There are many organic compounds that involve oxygen atoms. Among these compounds, ethers are the functional groups to which carbon atoms are connected by oxygen atoms.
There are only a few types of important organic reactions using ethers, and all you have to do is understand the cleavage reaction under acidic conditions. In other words, cleavage of the ether splits one molecule into two.
Also, in the case of triangular cyclic compounds in the ether, they are called epoxides. Epoxides are important compounds in organic chemistry, and it is necessary to understand how epoxides are synthesized and how they are subsequently reacted to obtain the desired compounds.
Therefore, we will explain the synthetic reactions of ethers and epoxides, and what products can be obtained.
Table of Contents
- 1 The Ether Is Cleaved by Acid Catalyst
- 2 Synthesis of Epoxides by Epoxidation Reaction with Oxidizing Agents
- 3 Synthetic Reactions of Ethers and Epoxides
The Ether Is Cleaved by Acid Catalyst
There are some functional groups that are difficult to chemically react with because they are not reactive as they are. One such group is the ether. The oxygen atom is strongly bonded to the carbon atom and the bond will not cleave. Therefore, in order for the ether to chemically react, the ether must be activated.
How can the ether be activated? One way to do this is through the use of an acid catalyst. By adding an acid catalyst such as sulfuric acid (H2SO4), hydrogen chloride (HCl), or hydrogen bromide (HBr), you can activate the ether.
Why does the presence of an acid catalyst activate ether? The presence of acid causes hydrogen atoms (protons) to bond to the oxygen atoms of the ether; as a result of the H+ bond, the ether becomes positively charged.
A positively charged oxygen atom can be an excellent leavening group. Therefore, ether cleavage causes them to split into two molecules.
Position Selectivity Is Considered in terms of Carbocation Stability
When acid-catalyzed ether cleavage occurs, it seems that several different types of compounds can be obtained. For example, the following two compounds seem to be produced as intermediates under strong acid conditions when reacting with hydrogen bromide。
However, the actual synthetic reaction produces only one intermediate. Therefore, only certain compounds can be synthesized, not many.
Why is only one intermediate produced? This is because of the stability of the intermediate, the carbocation. The stability of the carbocation is shown below.
In the case of the previous compound, after the ether is activated by the acid, a tertiary carbocation or a primary carbocation is formed. However, since tertiary carbocation is more stable, the tertiary carbocation is preferentially formed.
As a result, only certain compounds can be synthesized.
In ether cleavage reactions, there is regioselectivity. When hydrogen halides, such as hydrogen chloride (HCl) or hydrogen bromide (HBr), are used as acid catalysts, the location where ether cleavage occurs is determined.
-If the SN1 Reaction Is Impossible, the SN2 Reaction Will Occur
The synthetic reaction just described is the SN1 reaction, which is nucleophilic substitution reactions. In the SN1 reaction, carbocation intermediates are first formed, and then nucleophiles (e.g., bromoions) attack the intermediates to form compounds.
What happens in the case of unstable intermediates such as primary carbocations, which are rarely formed under normal conditions? In this case, the synthetic reaction proceeds by the SN2 reaction rather than the SN1 reaction in which the carbocation is formed as an intermediate.
The SN2 reaction is a reaction in which the product is obtained in a single reaction without the formation of a carbocation. For example, it is as follows.
In any case, understand that the SN1 or SN2 reaction causes ether cleavage and splits one molecule into two.
Sulfuric Acid Catalysis Produces Alkenes in the E1 Reaction
When hydrogen halide is used as an acid catalyst, alkyl halides can be synthesized by ether cleavage as described above. On the other hand, when sulfuric acid is used as an acid catalyst, alkenes (compounds with double bonds) can be synthesized.
In the case of hydrogen chloride (HCl) and hydrogen bromide (HBr), chloride or bromide ions are produced after providing hydrogen atoms (protons). These chloride or bromide ions attack the carbon atoms to produce alkyl halides.
In the case of sulfuric acid, on the other hand, providing H+ does not produce halogen ions. In addition, sulfate ions are stable and do not attack as nucleophiles.
Therefore, when sulfuric acid is used as a catalyst, the E1 reaction occurs. The E1 reaction is known as an elimination reaction, in which a compound with a double bond is formed when the sulfate ion pulls out a hydrogen atom (proton).
For example, in the case of the previous compound, the following compounds can be synthesized by using sulfuric acid instead of hydrogen bromide.
Understand that sulfuric acid cannot act as a nucleophilic reagent, so instead it pulls out protons and causes an elimination reaction as an E1 reaction.
Synthesis of Epoxides by Epoxidation Reaction with Oxidizing Agents
As we have discussed, the acidic conditions allow the ether to cleave to yield other compounds. On the other hand, there is a special ether, even though it is the same ether. It is an epoxide. It is a cyclic ether and has a triangular shape.
When learning about the reactivity of ethers, epoxides have a fairly specific organic reaction.
Triangular compounds have significant strain. They have a bonding angle of 60° and also have a torsional strain. In a triangular compound, it looks like the following.
Therefore, triangular cyclic compounds are known to be highly reactive compounds.
-Oxidation of Alkenes to Obtain Epoxides
So how do we synthesize epoxides? The synthetic reaction to obtain epoxides is called epoxidation reaction. In epoxidation, alkenes are used.
Peracids are also used in epoxidation. The famous peracids are hydrogen peroxide, and epoxides can be synthesized by using peracids. The reaction mechanism is as follows.
The reaction mechanism is a little more complex, and the synthetic reaction by epoxidation reaction occurs as a result of the transfer of electrons at once.
Syn Addition causes alkenes and peracids to react, and become epoxied. The synthetic reaction proceeds from the same side by syn addition. Therefore, in cis compounds, epoxides of cis compounds can be obtained. In trans compounds, on the other hand, epoxides of trans compounds can be synthesized.
In epoxidation reaction by peracids, the synthetic reaction proceeds with the three-dimensional structure of the alkene preserved by syn addition.
Reactivity of Epoxides under Acidic and Basic Conditions
After synthesizing epoxides, we will perform the next synthetic reaction. Epoxides are highly strained and unstable compounds. Therefore, epoxides are generally used to proceed to the next synthetic reaction.
Due to the high reactivity of epoxides, there are a large number of nucleophiles available.
- Alcohol (R-OH)
- Amine (R-NH2)
- Thiol (R-SH)
In addition to these nucleophiles, many other nucleophiles react with epoxides to obtain products. However, it is important to note that when a nucleophile attacks an epoxide, it can attack in two places.
Which carbon atom does the nucleophilic agent attack? This depends on whether the reaction conditions are basic or acidic.
Nucleophilic Attack on Substituent-Poor Carbons under Basic Conditions
When reacting epoxides with reagents, the carbon atoms with fewer substituents are attacked in the synthetic reaction under basic conditions. In other words, the synthetic reaction with epoxides is regioselective.
The synthetic reaction proceeds as follows.
Why do nucleophiles attack carbon atoms with few substituents? It’s because of a steric hindrance. If there are many substituents, the nucleophile cannot approach the carbon atom due to steric hindrance. Therefore, it selectively attacks carbon atoms that have few substituents.
By attacking the carbon atom with less steric hindrance, only specific products are obtained in the synthesis of epoxide under basic conditions.
Polysubstituted Carbons are Attacked in Acidic Conditions
On the other hand, how does it work under acidic conditions? In contrast to the basic condition, in the acidic condition, polysubstituted carbon atoms are attacked. Why are carbon atoms with large steric hindrances attacked by nucleophiles? The reason is that protons bind to oxygen atoms in the acidic reaction.
As already explained in the case of ether cleavage, the addition of an acid catalyst allows H+ to bond to the oxygen atom and activate the ether. In the same way, if it is acidic, a proton is attached to the oxygen atom of the epoxide, and the oxygen atom is positively charged.
In addition, focusing on the carbon atom, if the bond is broken to produce a carbocation, one is a tertiary carbocation and the other is a primary carbocation.
Considering the order of stability of the carbocations, the tertiary carbocations have more positive properties. Therefore, positively charged oxygen atoms and polysubstituted carbon atoms are repulsive. The image is as follows.
Compared to other bonds, the C-O bond is longer. The bond is also weaker than other bonds. As a result, the nucleophile attacks the polysubstituted carbon atoms.
For these reasons, under acidic conditions, the nucleophile attacks carbon atoms with many substituents.
Be sure to understand these reaction mechanisms because the compounds formed depend on whether the epoxide reaction conditions are acidic or basic.
Synthetic Reactions of Ethers and Epoxides
There are many compounds that have ethers as molecules with oxygen atoms present. So you have to learn how the ether is involved in organic chemistry.
The reactivity of the ether is poor. Therefore, it is necessary to activate it, and by using an acid catalyst, a proton bonds to an oxygen atom and causes ether cleavage. Also, by changing the acid catalyst used, nucleophilic substitution reactions or E1 reactions (elimination reactions) can occur.
In addition, be sure to learn about the reactivity of epoxides as well. By using a peracid, epoxidation reactions can take place to obtain an epoxide. Then you can react with the epoxide by adding a nucleophile.
These are the important details in the reactivity of the ether and epoxide. The ether produces different compounds and regioselectivity depending on the reaction conditions, so make sure you understand these.