Problem 96 - Chemistry An Atoms First Approach

Question

Indicate the chiral carbon atoms found in the monosaccharides D-ribose and D-mannose.

Step-by-Step Solution

Answer

The chiral carbon atoms in D-ribose are at positions 2, 3, and 4, while in D-mannose they are at positions 2, 3, 4, and 5.

Step 1: Molecular structure of D-ribose and D-mannose

D-ribose is an aldopentose having the following structure: \( CH_2OH(1) - CHOH(2) - CHOH(3) - CHOH(4) - CHO(5) \) D-mannose is an aldohexose with the following structure: \( CH_2OH(1) - CHOH(2) - CHOH(3) - CHOH(4) - CHOH(5) - CHO(6) \)

Step 2: Identifying Chiral Carbon Atoms in D-ribose

To identify the chiral carbon atoms, we need to find carbon atoms that have four distinct groups attached to them: 1. Carbon 1: \( CH_2OH \) - not chiral, as it has two hydrogen atoms attached to the carbon. 2. Carbon 2: \( CHOH(CH_2OH) \) - chiral, as it has four different groups attached: H, OH, \( CH_2OH \), and \( CHOH(3) \). 3. Carbon 3: \( CHOH \) - chiral, as it has four different groups attached: H, OH, \( CHOH(2) \), and \( CHOH(4) \). 4. Carbon 4: \( CHOH \) - chiral, as it has four different groups attached: H, OH, \( CHOH(3) \), and CHO. 5. Carbon 5: \( CHO \) - not chiral, as it has two bonds to the same oxygen atom. So, in D-ribose, the chiral carbon atoms are located at positions 2, 3, and 4.

Step 3: Identifying Chiral Carbon Atoms in D-mannose

To identify the chiral carbon atoms in D-mannose, we follow the same process: 1. Carbon 1: \( CH_2OH \) - not chiral, as it has two hydrogen atoms attached to the carbon. 2. Carbon 2: \( CHOH(CH_2OH) \) - chiral, as it has four different groups attached: H, OH, \( CH_2OH \), and \( CHOH(3) \). 3. Carbon 3: \( CHOH \) - chiral, as it has four different groups attached: H, OH, \( CHOH(2) \), and \( CHOH(4) \). 4. Carbon 4: \( CHOH \) - chiral, as it has four different groups attached: H, OH, \( CHOH(3) \), and \( CHOH(5) \). 5. Carbon 5: \( CHOH \) - chiral, as it has four different groups attached: H, OH, \( CHOH(4) \), and CHO. 6. Carbon 6: \( CHO \) - not chiral, as it has two bonds to the same oxygen atom. So, in D-mannose, the chiral carbon atoms are located at positions 2, 3, 4, and 5. In summary, the chiral carbon atoms in D-ribose are found at positions 2, 3, and 4, while in D-mannose, they are found at positions 2, 3, 4, and 5.