Question

The number of primary and secondary hydroxyl groups in ribose are-
A) $ 1,3 $
B) $ 2,3 $
C) $ 3,1 $
D) $ 3,2 $

Answer 1

Hint :The hydroxyl group (-OH) attached with a carbon bonded with only one carbon atom is called primary hydroxyl group. The hydroxyl group attached with carbon that is bonded to two other carbon atoms is called the secondary hydroxyl group. So count the number of both the hydroxyl groups to get the answer.

Complete Step By Step Answer:
Ribose is a monosaccharide carbohydrate. The molecular formula of ribose is $ {{\text{C}}_6}{{\text{H}}_5}{{\text{O}}_5} $ .
We know that –
The carbon attached to only one carbon is called primary carbon and the hydroxyl group attached with the primary carbon is called primary hydroxyl group.
 The carbon attached with two carbon atoms to satisfy its valency is called secondary carbon and the hydroxyl group attached with this carbon is called secondary hydroxyl group.
 First we need to draw the structure of ribose using the formula.

In this structure, we can see that the carbon atom at the bottom of the chain is attached with one carbon atom, two hydrogen and one hydroxyl group. So this carbon is primary carbon and the hydroxyl group attached with it is the primary hydroxyl group. So there is only one primary hydroxyl group.
Now, we can see that the other three carbon atoms over the primary carbon in the chain are attached with two carbon atoms so they are secondary carbons and the hydroxyl groups attached with them are secondary hydroxyl groups.
So there are three secondary hydroxyl groups.

It is clear from the image that the number of primary and secondary hydroxyl groups is $ 1 $ and $ 3 $ respectively.
Hence correct answer is option A.

Note :
Here student may get confused that why are we not counting the carbon on the top of the chain, so remember that it is aldehyde group(-CHO) not hydroxyl group(-OH) as explained in the structure-

Because in aldehyde there is a double bond between carbon and oxygen atom and the hydrogen atom is attached to carbon atom not oxygen atom whereas in hydroxyl group oxygen atom is attached with hydrogen atom with a single bond.