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Reactions of Epoxides: Ring Opening by Alcoholysis, Hydrolysis, HX Nucleophiles, Additional Stereochemical Considerations of Ring Opening

Reactions of Epoxides: Ring Opening by Alcoholysis, Hydrolysis, HX Nucleophiles, Additional Stereochemical Considerations of Ring Opening

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Table of content

  • Epoxide ring opening by alcoholysis
  • Epoxide ring opening by hydrolysis
  • Epoxide ring opening by HX
  • Epoxide ring opening by nucleophiles
  • Additional stereochemical considerations of ring opening
  • Practice problems
  • Frequently asked questions

Epoxide ring opening by alcoholysis:

Depending on the type of the epoxide and the reaction circumstances, ring-opening reactions can happen via SN2 or SN1 processes.If the epoxide is asymmetric, the structure of the product will vary according to which mechanism dominates.Depending on which mechanism predominates, the product's structure will change. When an asymmetric epoxide undergoes alcoholysis in basic methanol, an SN2 process opens the rings, and the less substituted carbon becomes the target of nucleophilic attack, yielding product B:

basic ring opening

When solvolysis occurs in acidic methanol, when solvolysis takes place in the presence of methanol however, the reaction follows an SN1 mechanism, with the more substituted carbon being the target of attack. As a result product A dominates:

acidic ring opening

Both of these reactions are regioselective in nature.

Basic Epoxide Ring-Opening:

Because there is no acid available to protonate the oxygen prior to ring opening in the basic SN2 reaction, the leaving group is an alkoxide anion. Because an alkoxide is a poor leaving group, poor leaving group is alkoxide the ring is unlikely to open without the nucleophile's help.

The nucleophile itself is potent, a deprotonated, negatively charged methoxide ion is extremely powerful. When a nucleophilic substitution process involves a weak leaving group and a strong nucleophile, the SN2 mechanism is very likely to be used.

Although the epoxide contains two electrophilic carbons, the least hindered carbon is the optimum target for the nucleophile in an SN2 reaction. This explains the observed regiochemical result. Nucleophilic assault occurs from the same source as in other SN2 reactions.

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Acid-Catalyzed Epoxide Ring-Opening:

The acid-catalyzed epoxide ring-opening process is best described as a hybrid, or cross, of the SN2 and SN1 mechanisms. The oxygen is first protonated, resulting in a good leaving group (step 1 below). The carbon-oxygen bond then begins to break (step 2), allowing positive charge to accumulate on the more substituted carbon. Remember that alkyl substituents can transfer electron density and stabilize a positive charge on a carbon by hyper conjugation.

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The nucleophile attacks the electrophilic carbon (step 3) before a complete carbocation intermediate can develop, unlike in an SN1 reaction.

Unlike in an SN1 reaction, the nucleophile attacks the carbon which is electrophilic (step 3) before the formation of complete carbocation intermediate.

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Attack occurs preferentially from the backside (as in an SN2 reaction) because the carbon-oxygen bond is still present to some extent and the oxygen prevents attack from the front side. However, the regiochemical outcome differs from the base-catalyzed reaction: in the acid-catalyzed reaction, the nucleophile attacks the more substituted carbon because it has a higher degree of positive charge.

Because the carbon-oxygen bond is still intact to some extent and the oxygen hinders attack from the front side, attack takes place predominantly from the backside (as in an SN2 reaction). However, the regiochemical outcome varies from the base-catalyzed reaction; the acid-catalyzed reaction nucleophile attacks the more substituted carbon because it has a higher degree of positive charge.

Epoxide ring opening by hydrolysis:

Hydrolysis can cleave epoxides to yield trans-1,2-diols (1,2 diols are also called vicinal diols or vicinal glycols). The reaction can be carried out under acidic or basic conditions, resulting in the same regioselectivity as previously discussed.

Trans-1,2-diols can be produced via hydrolysis of epoxides (1,2 diols are also called vicinal diols or vicinal glycols). The reaction can be performed under acidic or basic conditions, yielding the same regioselectivity as previously mentioned.

Acid catalyzed hydrolysis:

The epoxide oxygen is protonated gets protonated in aqueous acidic conditions and then attacked by a nucleophilic water. A 1,2-diol product is formed after deprotonation to reform the acid catalyst. If the epoxide is asymmetric, the incoming water nucleophile will preferentially attack the more substituted carbon carbon which is more substituted of the epoxide. An SN2 like mechanism opens the epoxide ring, causing the two -OH groups in the product to be trans to each other.

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Base catalyzed hydrolysis:

During an SN2 reaction in aqueous basic conditions, the epoxide is opened by the attack of a hydroxide nucleophile. The epoxide oxygen reacts with water to form an alkoxide, which is then protonated to form the 1,2-diol product. If the epoxide is asymmetric, the incoming hydroxide nucleophile will preferentially attack the epoxide carbon that is less substituted. Because the reaction is catalyzed by an SN2, the two -OH groups in the product will be trans to each other.

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Epoxide ring opening by HX:

Anhydrous acids (HX) can also open epoxides to form a trans halohydrin. When both epoxide carbons are primary or secondary, the halogen anion will attack the less substituted carbon via an SN2 reaction. If one of the epoxide carbons is tertiary, the halogen anion will primarily attack If one of the epoxide carbons is tertiary, the halogen anion will attack that carbon first in an SN1 -like reaction.

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Epoxide ring opening by nucleophiles:

A wide range of basic nucleophiles, including amines, Grignard reagents, acetylide anions, and hydride, can be used to open the ring of an epoxide. These ring openings are usually caused by an SN2 mechanism.

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The reaction of Grignard reagents with ethylene oxide is particularly useful because it produces a primary alcohol with two more carbon atoms than the original Grignard reagent.

Grignard reagent reactions with ethylene oxide are particularly useful because they yield a primary alcohol with two extra carbon atoms than the original Grignard reagent.

Because Grignard reagents are both strong nucleophiles and strong bases, this reaction follows the same SN2 mechanism as the opening of epoxide rings under basic conditions. The SN2 attack of the Grignard reaction opens the epoxide to form an alkoxide in the first step of this reaction's mechanism. The alkoxide is protonated to form alcohol in the second step of the mechanism.

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Additional stereochemical considerations of ring opening:

The regiochemical control of the reaction allows for the production of one stereoisomer during the ring-opening of an asymmetrical epoxide. If the epoxide is symmetrical, each carbon in the epoxide has roughly the same ability to accept the incoming nucleophile.The ability of each carbon in the epoxide to accept the incoming nucleophile is roughly the same. When this happens, the product usually contains a mixture of enantiomers.

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Practice problems:

Q1. Name a cyclic ether with a three-atom ring that resembles an equilateral triangle?

(A) Epoxide
(B) Diethyl ether
(C) Diphenyl ether
(D) Methyl ethyl ether

Answer: (A) 

Solution: One epoxide's basic structure consists of an oxygen atom linked to two neighboring hydrocarbon carbon atoms. The general ethers, on the other hand, can be thought of as a set of chemical molecules that contain an ether group. This group is made up of two alkyl or aryl groups linked by an atom of oxygen. In general, the formula is ROR', where R and R' denote the aryl or alkyl groups respectively. As a result, it may be determined that its fundamental structure has two hydrocarbon carbon atoms connected to an oxygen atom.

The cyclic ether with a three-atom ring is known as an epoxide. Now, because this ring approximates an equilateral triangle, it is strained and thus extremely reactive in comparison to other ethers.

Q2. Ethylene on reaction with oxygen in the presence of silver catalyst will give

(A) Epoxide
(B) Diethyl ether
(C) Trifluoroacetic acid
(D) None of the above

Answer: (A)

Solution: Epoxide is generated when ethylene combines with oxygen in the presence of a silver catalyst. It can be chemically represented as:

 Epoxide is generated when ethylene combines with oxygen in the presence of a silver catalyst. It can be chemically represented as:

 7 CH2 = CH2 + 6O2 + Ag → 6 C2H4O+ 2 CO2+ 2 H2O

Q3. Which of the following is a result of unsaturated carbon double bond oxidation?

(A) Aerobic composting
(B) Anaerobic composting
(C) Epoxide
(D) Anaerobic digestion

 

Answer: (B)

Solution: Epoxidation is the process by which microbes catalyze the oxidation of unsaturated carbon double bonds to form epoxide. Aerobic composting is typically used to treat solid wastes that are dry in nature rather than moist or wet wastes, whereas anaerobic composting is used to degrade moist solid wastes that are very easy to degrade.

Q4. Epoxides are

(A) Polar
(B) Non polar
(C) Both 
(D) None of the above

Answer: (A) 

Solution: A three-membered ring's epoxide unit contains two C atoms and one O atom. The atoms are linked together by Sigma bonds. Because of the high electronegativity difference between the C and O atoms, both C-O bonds are polar.

Frequently asked questions:

Question 1. Are epoxides electrophiles?
Answer: Because significant ring strain is relieved when the ring opens upon nucleophilic attack, the carbons in an epoxide group are highly reactive electrophiles. Epoxides are typically formed in the laboratory by the oxidation of an alkene. Epoxides are electrophilic due to their strained three-membered ring system.

Question 2. Why are some epoxides toxic?
Answer: The only ethers composed of ring strains are epoxides. Because of this, they are more reactive than other ethers. Because they are highly reactive, they are mutagenic. Mutagenic substances are those that react with the body and permanently alter or mutate the DNA or genetic materials. That is why ingesting epoxides is extremely harmful to all living things.

Question 3. What is the reagent that is used to form epoxides?
Answer: Epoxides are a significant substance that has applications in our daily lives. They are produced by treating an alkene with a specific type of peroxy acid. Peroxy acid is a carboxylic acid with an extra oxygen atom. One of the most commonly used peroxy acids for this purpose is m-CPBA. m-CPBA is also known as m-chloroperoxybenzoic acid. These peroxy acids, however, can also be used to make epoxides. These peroxyacids must be in the RCO3H form.

Question 4. Are epoxides ethers?
Answer: An epoxide is a three-atom ring cyclic ether. Because this ring resembles an equilateral triangle, it is more strained and thus highly reactive than other ethers. They are produced on a large scale for a variety of applications.

Related Topics:

Ether-Classification and Preparation

Methanol

Ethanol

Phenol

Important compounds of alcohol, phenol and ether

Epoxide-preparations








 

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