Monosaccharides - Oxidative Cleavage: Videos & Practice Problems

Periodic acid cleaves vicinal diols in sugars, leading to oxidative cleavage. Aldoses yield formic acid upon oxidation, while ketoses produce carbon dioxide. Internal alcohols also generate formic acid, whereas terminal alcohols result in formaldehyde. Understanding these cleavage patterns is crucial for predicting products when reacting monosaccharides with periodic acid. The mechanism involves forming a cyclic periodic ester, followed by breaking sigma bonds to create carbonyls, illustrating the relationship between oxidation and cleavage in carbohydrate chemistry.

Monosaccharides - Oxidative Cleavage

Periodic acid is a powerful reagent used to cleave vicinal diols, which are compounds containing two alcohol groups located on adjacent carbon atoms. This reaction is not exclusive to sugars; it applies broadly to any vicinal diol. When periodic acid interacts with these diols, it cleaves them, resulting in the formation of carbonyl compounds while leaving oxygen atoms on both sides of the cleavage site.

To understand this process, it's essential to recognize the structure of periodic acid, which can be represented in various forms, including periodic acid with water, iodic acid, and the periodate anion (IO₄^-). Regardless of the specific representation, the key takeaway is that these forms of periodic acid facilitate the cleavage of vicinal diols.

The mechanism begins with the formation of a cyclic periodic ester as an intermediate. This cyclic structure is crucial in the oxidative cleavage process. The next step involves breaking the sigma bond between the two carbon atoms of the diol. This bond cleavage transforms one of the carbon atoms into a carbonyl group (C=O), while the electrons from the broken bond are redistributed to maintain the formal charge of the iodine atom, ultimately leading to the formation of two carbonyl compounds.

The result of this reaction is the conversion of alcohols into aldehydes, exemplifying the concept of oxidative cleavage. The byproduct of this reaction is iodic acid. Understanding this mechanism is vital, as it lays the groundwork for recognizing the various cleavage patterns that can occur during oxidative cleavage with periodic acid.

Monosaccharides - Oxidative Cleavage

Understanding the reactions of sugars with periodic acid is crucial for predicting the products formed during oxidation. Sugars can be classified as either aldoses or ketoses, which significantly influences their reactivity. An aldose contains an aldehyde group at the top, while a ketose has a ketone group. When these sugars react with periodic acid, the outcomes differ based on their functional groups.

For aldehydes, the oxidation process leads to the formation of formic acid, also known as methanoic acid, which is the simplest carboxylic acid with the formula HCOOH. This occurs because aldehydes are always terminal, meaning they are located at the end of the carbon chain.

In contrast, when a ketone undergoes oxidation, the product is carbon dioxide (CO_2). This distinction is essential, as it highlights that the oxidation of carbonyls results in different products based on their structure.

Moving on to alcohols, the type of alcohol also affects the reaction outcome. Internal alcohols, which have substituents on both sides, yield one equivalent of formic acid upon oxidation. Conversely, terminal alcohols produce formaldehyde, the simplest aldehyde, with the formula CH_2O. This means that depending on whether the alcohol is internal or terminal, the products will vary between formic acid and formaldehyde.

In summary, when reacting monosaccharides with periodic acid, the key products to remember are formic acid for terminal aldehydes and internal alcohols, formaldehyde for terminal alcohols, and carbon dioxide for ketones. By mastering these four cleavage patterns, one can accurately predict the products of any sugar reaction with periodic acid.

Predict the products of the following oxidative cleavage

In the reaction between periodic acid and a ketose sugar, oxidative cleavage occurs, resulting in the breakdown of the sugar into smaller molecules. The process involves the cleavage of each carbon atom in the monosaccharide, leading to the formation of distinct oxidation products. For a ketose with five carbon atoms, the expected products can be predicted based on the structure of the sugar.

Starting with the terminal carbon (carbon A), the oxidation product is formaldehyde, a simple aldehyde. Moving to carbon B, which is a ketone, the reaction with periodic acid yields carbon dioxide (CO₂). For carbon C, being an internal alcohol, the product is formic acid, a carboxylic acid. The same applies to carbon D, which is also an internal alcohol, resulting in another molecule of formic acid. Finally, carbon E, another terminal alcohol, produces formaldehyde again.

Summarizing the products from the oxidative cleavage, the overall reaction yields 1 mole of CO₂, 2 moles of formic acid, and 2 moles of formaldehyde. This reaction illustrates the utility of periodic acid in the oxidative cleavage of monosaccharides, providing a clear pathway to predict the products based on the structure of the sugar involved.

Predict the structure of the glycoside products

In aqueous base, D-glucose can undergo two significant transformations: epimerization to form small amounts of D-mannopyranose and rearrangement to produce D-fructofuranose. These processes lead to the formation of cyclic sugars, specifically a pyranose (six-membered ring) and a furanose (five-membered ring). The focus of the analysis is on the subsequent reactions of these sugars when treated with acid and methanol, leading to the formation of O-glycosides.

During O-glycosidation, only the anomeric carbon of the sugar can react with methanol in the presence of acid, resulting in the addition of a methyl group (–CH₃) to the anomeric oxygen. This means that for both D-mannopyranose and D-fructofuranose, the resulting O-glycosides will have the anomeric position modified while the other hydroxyl groups remain unchanged. The products can be represented as D-mannopyranosyl methyl ether and D-fructofuranosyl methyl ether, respectively, with the anomeric carbon now bearing the methyl group.

The next part of the question involves the treatment of these glycosides with periodic acid (HIO₄), which is known to cleave vicinal diols. The ability to distinguish between the two products formed from the initial sugars relies on the number of cleavage sites available. In the case of the pyranose, there are two vicinal diols, allowing for two cleavage points, which would yield two distinct products, one of which is formic acid due to the cleavage of an internal alcohol. Conversely, the furanose structure has only one vicinal diol, leading to a single cleavage point and resulting in one linear chain product without the formation of formic acid.

Thus, the presence of formic acid in the reaction mixture after treatment with periodic acid indicates that the pyranose form is favored, while the absence of formic acid suggests the furanose form predominates. This distinction is crucial for understanding the relative stability and prevalence of these cyclic forms of sugars in biochemical processes.

Here’s what students ask on this topic:

What is the role of periodic acid in the oxidative cleavage of monosaccharides?

Periodic acid (HIO₄) plays a crucial role in the oxidative cleavage of monosaccharides by cleaving vicinal diols. When periodic acid reacts with vicinal diols in sugars, it forms a cyclic periodic ester intermediate. This intermediate then undergoes cleavage, breaking the sigma bond between the two carbons and forming carbonyl compounds. The products depend on the type of alcohol or carbonyl present in the sugar: aldoses yield formic acid, ketoses produce carbon dioxide, internal alcohols generate formic acid, and terminal alcohols result in formaldehyde.

How does periodic acid cleave vicinal diols in sugars?

Periodic acid cleaves vicinal diols in sugars through a mechanism that involves the formation of a cyclic periodic ester. Initially, periodic acid reacts with the vicinal diol to form this cyclic ester. The ester then undergoes oxidative cleavage, breaking the sigma bond between the two carbons. This cleavage results in the formation of carbonyl compounds. The specific products depend on the type of diol: internal diols yield formic acid, terminal diols produce formaldehyde, and carbonyl groups in aldoses and ketoses yield formic acid and carbon dioxide, respectively.

What are the products of oxidative cleavage of aldoses and ketoses with periodic acid?

The products of oxidative cleavage of aldoses and ketoses with periodic acid differ based on the type of sugar. For aldoses, which have an aldehyde group at the terminal end, the oxidative cleavage results in the formation of formic acid (HCOOH). For ketoses, which have a ketone group, the cleavage produces carbon dioxide (CO₂). These products are formed due to the specific oxidation patterns of the carbonyl groups in these sugars when exposed to periodic acid.

What is the mechanism of oxidative cleavage of vicinal diols by periodic acid?

The mechanism of oxidative cleavage of vicinal diols by periodic acid involves several steps. First, periodic acid reacts with the vicinal diol to form a cyclic periodic ester. This intermediate then undergoes cleavage, breaking the sigma bond between the two carbons. The electrons from the broken bond are used to form carbonyl groups, resulting in the formation of aldehydes or ketones. The overall process involves the redistribution of electrons and the formation of carbonyl compounds, illustrating the oxidative nature of the cleavage.

How do internal and terminal alcohols in sugars react with periodic acid?

Internal and terminal alcohols in sugars react differently with periodic acid. Internal alcohols, which have substituents on both sides, are oxidized to form formic acid (HCOOH). Terminal alcohols, located at the end of the sugar molecule, are oxidized to produce formaldehyde (CH₂O). These reactions are part of the oxidative cleavage process, where periodic acid cleaves the vicinal diols in the sugar, leading to the formation of specific oxidation products based on the position and type of alcohol.