Table of Content:
^ Epoxidation of Alkenes
^ Peroxyacid as oxidizing agent / oxidant
^ Epoxidation of a,b- unsaturated Carbonly Compounds
^ Characteristics of Epoxidation
^ Reactions of Epoxides
^ Practice Problems
^ Peroxyacid as oxidizing agent / oxidant
^ Epoxidation of a,b- unsaturated Carbonly Compounds
^ Characteristics of Epoxidation
^ Reactions of Epoxides
^ Practice Problems
Epoxidation of Alkenes:
Peroxyacid is used as oxidizing agent/oxidant. Alkenes react with peroxyacids to give epoxides.Mechanism of reaction is usually expressed as;
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| epoxidation of alkene with peroxyacid |
- There is partial transfer of electrons from alkene to peroxyacid at the transition state. So, alkene is loosing electrons i.e., oxidation of alkene.
- Electron attracting groups in the peroxyacid increases the reactivity.
- Electron releasing groups in the alkene should also increase the reactivity but that's not the case in this reaction i.e., no affect on reactivity.
Now question arises;
How do you make peroxyacid reagent?
The best general method is the oxidation of carboxylic acids with hydrogen peroxide in the presence of an acid catalyst. |
| formation of peroxyacid |
- The most common catalyst for aliphatic R is concentrated sulfuric acid.
- For aromatic R the best catalyst is methanesulfonic acid, which is also used as the solvent.
- A peroxyacid from a relatively strong acid ( e.g., CF3COOH ) is unsuitable since the acid released during epoxidation and is sufficiently string to bring about ring opening of the epoxide (product).
- The reaction is an equilibrium and is driven to the right by removal of water or by the use of excess reagents.
Peroxyacid Examples:
Two examples are given below;
- One which has been widely is m-chloroperoxybenzoic acid.
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| Mg salt of monoperoxyphthalic acid |
Epoxidation of a,b- Unsaturated Carbonyl Compounds:
These can be epoxidized by an alkaline solution of hydrogen peroxide through nucleophilic
addition.
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| epoxidation of a,b- unsaturated carbonyl compounds |
What are the characteristics of epoxidation?
Related Search: What are chemical properties of epoxides?
The important characteristics of epoxidation are given below;
- Regioselectivity:
When more than one double bond is present, it is regioselective for more electron rich double bond. for example,
 |
| epoxidation with peroxyacid |
- Stereospecific:
It is stereospecific; as cis-reactant will give cis-product and trans-reactant will give trans-product. For example, cis-2-butene gives only the cis-product.
- Steric Hindrance:
Cyclic alkenes are attacked predominantly from less hindered side. For example,
Position of epoxide and hydrogen is same i.e., below the plane. It shows peroxyacid attack from less hindered side and not from alkylated side.
What are reactions of epoxides?
Related Searches:
- Why is epoxidation important?
- Why do epoxides form?
- What are uses of epoxides?
The significance of epoxidation lies in the following chemical syntheses.
1. Formation of Alcohol:
Reduction with LiAlH4 gives an alcohol;
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| Reduction of epoxide with LiAlH4 |
2. Formation of Carbonyl Compound:
Treatment with Lewis acid gives a carbonyl compound.
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| Formation of carbonyl compound from epoxide |
3. Formation of a-ketol:
Epoxide undergoes hydrolysis (either acidic or basic) to form 1,2-diol.
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| hydrolysis of epoxide to form 1,2-diol |
- The reactivity of the epoxide to a weak nucleophile (water) is enhanced by protonation of the ring oxygen.
- The product has trans stereochemistry as a result of the stereospecificity of SN2 displacements.
FOR NUCLEOPHILIC SUBSTITUTION REACTIONS:
In nucleophilic substitution reactions of epoxides (e.g., above given hydrolysis), mechanism usually depends on different combinations of nucleophile and acid.
Strong Nucleophile + Strong Acid
- Nucleophile attacks on less substituted carbon.
- In this case, nucleophile first attacks and then ring opens.
- SN2 mechanism usually takes place.
For example, the formation of bromohydrin (Halohydrin, a compound in which -OH group and halogen are present on adjacent carbon atoms)
Weak Nucleophile + Strong Acid
- Nucleophile attacks on high substituted carbon.
- In this case, ring opens before the attack of nucleophile.
- SN1 mechanism usually takes place.
For example,
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| Nucleophilic substitution reaction of epoxide |
Practice Problems:
Problem 1;
SHOW ANSWER
Problem 2;
Suggest the reagent and possible mechanism for following A and B epoxides.
References:
- Principles of Organic Synthesis, R.O.C. Norman and J.M. Coxon,, third edition, 1993.
- Advanced Organic Chemistry: reactions, mechanisms, and structure/ Jerry March, 4th edition, 1929.
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