Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Oxidation Reactions in Organic Chemistry
11.4 Oxidation of Alkenes Without Bond Cleavage
There are two types of oxidation reactions involving alkenes, the first is oxidation reaction without cleavage of the double bond and the second is oxidation with cleavage of the double bond. In this section, we will examine oxidation of alkenes without cleavage of the double bond, and in the next section, we will examine oxidation of alkenes with cleavage of the double bond. Reaction (11-41) shows the general oxidation reaction of alkenes without bond cleavage to form an epoxide. Reaction (11-42) also shows oxidation of the double bond without cleavage but forms a 1,2-diol. As you can imagine, the oxidizing agents and reaction conditions for these reactions are different to enable different types of products.
(11-41)
(11-42)
11.4.1 Epoxidation of Alkenes
The epoxidation of ethylene to form ethylene oxide is one of the most important reactions of industrial importance. Each year, millions of pounds of ethylene oxide are produced by the reaction of ethylene with oxygen in the presence of silver catalyst, as shown in Reaction (11-43).
(11-43)
Peroxyacids, also known as peracids, are oxidizing agents. One that is highly used in industry for the bleaching of pulp is peroxysulfuric acid, but a more commonly used peracid is hydrogen peroxide. Hydrogen peroxide can be used to oxidize organic carboxylic acids to peroxyacids, which are commonly used in organic chemistry as an oxidizing reagent, the reaction that shows the synthesis of peroxyacids from carboxylic acids is shown in Reaction (11-44).
(11-44)
Compared to organic carboxylic acids, peroxyacids have an additional oxygen, and peroxyacids are known to be mild oxidizing agents. Alkenes are oxidized by peracids (RCOOOH) to give oxiranes. The solvent used for these reactions is usually a very inert solvent, such as CHCl3 or CH2Cl2. The peroxyacid that is typically used to oxidize the large majority of alkenes is meta-chloroperoxybenzoic acid (MCPBA), and Reaction (11-45) gives an example of this oxidation reaction.
(11-45)
The mechanism for epoxidation of alkenes with a peracid is a concerted one in which bonds are broken and formed simultaneously as shown in Reaction (11-46).
(11-46)
Problem 11.8
i. Complete the following reactions by supplying the major organic products.
ii. Give the reactants needed to complete the following reactions.
11.4.1.1 Reactions of Epoxides
Epoxides are very reactive, and we have seen the reactions of epoxides with different reducing agents in the previous chapter; epoxides can be easily reduced to undergo ring opening with a reducing agent, such as an organometallic reagent or metal hydride. Epoxides can also undergo ring opening reactions under aqueous acidic or basic conditions to produce trans-diols. The acid catalyzed opening of epoxides in water is shown in Reaction (11-47).
(11-47)
Note that attack of the nucleophilic water occurs from the opposite side of the epoxide, resulting in a trans-diol as product. An important ring opening reaction of ethylene oxide in the presence of water is to produce ethylene glycol, which is the main component of anti-freeze as shown in Reaction (11-48).
(11-48)
You will notice from the above mechanism that the nucleophilic water can react with any of the electrophilic carbons of the protonated epoxide to produce the final product. In the case of a ring opening reaction in which HCl is used, the chloride anion is the nucleophile, instead of water, and the attack can take place on either electrophilic carbon atoms of the protonated epoxide. In the case of a symmetrical epoxide, only one product will result, but in the case of an unsymmetrical oxirane, there are two possible products, as shown in Reaction (11-49).
(11-49)
We will have to examine the mechanism in order to determine a rationale for the formation of the product shown above as the major product. Under acidic conditions, the carbon that contains the most groups, i.e. the one with the least hydrogens, is the carbon that is usually attacked by the nucleophile. The carbon with the most surrounding alkyl groups in an acidic medium is more carbocationic-like and we know that the order of stability is that a tertiary carbocation is more stable than a secondary. This concept is illustrated below for the resonance structures shown in Reaction (11-50).
(11-50)
Based on this concept, the chloride anion will attack the carbon that has the methyl and ethyl groups, compared to the other electrophilic carbon that has one hydrogen and an ethyl group.
Problem 11.9
Give the final organic product for each of the following sequence of reactions. Show appropriate stereochemistry of the final product.
The ring opening of oxiranes in a basic medium involves the attack of the nucleophile on one of the electrophilic carbons, followed by protonation in a second step to give the final trans-diol, as shown below for the reaction using water as the nucleophile.
(11-51)
For the ring-opening reactions in basic media, the attack of the nucleophile on an unsymmetrical oxirane occurs at the electrophilic carbon that is less crowded; that is, the carbon with the least number of alkyl groups, as shown in Reaction (11-52).
(11-52)
As we have seen from the previous chapter, the Grignard reagent and other reducing agents react with epoxides in this manner.
Problem 11.10
Give the final organic products for each of the following sequence of reactions. Show appropriate stereochemistry of the final product.
11.4.2 Oxidation of Alkenes with KMnO4
The reaction of an alkene with KMnO4 at room temperature proceeds without cleavage of the double bond, even though KMnO4 is a strong oxidizing agent. Note from Reaction (11-53) that the addition is a cis-addition and not a trans-addition as observed using an aqueous ring opening of an epoxide.
(11-53)
In order to understand the stereochemical outcome of this type of reaction, we will have to examine the reaction mechanism. In the first step of the reaction mechanism, the KMnO4 adds to the alkene double bond as shown in Reaction (11-54).
(11-54)
In the next step of the reaction mechanism, water hydrolyzes the Mn complex to give the cis-diol and MnO2, as shown in Reaction (11-55).
(11-55)-
Note that the syn-addition results because both oxygen atoms of the diol are from the oxidizing reagent, KMnO4. Based on the above observations, you should be able to predict the organic product of any oxidation reaction involving KMnO4 and any alkene at room temperature. An example of an oxidation reaction of cyclohexene is shown in Reaction (11-56). Note the stereochemistry of the OH groups in the product.
(11-56)
11.4.3 Oxidation of Alkenes with OsO4
The reaction of an alkene with osmium tetroxide (OsO4) at room temperature proceeds without cleavage of the double bond. Like KMnO4, OsO4 is a strong oxidizing agent, and as you can suspect from the similar structure of both oxidizing reagents, the addition is also a cis-diol addition as shown in Reaction (11-57).
(11-57)
A compressed mechanism to show the cis-addition for this reaction is shown in Reaction (11-58).
(11-58)
Osmium tetroxide in the presence of hydrogen peroxide can be used to give the same 1,2-cis diol from cyclohexene in which KMnO4 was used, Reaction (11-56). Reaction (11-59) shows the oxidation of cyclohexene with osmium tetroxide.
(11-59)
For the oxidation of alkene, with KMnO4 (room temperature), or with OsO4 in the presence of hydrogen peroxide the reaction proceeds without cleavage of the double bond and the product is a syn-addition of two OH groups to form a cis-diol.
Problem 11.11
i. Give the final organic products for each of the following reactions. Show appropriate stereochemistry of the final product.
ii. Give the final organic products for each of the following reactions. Show appropriate stereochemistry of the final product.