Glucose and galactose differs on which carbon atom.

Options

A. `2^(nd)` carbon atom
B. `3^(rd)` carbon atom
C. `4^(th)` carbon atom
D. `5^(th)` carbon atom

Answer

To determine the carbon atom at which glucose and galactose differ, we can follow these steps:

### Step 1: Draw the structure of glucose.
- Glucose is an aldohexose, meaning it has an aldehyde group at one end and six carbon atoms. The structure can be represented as follows:
- C1: CHO (aldehyde group)
- C2: CH2OH
- C3: CH(OH)
- C4: CH(OH)
- C5: CH2OH
- C6: CH3 (if in the cyclic form, it will be a ring structure)

### Step 2: Draw the structure of galactose.
- Galactose is also an aldohexose and has the same molecular formula as glucose but differs in the arrangement of hydroxyl (OH) groups. The structure can be represented as follows:
- C1: CHO (aldehyde group)
- C2: CH2OH
- C3: CH(OH)
- C4: CH(OH)
- C5: CH2OH
- C6: CH3 (if in the cyclic form, it will be a ring structure)

### Step 3: Identify the carbon atoms.
- For both glucose and galactose, we can label the carbon atoms from the aldehyde group:
- C1: Aldehyde carbon
- C2: Second carbon
- C3: Third carbon
- C4: Fourth carbon
- C5: Fifth carbon
- C6: Sixth carbon

### Step 4: Compare the structures of glucose and galactose.
- When we compare the structures of glucose and galactose, we notice that the hydroxyl (OH) group on the fourth carbon (C4) is positioned differently:
- In glucose, the OH group on C4 is on the right side.
- In galactose, the OH group on C4 is on the left side.

### Step 5: Conclude the difference.
- Therefore, glucose and galactose differ at the fourth carbon atom (C4). This type of difference is known as an epimeric difference, specifically C4 epimers.

### Final Answer:
Glucose and galactose differ at the **4th carbon atom**.

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