Hemiacetal formation of carbohydrates
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Hemiacetal formation of carbohydrates
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[post_date] => 2025-01-14 06:04:49
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Generally, hemiacetal formation occurs when the nucleophilic hydroxyl group of an alcohol attacks the carbonyl carbon of an aldehyde functional group. In the case of glucose specifically, this reaction occurs intramolecularly between the hydroxyl group of Carbon-5 and the aldehyde carbon resulting in the formation of a more stable, six-membered ring. In the process, a new chiral center is formed at the former carbonyl carbon, which is now called the anomeric carbon. This nucleophilic attack can occur above or below the pi bond resulting in two diastereomer products called anomers. In the β-anomer of glucose, the anomeric (Carbon-1]) hydroxyl group is cis with respect to Carbon-6. Alternatively, the α-anomer contains an anomeric hydroxyl group that is trans with respect to Carbon-6. These are represented in Figure 1. In water, the ring-closing hemiactel reaction of glucose occurs in the forward and reverse direction, resulting in the spontaneous opening and closing of the ring and an equilibrium concentration of the two anomers. Due to steric hindrance, the β-anomer of glucose accounts for ~ 64% of concentration, and the α-anomer accounts for ~34%. The straight chain form accounts for <1% of concentration.
Carbohydrates that form six membered rings (such as glucose) are referred to as pyranoses, while those forming five membered rings are referred to as furanoses.
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[question] => An aldohexose has how many units of unsaturation?
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[answer] => 2
[description] => Reason for the Correct Answer:
In the straight chain form, an aldohexose has one pi bond associated with the aldehyde functional group.
In the ring form, an aldohexose has no pi bonds, but a cyclical structure.
In the straight chain form, an aldohexose such as glucose contains one pi bond yielding one unit of unsaturation. While the pi bond does not exist in the ring structure, the cyclic structure also accounts for one unit of unsaturation. In either form there is one unit of unsaturation.
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[quiz_unique_key] => 3873426850
[question] => What is the mass percent of oxygen in β-D-Glucose?
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[answer] => 3
[description] => Reason for the Correct Answer:
The molecular formula of β-D-Glucose is C6H12O6.
The molar mass of β-D-Glucose is 180 grams/mol.
53%. The molar mass of Oxygen atoms in β-D-Glucose is 96 g/mol (6 O atoms * 16g/mol). This accounts for 53%] of glucose’s total molar mass (96÷180).
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[each_answer] => A.43%
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[each_answer] => B.47%
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[each_answer] => C.53%
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[each_answer] => D.57%
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[quiz_unique_key] => 83407773
[question] => When two monosaccharides are joined by the nucleophilic attack of one sugar’s hydroxyl group on the other sugar’s carbonyl carbon, the result is a disaccharide containing a glycosidic (acetal) linkage. What would be the molecular formula of a disaccharide made from two units of glucose?
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[description] => Reason for the Correct Answer:
The glycosidic linkage forms as a result of a dehydration reaction.
In a dehydration reaction, the equivalent of one water molecule is lost as two molecules are joined.
C12H22O11. The molecular formula for glucose is C6H12O6. Two glucose molecules would together have a molecular formula of C12H24O12; however, accounting for the loss of a water molecule, the disaccharide formed would have a molecular formula of C12H22O11. The sugar formed is maltose.
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[each_answer] => A.C12H22O11
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[each_answer] => B.C11H22O12
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[each_answer] => C.C12H24O12
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[each_answer] => D.(C6H12O6)2
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[question] => Which mechanism best describes the opening of glucose’s ring structure?
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[description] => Reason for the Correct Answer:
This is a specific type of addition reaction.
Water plays a role in “cutting” the bond such that hydrogen is added to one side of the open bond, and a Hydroxyl group is added to the other side.
Hydration. The splitting of a bond via the addition of water (hydration) is called hydrolysis, which may be a more familiar term, but this is best described by the answer choices as a hydration reaction.
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[question] => Which molecular stereochemistry describes D-Glucose?
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[answer] => 4
[description] => Reason for the Correct Answer:
Remember, D-Glucose! This is a reminder to use your middle finger out with palm facing up as a reference for the D-Glucose structure.
Remember that in a Fischer projection, the horizontal substituents are coming out of the page, while those vertical substituents are going into the page.
2R 3S 4R 5R. If you imagine your right hand giving the page the middle finger as a representative of D-glucose, then only the 3-carbon is directed to the left. In the fisher projection of glucose, those carbons with their priority substituents on the left have “S” configurations, and if the highest priority substituent is on the right, the carbon has “R” stereochemistry.
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[each_answer] => A.2S 3S 4S 5R
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[each_answer] => B.2S 3S 4R 5R
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[each_answer] => C.2R 3R 4S 5R
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[each_answer] => D.2R 3S 4R 5R
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