Module 16: Unit 7: Chemical Reactions of Monosaccharides, Disaccharides and Polysaccharides

1. The formation of carbohydrates can be carried out naturally by the photosynthesis process:

n CO₂   +   n H₂O   +  Energy    C_nH_2nO_n   +   n O₂

The chemical reactions of carbohydrates are following the chemical reactions of the aldehydes and ketones since carbohydrates are special aldehydes and ketones with many hydroxyl groups.

2. Glucose as an important monosaccharide can undergo many chemical reactions:

Reference: https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/carbhyd.htm

3. Monosaccharides cannot undergo hydrolysis but disaccharides as well as polysaccharides can undergo hydrolysis.

Hydrolysis is carried out using an acid or an enzyme as a catalyst;

Monosaccharides   +   H₂O     à  No Reaction

Disaccharides +   H₂O         Two molecules of monosaccharides.

Polysaccharides +   H₂O          many molecules of monosaccharides.

4. Combustion reaction of carbohydrates with pure oxygen yield CO₂ and H₂O and Energy. The reaction is exothermic.

Carbohydrates   +   O₂ (gas)       n CO₂ (gas) + n H₂O (gas)   +   Energy

5. Oxidation Reaction using an oxidizing agent:

Carbohydrates (aldehydes)  + Oxidizing agent  à  carboxylic acid

Carbohydrates (ketones)  +  Oxidizing agent à  carboxylic acid

[Exception is:  sucrose  +  Oxidizing agent à  No Reaction because sucrose is not a reducing agent]. There is no free carbonyl group available in sucrose.

6. Reduction of Carbohydrates yields secondary alcohols (with ketone carbohydrates) and primary alcohols (with aldehyde carbohydrates).

Reference: http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch25/ch25-5-3.html

• 7. Formation of Hemiacetal: Reaction with ONE molecule of an alcohol with ketone or aldehyde carbohydrates:

Reference: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Aldehydes_and_Ketones/Reactivity_of_Aldehydes_and_Ketones/Addition_of_Alcohols_to_form_Hemiacetals_and_Acetals

• Formation of Hemiacetal: Reaction with TWO molecule of an alcohol with ketone or aldehyde carbohydrates:

Reference: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Aldehydes_and_Ketones/Reactivity_of_Aldehydes_and_Ketones/Addition_of_Alcohols_to_form_Hemiacetals_and_Acetals

Glucose can undergo conversion from Fischer structure or projection to the cyclic structure of Haworth and a hemiacetal is formed:

Reference: https://www.khanacademy.org/test-prep/mcat/chemical-processes/aldehydes-and-ketones/a/cyclic-hemiacetals-and-hemiketals

Fructose can undergo two possibilities to form a cyclic ring and hence two possibilities to form hemiketal:

The first possibility is shown below:

The second possibility is shown below:

Both hemiacetal and hemiketal can react further with one molecule of alcohol to form the acetal or the ketal forms of glucose or fructose respectively.

The videos below illustrate the different reactions of carbohydrates: