Carbohydrates - Haworth & Fischer Projections With Chair Conformations

Summary

This educational video delves into the world of carbohydrates, starting with their historical naming to their crucial role in energy storage. It explains the structure and types of monosaccharides, highlighting glucose and fructose. The video then explores disaccharides like sucrose, maltose, and lactose, before moving on to polysaccharides such as starch and cellulose. It also covers the concept of epimers, focusing on glucose and galactose, and concludes with an in-depth look at glucose's cyclic form, its anomers, and the stability of its chair conformation.

Takeaways

• 🔍 Carbohydrates are biomolecules known for their role in storing energy and are historically named for their carbon-hydrate composition, as seen in glucose's formula C6H12O6.
• 🍬 Monosaccharides are the simplest form of carbohydrates, with common examples including glucose, fructose, and galactose.
• 🔗 Disaccharides are formed by the combination of two monosaccharide units, such as sucrose (glucose + fructose), maltose (two glucose units), and lactose (glucose + galactose).
• 🌐 Polysaccharides are complex carbohydrate polymers, with starch being a storage form in animals and cellulose providing structure in plants, both composed of many glucose units.
• 🧪 Glucose is a key monosaccharide, identified as D-glucose due to the hydroxyl group's position on the chiral carbon atom furthest from the aldehyde group.
• 🔄 Fructose differs from glucose by having a ketone functional group instead of an aldehyde, making it a ketose, and it is also a D-isomer with the hydroxyl group on the right at the bottom carbon.
• 🔑 The term 'epimer' refers to molecules that are identical except for the configuration at one chiral center, exemplified by glucose and galactose differing at carbon 4.
• 🔁 Glucose predominantly exists in a cyclic form rather than a straight chain, with the cyclic form involving the reaction of the aldehyde group with a hydroxyl group to form a ring.
• 🌀 The cyclic form of glucose can exist as two interconvertible anomers, alpha and beta, determined by the orientation of the hydroxyl group on the anomeric carbon.
• 📉 The specific rotation of glucose in its beta form is 18.7 degrees, and in its alpha form is 112.2 degrees, indicating how it rotates plane polarized light.
• 🪑 The chair conformation of beta D-glucose is more stable due to the equatorial positioning of the hydroxyl groups, which minimizes 1,3-diaxial strain, making it the predominant form in nature.

Q & A

What is the origin of the name 'carbohydrates'?

The name 'carbohydrates' comes from the early chemists' view of them as 'hydrates of carbon'. This is derived from the molecular formula of glucose, C6H12O6, which simplifies to CH2O when each subscript is divided by 6, resembling carbon plus water.

What are monosaccharides and what are the most common ones?

Monosaccharides are simple sugars, and the most common ones include glucose, fructose, galactose, mannose, ribulose, and xylose. Among these, glucose and fructose are the most frequently encountered.

Define disaccharides and give some examples.

Disaccharides are carbohydrates that consist of two simple sugar units linked together. Examples include sucrose (glucose + fructose), maltose (two glucose units), and lactose (glucose + galactose).

What is a polysaccharide and what are two common examples?

A polysaccharide is a long polymer molecule made up of many monosaccharide units. Two common polysaccharides are starch, which is found in animals and used for energy storage, and cellulose, found in plants and serving a structural purpose.

How are glucose and fructose structurally different?

Glucose and fructose differ structurally by their functional groups; glucose has an aldehyde group, while fructose has a ketone group. This is the primary distinction between the two.

What is the significance of the 'D' and 'L' prefixes in the context of glucose?

The 'D' and 'L' prefixes in glucose indicate the configuration of the hydroxyl group at the chiral carbon atom farthest from the aldehyde group. 'D' glucose has the hydroxyl group on the right at this carbon, while 'L' glucose has it on the left.

What is an epimer and how does it relate to glucose and galactose?

Epimers are molecules that differ in configuration at only one chiral center. Galactose is a C4 epimer of glucose, meaning it differs from glucose only at the fourth carbon atom's chiral center.

Why does glucose predominantly exist in a cyclic form rather than a straight chain form?

Glucose predominantly exists in a cyclic form because it can undergo an intramolecular reaction where the hydroxyl group on carbon five attacks the carbonyl carbon, forming a ring structure. This cyclic form is more stable and common in nature.

What are the alpha and beta anomers of glucose, and which is more prevalent?

The alpha and beta anomers of glucose refer to the orientation of the hydroxyl group on the anomeric carbon. The beta anomer, with the hydroxyl group in the equatorial position, is more prevalent, making up about 64% of glucose in solution, due to its greater stability.

Why is the beta D-glucose more stable than the alpha D-glucose?

Beta D-glucose is more stable than alpha D-glucose because in the beta form, the hydroxyl group is in the equatorial position, which minimizes 1,3-diaxial strain. This results in a more stable chair conformation, making beta D-glucose the most common form of glucose.

How does the specific rotation of glucose change when it goes from the beta form to a solution?

The specific rotation of beta D-glucose changes from 18.7 degrees to 52.7 degrees when dissolved in water, indicating a mixture of alpha and beta forms, with the beta form being more prevalent.