Swern Oxidation

Introduction

Oxidation reaction states a chemical reaction, which proceeds when substances tend to come in close proximity with the element of oxygen. However, the reaction also takes place when there lays oxidising agents other than oxygen. The result of the oxidation is the synthesis of variable organic compound. More this reaction results in the creation of varied new functional groups as well as tends to modify the structure that is pre-existing.

What is Swern Oxidation?

Swern Oxidation refers to a crucial oxidation process that tends to avoid the usage of varied metals such as, chromium. The reaction takes place on a mild level and contributes advantages over other oxidation reactions.

Figure 1 − Swern Oxidation

This is an organic reaction that makes usage of oxalyl chloride that works as an organic base. In this dimethyl sulfoxide is involved in oxidations of primary and secondary alcohols associated with aldehydes as well as ketones. However, it is noticed that in this, aldehydes does not conduct the further reaction. In the oxidation process carboxylic acids is formed gradually. This reaction is called redox reaction.

This reaction was discovered by the famous chemist of American chemist named, Daniel Swern. An example of this Swern Oxidation reaction is given in the above image that shows, DMSO or dimethyl sulfoxide.

By-products formed

In this oxidation reaction is quite popular as this reaction displays a wide range of tolerance for varied functional groups. As this reaction proceeds, there occurs the formation of certain by-products that includes the followings.

• Dimethyl sulfide ($\mathrm{Me_{2}S}$, also known as DMS)

• Triethylammonium chloride $\mathrm{Et_{3}NHCl}$

• Carbon dioxide or $\mathrm{CO_{2}}$

• Carbon monoxide or CO

Among the given by-products, both the products named carbon monoxide and dimethyl sulfide are considered as quite dangerous. This is because carbon monoxide is considered as highly toxic as if it is inhaled by a human it may result in an effect. This gets into the blood and thereby produces carboxyhaemoglobin which is quite dangerous for tissues. On the other Dimethyl sulfide seems to be highly volatile in nature resulting in the creation of a strong odour.

Mechanism of Swern Oxidation

The mechanisms of Swern Oxidation are followed along with its images. The first step considers reaction at 60o C among the reactants namely, molecules of dimethyl sulfoxide that show a resonance supported structure with oxalyl chloride. This tends to quickly tend to decompose resulting in the formation of by-products.

Figure 2 − Mechanism of Swern Oxidation (a)

The second step shows the addition of alcohol within the reaction of chloro (dimethyl) sulfonium chloride. This results in the formation of both carbon monoxide and carbon dioxide as the by-products.

Figure 3 − Mechanism of Swern Oxidation (b)

The next step allows the notion of the deprotonation, for the ions of alkoxy sulfonium as intermediates. The decomposition leads to the formation of carbonyl compound and dimethyl sulfide. This result in the formation of compounds of carbonyl, however, Tetrahydrofuran or THF is most often found as a solvent in this reaction.

Advantages of Swern Oxidation

Varied advantages are noticed in this Swern Oxidation, the most important of which is that, this reaction does not proceed further as aldehydes are formed. In low temperatures too, the reaction undergoes thereby oxidising the alcohols. More to this the condition for this reaction is quite mild as it takes place in -78o C and most often it is stated that these are requires acetone or dry-ice bath.

Drawbacks of Swern Oxidation

Although a multiple number of advantages are noticed, however, certain disadvantages are also there. The first of which is dimethylsulfide being a smelly gas and the other include, carbon monoxide being dangerous if inhaled by individuals.

Lab preparation of Swern Oxidation and its Application

The reaction of Swern Oxidation can be conducted in a laboratory thereby following a few steps that include the following. The first one is preparation in a flask that is charged with a stirring bar, followed by activation of DMSO. This results in the addition of alcohol followed by the addition of a base. However, the last step is Work-up and purification. Application of Swern Oxidation as it provides a distinctive feature for synthetic chemists.

Conclusion

In this tutorial, an in-depth focus has been given to determining the entire process that is conducted through Swern Oxidation. It is noticed that this is one of the most suitable processes in oxidising of primary and secondary alcohols. However, this reaction has multiple advantages as well as disadvantages. The major disadvantage is the nasty smell that belongs to dimethylsulfide.

FAQs

1. Why Swern Oxidation is stated as the selective method?

Swern Oxidation is stated as the selective method as it is one of the processes that is required for the oxidation of 1o or primary alcohols. Through this process, the alcohols turn into aldehydes. More to this, the Swern Oxidation chemical reaction tends to cease reaction as soon as groups of carbonyl are formed within the reaction. However, this nature is similar to the other present agents of oxidisation that are mild in nature, for example, the Dess-Martin (DMP) oxidation, pyridinium dichromate (PDC), and pyridinium chlorochromate (PCC).

2. What are the by-products formed in the Swern Oxidation reaction?

Swern Oxidation reaction tends to produce by-products. As the reaction proceeds, that involves, Dimethyl sulfide (Me2S or DMS) together with CO or carbon monoxide, Triethylammonium chloride $\mathrm{(Et_{3}NHCl)}$ and $\mathrm{CO_{2}}$ or carbon dioxide.

3. What is the reason behind calling this chemical reaction as Swern Oxidation?

The reason behind calling this chemical reaction as Swern Oxidation, is as this reaction was first discovered in the year 1978 by two renowned chemists named, Daniel Swern and Kanji Omura. However, after that discovery, this reaction is considered as the most used strategy for oxidation of both primary as well as secondary alcohol.