24.2 Reactions of Monosaccharides | Organic Chemistry
TLDRThis comprehensive lesson delves into the chemical reactions of monosaccharides, focusing on ester and ether formation, glycoside synthesis, isomerization, and polymerization. It explores oxidation and reduction processes, highlighting the selective oxidation of aldehydes to carboxylic acids using bromine water or Tollens' and Benedict's reagents. The video explains the concept of reducing sugars and how they react with these reagents, contrasting them with non-reducing sugars like glycosides. It also covers the Kiliani-Fischer synthesis for extending the carbon chain of an aldose by one carbon and the Wohl degradation for shortening it. The lesson is enriched with practical examples and concludes with study tips for organic chemistry finals.
Takeaways
- 🍬 Ester formation with monosaccharides involves a nucleophilic attack by an alcohol on an acid chloride or anhydride, resulting in the replacement of a leaving group with an alkoxy group to form an ester.
- 🥚 Ether formation is similar to ester formation but uses an alkyl halide instead, with silver oxide facilitating the deprotonation of the hydroxyl group for the nucleophilic attack.
- 🍬 Glycoside formation is specific to the hemiacetal hydroxyl group at the anomeric carbon, leading to the creation of a glycosidic linkage when another alcohol is added.
- 🔁 Isomerization and epimerization of monosaccharides can occur under basic conditions, allowing for the conversion between different sugar structures.
- ➡️ Reduction of monosaccharides using sodium borohydride converts aldehydes and ketones into alcohols (alditols), which are used as sugar substitutes due to their different metabolic pathways.
- ⚖️ Mild oxidations, such as with aqueous bromine at pH 6, selectively oxidize aldehydes to carboxylic acids without affecting alcohols, resulting in aldonic acids.
- 🔑 Tollens and Benedict's reagents, both mild oxidizing agents, oxidize aldehydes to carboxylic acids and can be used to test for the presence of reducing sugars, which include aldoses and some ketoses.
- ⛔ Non-reducing sugars are those where the anomeric carbon is part of an acetal, such as in glycosides, and do not react with Tollens or Benedict's reagents.
- ⚠️ Stronger oxidizing agents like nitric acid can oxidize both aldehydes and primary alcohols to carboxylic acids, leading to the formation of aldaric acids.
- 🔄 Kiliani-Fischer synthesis is a method to lengthen the carbon chain of an aldose by one carbon through the formation of cyanohydrins, which are then reduced and converted to aldoses with an additional carbon.
- ⏪ Wohl degradation is the reverse of Kiliani-Fischer synthesis, reducing the carbon chain of an aldose by one carbon through the formation of an oxime, followed by oxidation to a nitrile and base-induced cleavage.
Q & A
What are the main reactions of monosaccharides covered in the lesson?
-The main reactions of monosaccharides covered include ester formation, ether formation, glycoside formation, isomerization, polymerization, oxidation, reduction, and classic syntheses reactions such as the Kiliani-Fischer synthesis and the Wohl degradation.
How does ester formation occur with monosaccharides?
-Ester formation occurs when an alcohol reacts with an acid chloride or an acid anhydride. The alcohol acts as a nucleophile, attacking the carbonyl carbon, and the leaving group (chloride or oxygen from the acid chloride/anhydride) departs, resulting in the formation of an ester.
What is the difference between ester and ether formation in monosaccharides?
-Ester formation involves the reaction of an alcohol with an acid derivative (acid chloride or anhydride), while ether formation involves the reaction of an alcohol with an alkyl halide. Ether formation typically utilizes a strong base to deprotonate the alcohol, making it a better nucleophile for the subsequent reaction.
What is glycoside formation and where does it occur in a monosaccharide?
-Glycoside formation is a specific reaction that occurs only at the hemiacetal hydroxyl group, specifically at the anomeric carbon. This carbon is unique as it is bonded to two oxygens, one of which is part of the hemiacetal and the other is the hydroxyl group that participates in the glycoside formation.
How does isomerization of monosaccharides occur under basic conditions?
-Isomerization occurs through keto-enol tautomerism, where an aldehyde or ketone is converted to an enol form. In the case of monosaccharides, this can lead to the formation of an enediol, which can then shift to form either the original sugar (epimerization) or a different sugar (isomerization), resulting in constitutional isomers.
What is the role of sodium borohydride in the reduction of monosaccharides?
-Sodium borohydride is used to reduce ketones or aldehydes in monosaccharides to alcohols. It is a milder reagent than lithium aluminum hydride (LiAlH4) and does not deprotonate hydroxyl groups, making it suitable for use with monosaccharides that contain multiple hydroxyl groups.
How does the oxidation of monosaccharides with aqueous bromine (Br2) differ from oxidation with Tollens' or Benedict's reagent?
-Aqueous bromine (Br2) is used under mildly acidic conditions, which selectively oxidizes aldehydes to carboxylic acids without promoting isomerization. In contrast, Tollens' and Benedict's reagents are basic and can cause isomerization of ketoses to aldoses, which can then be oxidized, leading to the oxidation of both aldoses and ketoses to aldonic acids.
What is the difference between a reducing sugar and a non-reducing sugar?
-A reducing sugar is a sugar that can be oxidized, typically because it has a free aldehyde or ketone group that can act as a reducing agent. A non-reducing sugar, such as a glycoside, does not have a free aldehyde or ketone group and thus cannot be oxidized under typical conditions.
How does the Kiliani-Fischer synthesis extend the carbon chain of an aldose?
-The Kiliani-Fischer synthesis starts with the formation of a cyanohydrin from the aldose and hydrocyanic acid (HCN). The cyanohydrin is then reduced to an amine, which under aqueous acidic conditions, is converted back to an aldehyde, effectively adding one carbon to the original aldose to form a longer aldose.
What is the Wohl degradation and how does it differ from the Kiliani-Fischer synthesis?
-The Wohl degradation is a reaction that shortens the carbon chain of an aldose by one carbon. It involves the formation of an oxime from the aldose and hydroxylamine, followed by oxidation to a nitrile and then hydrolysis in the presence of a strong base like sodium methoxide to remove the nitrile group, resulting in an aldose with one less carbon.
What is the significance of the anomeric carbon in the context of glycoside formation and disaccharides?
-The anomeric carbon is the carbon involved in the formation of a glycosidic bond in a glycoside. It is the carbon bonded to two oxygens, one being the hemiacetal oxygen and the other being part of the glycosidic bond. In disaccharides, the anomeric carbon can be part of an acetal, which is non-reactive and makes the sugar non-reducing. However, if a hemiacetal is present, the sugar can be reducing.
Outlines
📚 Ester and Ether Formation from Monosaccharides
This paragraph introduces the chemistry of monosaccharides, focusing on ester and ether formation. It explains the reaction of alcohols with acid chloride or anhydride to form esters, noting the nucleophilic attack and the role of a good leaving group. The paragraph also covers the formation of ethers using alkyl halides and the specific reactivity of the hemiacetal hydroxyl group in glycoside formation. It emphasizes the anomeric carbon's role and how it differs from other hydroxyl groups in the monosaccharide.
🔄 Isomerization and Epimerization of Monosaccharides
The second paragraph delves into isomerization and epimerization, processes that occur in aqueous solutions under basic conditions. It discusses keto-enol tautomerization and how it leads to the formation of enols, which can revert to their original keto or aldehyde forms. The text also explains epimerization, where a change in configuration at one chiral center results in a new sugar, and how this differs from isomerization, which produces constitutional isomers.
➡️ Reducing and Oxidizing Monosaccharides
This section covers the reduction of ketones and aldehydes in monosaccharides using sodium borohydride to form alditol, which are sugar alcohols. It also explores various oxidation reactions, such as the selective oxidation of aldehydes to carboxylic acids using aqueous bromine, and the oxidation of aldehydes to carboxylic acids and primary alcohols to carboxylic acids using nitric acid. The paragraph explains the conditions under which these reactions occur and the specificity of the reagents involved.
🔍 Identifying Reducing and Non-Reducing Sugars
The fourth paragraph explains the concept of reducing and non-reducing sugars. It describes how the presence of a hemiacetal group in a sugar molecule allows it to act as a reducing sugar, capable of being oxidized by Tollens' or Benedict's reagents. In contrast, non-reducing sugars, such as glycosides, do not have a free hemiacetal group and thus do not react with these reagents. The paragraph also discusses the identification of anomeric carbons in disaccharides and polysaccharides and how the presence of even a single hemiacetal group can render the molecule a reducing sugar.
🔬 Advanced Oxidation and Carbohydrate Chemistry
This paragraph discusses advanced oxidation reactions involving monosaccharides, including the use of nitric acid, which can oxidize both aldehydes to carboxylic acids and primary alcohols to carboxylic acids, resulting in the formation of aldaric acids. It also touches on the Kiliani-Fischer synthesis, which extends the carbon chain of an aldose by one carbon through the formation of cyanohydrins and subsequent reduction. Lastly, the Wool degradation is introduced, which shortens the carbon chain of an aldose by one carbon, involving the formation of an oxime, its conversion to a nitrile, and finally the removal of a cyanide group to form a new aldose with one less carbon.
🎓 Conclusion and Study Resources for Organic Chemistry
The final paragraph concludes the lesson and offers study resources for those preparing for organic chemistry exams. It encourages viewers to like and comment to help with the YouTube algorithm and mentions the availability of a study guide, an Organic Chemistry Master course, rapid review sessions for final exams, and a refresher course for the ACS standardized final exam. Links to these resources are provided in the video description.
Mindmap
Keywords
💡Monosaccharides
💡Ester Formation
💡Ether Formation
💡Glycoside Formation
💡Isomerization
💡Polymerization
💡Oxidation
💡Reduction
💡Kiliani Fission Synthesis
💡Wohl Degradation
💡Aldonic Acid
Highlights
The lesson covers key reactions of monosaccharides, including ester formation, ether formation, glycoside formation, isomerization, polymerization, oxidation, reduction, and classic syntheses reactions.
Ester formation can be achieved by reacting an alcohol with an acid chloride or anhydride, with a good leaving group being replaced by an alkoxy group.
Ether formation involves an SN2 backside attack using a methyl halide, with silver oxide acting as a base to deprotonate the hydroxyl group.
In glycoside formation, the reaction is specific to the hemiacetal hydroxyl group at the anomeric carbon.
Epimerization and isomerization can occur with monosaccharides in aqueous solution under basic conditions, leading to different stereoisomers or constitutional isomers.
Reduction of an aldehyde or ketone using sodium borohydride yields an alditol, which can be used as a sugar substitute.
Oxidation of an aldose using aqueous bromine (buffered at pH 6) selectively oxidizes the aldehyde to a carboxylic acid, forming an aldonic acid.
Tollens reagent and Benedict's reagent, both mild oxidizing agents, oxidize aldehydes to carboxylic acids and can be used to test for the presence of reducing sugars.
Ketoses can isomerize to aldoses under basic conditions, allowing them to be oxidized to aldonic acids, which is why they are also reducing sugars.
Glycosides are non-reducing sugars as the anomeric carbon is part of an acetal, making it unreactive under basic conditions.
Nitric acid is a stronger oxidizing agent that can oxidize both aldehydes and primary alcohols to carboxylic acids, forming an aldaric acid.
The Kiliani-Fischer synthesis lengthens the carbon chain of an aldose by one carbon through cyanohydrin formation and reduction.
In the Kiliani-Fischer synthesis, a new chiral center is formed, resulting in a pair of epimers due to the lack of stereospecificity.
Wohl degradation shortens the carbon chain of an aldose by one carbon through the formation of an oxime, oxidation to a nitrile, and hydrolysis.
During Wohl degradation, a chiral center is lost rather than formed, simplifying the process and yielding a single aldose with one fewer carbon.
The lesson provides a comprehensive overview of key monosaccharide reactions, including detailed mechanisms and practical applications.
The instructor emphasizes the importance of understanding reaction conditions, such as pH, in determining the extent and type of monosaccharide reactions.
The lesson concludes with helpful resources for students studying for organic chemistry exams, including a master course and rapid review videos.
Transcripts
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