In today’s post, we will discuss the conversion between Fischer, Haworth, and Chair forms of carbohydrates as well the mechanism that leads to the formation of ɑ-D-glucose and β-D-glucose from the open form of glucose.
We mentioned the formation of the cyclic forms ɑ-D-glucose and β-D-glucose last time, however, did not pay close attention to how the Fischer forms are transformed into the cyclic anomers of glucose:
Let’s now see how this happens. First, we are going to draw the Haworth projection of the D glucose. To get this, simply flip the glucose by 90o and then rearrange the carbon chain to resemble a six-membered ring:
After this, rotate about C4-C5 bond such that the C5-OH points to the carbonyl for a nucleophilic attack. The reason for this preference of the C5-OH attack is that it allows for the formation of a six-membered ring:
The nucleophilic attack of C5-OH group on the carbonyl forms a new asymmetric center which is formed in both configurations. Carbon 1 is the new stereogenic center shown with a wiggly line which indicates the formation of both configurations ɑ and β anomers.
This carbon is called the anomeric carbon (carbon 1 in the picture above) and the configuration about it is denoted by prefixes ɑ – and β.
So, how exactly these two isomers are formed?
This happens as a result of a 180o rotation about the C1–C2 single bond which places the C=O group pointing up or down. The attack of C5-OH group on the carbonyl carbon forms the ɑ-D-glucose and β-D-glucose anomers:
Notice that all the initial chiral centers remain intact, and the two cyclic forms differ in the configuration of only one chiral center. We classify these as epimers, and in the case of sugars, they are said to be anomers. So, an anomer is type of epimer characterized by the carbon in two possible configurations of a cyclic saccharide.
The cyclic forms of D-Glucose, as six-membered rings, adopt chair conformations so, in the last step, we need to convert the Haworth projections to the corresponding chair conformations. For this, simply draw a chair conformation with an oxygen in the ring and add the Oh groups one by one keeping the configurations consistent: The “up” in a Haworth stay “up” in the chair and “down” stays down (either axial or equatorial):
If the newly formed OH group on the asymmetric center is pointing down (trans to CH2OH group at C-5), then it is ɑ-D-glucose. On the other hand, if the OH group is pointing up (cis to the CH2OH group at C-5), then the hemiacetal is β-D-glucose.
Keep in mind that “up” and “down” is always ambiguous unless we specify what it is relative to. For example, if we flip the chairs 180o (not ring-flip), all the “up” and “down” notation will change. That is why the cis and trans orientation of the Oh group to the CH2OH on carbon 5 is emphasized.
One statement that is true regardless of the way we draw the chair is that all the OH groups in β -D-glucose are equatorial.
Let’s now put the conversion of Fischer-Haworth-Chair forms of glucose in a little summary chart:
Can This be Even Shorter?
Now that we know how the conversion happens, we can shorten this process by skipping the first two steps and instead draw the Haworth projection of the sugar. The skeleton is standard for the pyranose and furanose rings.
For example, let’s convert D-Galactose into β-D-Galactose pyranose in the Haworth and chair forms.
Step 1. Draw the Haworth projection for pyranose rings by placing the oxygen in the upper right corner and pointing the C6 CH2OH on carbon up:
Step 2. Add the OH on the anomeric carbon pointing up for the β isomer, and pointing down for the ɑ isomer:
Step 3. Draw the H’s and OH groups: all the groups on the right side in the Fischer projection point down, the groups on the left are pointing up.
Step 4. Convert the Haworth to a chair conformation if needed. The groups pointing “up” in a Haworth stay “up” in the chair and “down” stay down (either axial or equatorial).
Putting this together in a three-step process, here is a guide you can use in the practice problems below for converting Fischer projection to Haworth and Chair forms:
Furanose Ring Formation
Just like we have seen the formation of pyranose rings, many monosaccharides, typically ketoses tend to also form five-membered rings, classified as furanose rings. For example, D-fructose can form a furanose ring as a result of an intramolecular reaction between the carbonyl group and the C5 hydroxyl group. It follows the same mechanism and patterns as the formation of pyranoses and the new stereogenic center at the anomeric carbon is formed:
Here as well, if the OH group bonded to the new asymmetric center is trans to the CH2OH group on C6, then the compound is ɑ anomer of D-fructofuranose and if it is cis to the CH2OH group, then it is the β–D-fructofuranose.
D-Fructose can also form a six-membered ring if the nucleophilic attack on the carbonyl occurs by the C-6 OH group:
Need some practice on carbohydrates?
Check this Multiple-Choice, summary quiz on the structure and reactions of carbohydrates with a 40-min video solution!
Check also in Carbohydrates
- Carbohydrates – Structure and Classification
- Erythro and Threo
- D and L Sugars
- Aldoses and Ketoses: Classification and Stereochemistry
- Epimers and Anomers
- Mutarotation
- Glycosides
- Isomerization of Carbohydrates
- Ether and Ester Derivatives of Carbohydrates
- Oxidation of Monosaccharides
- Reduction of Monosaccharides
- Kiliani–Fischer Synthesis
- Wohl Degradation
Practice
Convert the following Fischer projections of carbohydrates to their ɑ and β anomeric Haworth and chair forms:
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This content is for registered users only.
By joining Chemistry Steps, you will gain instant access to the answers and solutions for all the Practice Problems including over 20 hours of problem-solving videos, Multiple-Choice Quizzes, Puzzles, and the powerful set of Organic Chemistry 1 and 2 Summary Study Guides.
This content is for registered users only.
By joining Chemistry Steps, you will gain instant access to the answers and solutions for all the Practice Problems including over 20 hours of problem-solving videos, Multiple-Choice Quizzes, Puzzles, and the powerful set of Organic Chemistry 1 and 2 Summary Study Guides.
This content is for registered users only.
By joining Chemistry Steps, you will gain instant access to the answers and solutions for all the Practice Problems including over 20 hours of problem-solving videos, Multiple-Choice Quizzes, Puzzles, and the powerful set of Organic Chemistry 1 and 2 Summary Study Guides.
This content is for registered users only.
By joining Chemistry Steps, you will gain instant access to the answers and solutions for all the Practice Problems including over 20 hours of problem-solving videos, Multiple-Choice Quizzes, Puzzles, and the powerful set of Organic Chemistry 1 and 2 Summary Study Guides.
This content is for registered users only.
By joining Chemistry Steps, you will gain instant access to the answers and solutions for all the Practice Problems including over 20 hours of problem-solving videos, Multiple-Choice Quizzes, Puzzles, and the powerful set of Organic Chemistry 1 and 2 Summary Study Guides.
Convert the Fischer projections of D-Erythrose and D-Threose to their ɑ and β furanose rings in Haworth and chair forms:
This content is for registered users only.
By joining Chemistry Steps, you will gain instant access to the answers and solutions for all the Practice Problems including over 20 hours of problem-solving videos, Multiple-Choice Quizzes, Puzzles, and the powerful set of Organic Chemistry 1 and 2 Summary Study Guides.
This content is for registered users only.
By joining Chemistry Steps, you will gain instant access to the answers and solutions for all the Practice Problems including over 20 hours of problem-solving videos, Multiple-Choice Quizzes, Puzzles, and the powerful set of Organic Chemistry 1 and 2 Summary Study Guides.
Check also in Carbohydrates
- Carbohydrates – Structure and Classification
- Erythro and Threo
- D and L Sugars
- Aldoses and Ketoses: Classification and Stereochemistry
- Epimers and Anomers
- Converting Fischer, Haworth, and Chair forms of Carbohydrates
- Mutarotation
- Glycosides
- Isomerization of Carbohydrates
- Ether and Ester Derivatives of Carbohydrates
- Oxidation of Monosaccharides
- Reduction of Monosaccharides
- Kiliani–Fischer Synthesis
- Wohl Degradation
