Alkene Oxidation To 1,2 Diols | Alkene Hydroxylation

Different reagents are used for alkene oxidation to 1,2-diol or vicinal diol (commonly called glycol) either in cis-form or trans-form. Following mechanisms will be discussed under this section;

• For cis-1,2-diol;

     1.  Osmium tetraoxide
     2.  Potassium permanganate (cold dilute)
     3.  Iodine-silver acetate (wet)

• For trans-1,2-diol;

     4.  Iodine-silver acetate (dry), Prevost reaction
     5.  Through epoxide

• Self assessment;

1.  Osmium Tetraoxide

• An expensive and quite toxic reagent.
• More selective as compared to reactivity; according to general rule,

Reactivity is inversely proportional to the selectivity.

• Use in laboratory.

Mechanism:

(a)  Reaction with OsO4 takes place in the presence of neutral conditions.

(b)  Ether as solvent.

(c)  Pyridine as catalyst and complexes osmium in the ester.

(d)  Steps;

(I)  Formation of osmate ester

(II) Hydrolysis with aqueous sodium sulfite i.e., Na2SO3 + H2O  

OsO4 oxidation to 1,2 diol

(e)  Syn-stereosoecific

(f)  Regioselective for more electron rich double bond, when more than one double bond is present.

(g)  Attacks from less hindered side in rigid cyclic systems, thereby yielding the more stable of the two possible cis-diols.

Because of both its expense and toxicity, it is best to use osmium tetraoxide in catalytic quantities by carrying out the reaction in the presence of a;

• tertiary amine oxide

                                                                     OR                                                                                            

• alkaline t-butyl hydroperoxide (hydrogen peroxide in t-butyl alcohol)

                                                                      OR                                                                                           

• N-methylmorpholine-N-oxide (NMO)

OsO4 oxidation to diol

                                                                                                                                                                                                                                                                                                                                            

2.  Potassium Permanganate

• KMnO4 is cheaper (as compared to OsO4).
• Less hazardous to use
• More reactive and less selective. It also oxidizes other bonds in addition to the C-C double bond.
• Use in industry.

Mechanism:

(a)  Reaction takes place with potassium permanganate in presence of basic conditions (alkaline KMnO4).

(b)  In alkaline conditions, reaction stops at diol level and further oxidation (formation of hydroxyketone) is declined.

(c)  Potassium permanganate acts similarly to osmium tetraoxide;

Steps;

(I)  Formation of manganate ester

(II) Hydrolysis

Mechanism of KMnO4 oxidation to diol

(d)  This reaction is used as a  test for the presence of a double bond (unsaturation) in an organic molecule and is known  as Baeyer's test.

3.  Iodine-Silver Acetate (wet)

wet method of iodine-silver acetate

Following steps are involved in the mechanism;

• Formation of iodonium ion by reaction of iodine with double bond.
• Iodonium ion undergoes displacement by acetate in the SN2 manner, giving a trans-iodoacetate.
• Formation of cyclic acetoxonium ion.
• Reaction with water to give a cis-hydroxyacetate.
• Final hydrolysis gives the cis-diol.

Mechanism of wet method for diol formation

4.  Iodine-Silver Acetate (dry) OR Prevost Reaction

dry method (prevost reaction) of iodine-silver acetate

The same above reaction carried out in the absence of water. In forth step, acetate ion reacts instead of water and gives trans ester (trans-1,2-diacetate). Reaction stops at this point then we work up by adding H+/H2O (hydrolysis). Hydrolysis gives the final trans-diol.

Steps can be arranged in following way;

• Formation of iodonium ion by reaction of iodine with double bond.
• Iodonium ion undergoes displacement by acetate in the SN2 manner, giving a trans-iodoacetate.
• Formation of cyclic acetoxonium ion.
• Reaction with acetate ion to form trans-ester.
• Final hydrolysis gives the trans-diol.

Mechanism of dry method (prevost reaction ) for diol formation

5.  Through Epoxide

When a nucleophile opens an epoxide, it generates an alcohol. If the nucleophile is water, the product is the diol. The two hydroxyl groups end up on opposite sides of the six membered ring; the product is anti-diol. The epoxide opening reaction can be done in acid or base.

diol formation from epoxide

Click this for more examples

Self Assessment:

Practice problem for alkene to 1,2 diol

SHOW ANSWER

Related:

• Epoxidation of Alkenes | Reactions of Epoxides | Peroxyacid
• Practice Problems For Epoxidation and Reactions of Epoxides

Reference:

Principles of organic synthesis (Norman R. O. C.), Marshall W. Cronyn, Journal of Chemical Education 1972 49 (8), A434

Clayden, Jonathan, et al. Organic Chemistry. 2nd ed., Oxford University Press, 2012.Author(s): Jonathan Clayden, Nick Greeves, St...Publisher: Oxford University Press, Year of publication: 2012

Advanced Organic Chemistry by Dr. Haq Nawaz Bhatti , 2nd edition, published by Caravan Book House Lahore, 2016.

Practice Problems For Epoxidation and Reactions of Epoxides:

Given practice problems are related to epoxidation of alkenes, epoxidation of a,b- unsaturated carbonyl compounds and epoxide reactions.

One can easily slove these problems in just 5 minutes after revising the following topic;

Epoxidation of Alkenes | Reactions of Epoxides | Peroxyacid

PROBLEM 1:

Practice problem for epoxide reactions

ANSWER:

Epoxidation and epoxide reactions

PROBLEM 2:

Practice problem for epoxidation

ANSWER:

Epoxidation of unsaturated carbonyls

Epoxidation of alkenes

For Content Guidance;

Epoxidation of Alkenes | Reactions of Epoxides | Peroxyacid

Epoxidation of Alkenes | Reactions of Epoxides | Peroxyacid

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

^  Reference

Epoxidation of Alkenes:

Peroxyacid is used as oxidizing agent/oxidant. Alkenes react with peroxyacids to give epoxides.
Mechanism of reaction is usually expressed as;

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;

1.        One which has been widely is m-chloroperoxybenzoic acid.

m- Chloroperoxybenzoic acid (m-CPBA)

 2.     Safety considerations have led to displacement of m- CPBA by the Mg salt monoperoxyphthalic acid.

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.

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.

stereospecific epoxidation of alkene

• Steric Hindrance:

Cyclic alkenes are attacked predominantly from less hindered side. For example,

Epoxidation from less hindered side

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;

Reduction of epoxide with LiAlH4

2.  Formation of Carbonyl Compound:

Treatment with Lewis acid gives a carbonyl compound.

Formation of carbonyl compound from epoxide

3.  Formation of a-ketol:

Epoxide gives an a-ketol on treatment with dimethyl sulfoxide (DMSO).

formation of ketol  from epoxide

 
4.  Formation of 1,2-diol:

Epoxide undergoes hydrolysis (either acidic or basic) to form 1,2-diol.

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)

Reaction of epoxide with HBr

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,

Nucleophilic substitution reaction of epoxide

Practice Problems:

Problem 1;

Practice problem for epoxide reactions

SHOW ANSWER

Problem 2;

Suggest the reagent and possible mechanism for following A and B epoxides.

Practice problem for epoxidation

SHOW ANSWER

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.