Overview:
The general form of Fischer esterification mechanism is as follows:
The first step involves protonation of the carbonyl oxygen, followed by the nucleophillic attack of the alcohol.
Then a loss and regain of a proton,
followed by loss of water as electrons from the alcohol oxygen kick down to form the double bond. Loss of a proton yields the ester.
Example of fischer esterification:
Wednesday, March 21, 2007
Fischer esterification mechanism
Alcohol dehydration
Overview:
The general form of alcohol dehydrations is as follows:
The first step involves the protonation of the alcohol by an acid, followed by loss of water to give a carbocation.
Elimination occurs when the acid conjugate base plucks off a hydrogen. Alcohol dehydrations generally go by the E1 mechanism.
Example of alcohol dehydration:
Nucleophilic addition to carbonyl groups
Overview:
The general form of the nucleophilic addition to carbonyl group mechanism is as follows:
Nucleophilic Addition to Carbonyl Groups
First step is the attack of the nucleophile on the partially positive carbon to make the tetrahedral intermediate with the full negatively charged oxygen. The oxygen then becomes protonated to yield the alcohol.
Variety of nucleophiles:
* Grignard Reagents
* Alcohols
* Amines
* Alkyl Lithium Reagents
* Acetylide Ions
Example of nucleophilic addition to carbonyl groups:
In this case, acetylide anion is acting as the nucleophile
Hydroboration of alkenes
Overview:
The general form of the hydroboration of alkenes mechanism is as follows:
First step is the attack of the alkene on BH3, which then forms a four membered ring intermediate of partial bonds. It is because of this intermediate that hydroboration forms the anti-Markovnikov product. The boron atom is highly electrophilic because of its empty p orbital (ie. it wants electrons), and forms a slight bonding interaction with the pi bond. Since some electron density from the double bond is going towards bonding with the boron, the carbon opposite the boron is slightly electron deficient, left with a slightly positive charge. Positive charges are best stabilized by more highly substituted carbons, so the carbon opposite the boron tends to be the most highly substituted. Once the transition state breaks down, BH2 is attached to the least substituted carbon.
Peroxide then removes the borane and replaces it with the alcohol to form the anti-markovnikov product.
An example of the hydroboration mechanism:
Electrophilic Addition to Alkenes Mechanism
Overview:
Electrophilic addition to alkenes takes the following general form:
electrophilic addition to alkenes overview
nuc: = nucleophile
E+ = electrophile
Electrophilic addition to alkenes starts with the pi electrons attacking an electrophile, forming a carbocation on the most stable carbon. A nucleophile then attacks the carbocation to form the product. There are many different kinds of such addition, including:
* Hydroxylation
* Hydrogenation
* Halogenation
* Oxidative Cleavage
* Hydration
* Epoxidation
* Cyclopropanation
* Halohydrin Formation
Clearly, there are numerous kinds of products that can be formed as a result of this mechanism.
Orientation of Addition: Electrophilic Addition adds to give the Markovnikov Product, with the nucleophile added to the more highly substituted carbon. This is because the carbocation intermediate is significantly stabilized by alkyl substituents.
Example of electophilic addition to alkenes:
First, formation of the carbocation on the most highly substituted carbon
Followed by attack of chloride on the carbocation to give the addition product
E2 Mechanism
Overview:
The general form of the E2 mechanism is as follows:
General form of the E2 mechanism
B: = base
X = leaving group (usually halide or tosylate)
In the E2 mechanism, a base abstracts a proton neighboring the leaving group, forcing the electrons down to make a double bond, and, in so doing, forcing off the leaving group. When numerous things happen simultaneously in a mechanism, such as the E2 reaction, it is called a concerted step.
An example of the E2 reaction:
Example of the E2 mechanism
Base Strength: A strong base is required since the base is involved in the rate-determining step.
Leaving groups: A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step.
Stereochemistry requirements: Must occur with antiperiplanar stereochemistry.
E1 Mechanism
Overview:
The general form of the E1 mechanism is as follows:
B: = base
X = leaving group (usually halide or tosylate)
In the E1 mechanism, the the first step is the loss of the leaving group, which leaves in a very slow step, resulting in the formation of a carbocation. The base then attacks a neighboring hydrogen, forcing the electrons from the hydrogen-carbon bond to make the double bond. Since this mechanism involves the formation of a carbocation, rearangements can occur.
An example of the E1 reaction:
E1 Reaction
Base Strength: A strong base not required, since it is not involved in the rate-determining step
Leaving groups: A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step.
Rearangements: Since the mechanism goes through a carbocation intermediate, rearangements can occur.
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