Question

Which of the following Fischer’s projection formula is identical to D-glyceraldehyde?

(a)

(b)

(C)

(D)

Answer 1

We know that there are two types of glyceraldehydes namely L-glyceraldehyde and D-glyceraldehyde. Both are enantiomers. In L-Glyceraldehyde, OH group is present at the left and H group is at the right but in D-glyceraldehyde OH is present at the right and H is at the left.

Complete step by step answer:
Let’s understand the difference of L-glyceraldehyde and D-glyceraldehyde with the help of diagrams.

From the above structure, we see that in both the enantiomers CHO group is on the upper side and $CH_2OH$ is on the lower side. But, the position of OH and H is different in both the enantiomers.

Now, come to the question. Here, we have to identify the Fisher projection which is identical to D-glyceraldehyde.

We know that, if even number of interchanges is done in the Fisher projection formula we get the identical molecule.

For the Fischer projection in option (C),

Here, CHO group is in the vertical position. So, we don’t need to exchange it. Now, we have to interchange H and $CH_2OH$. Then, L-glyceraldehyde forms. Now, if we interchange the position of H and OH D-glyceraldehyde obtained. So, 2 times interchanges give the D-glyceraldehyde.

All other Fisher projections result in D-glyceraldehyde in an odd number of interchanges.

So, the correct answer is Option c .

Note: Always remember that Fischer projection represents three dimensional carbon atoms in 2-D. The molecule is drawn the form of a cross. The tetrahedral carbon is in the plane of the paper at the centre of the cross. Atoms bonded horizontally to the tetrahedral carbon are behind the plane of paper and vertically bonded atoms are in front of the paper.