. Intermediates occur at a minimum of an Energy Profile. They are potentially isolatable species.
