How many structural isomers of a hydroxyaldehyde (C4H8O2) show optical activity ?

To answer this question, we need to understand a few concepts:

1. Structural Isomers: These are compounds with the same molecular formula but different structural formula. They have different physical and chemical properties.

2. Hydroxyaldehyde: This is a type of organic compound that contains both a hydroxyl group (-OH) and an aldehyde group (-CHO). In this case, the molecular formula is C4H8O2.

3. Optical Activity: This refers to the ability of a molecule to rotate the plane of polarized light. For a molecule to show optical activity, it must be chiral, meaning it cannot be superimposed on its mirror image.

Now, let's find the structural isomers of C4H8O2 that are hydroxyaldehydes:

1. 2-Hydroxybutanal
2. 3-Hydroxybutanal

Both of these isomers have a chiral center (the carbon atom bonded to the -OH group), so they can exist as two different enantiomers (mirror images). Therefore, both of these isomers can show optical activity.

So, there are 2 structural isomers of a hydroxyaldehyde with the formula C4H8O2 that show optical activity.