[chenglonglo]

Acetaldehyde, CH3CHO is considered prochiral because it can be converted into a chiral compound in a single step. Which of the following would cause acetaldehyde to form a chiral product?

A   Addition of an -OH group
B   Addition of an -H group
C   Replacement of the C=O group with 2 -H groups
D   Addition of an -OCH3 group

Why?   
Addition of an -OCH3 group would make 4 different substituent groups on a carbon atom, making the molecule chiral. It would have a -CH3 group, an -OH group, an -H group, and an -OCH3 group. Options (A) through (C) would each result in the product having at least two identical substituents and therefore achiral.

The question I am wondering is why it is not A. Because the explanation for D makes it seems as if there should be a total of 8 Hydrogens after adding an OCH3 group because the explanation states it would have -CH3, -OH, -OCH3, and an -H group making 8 Hydrogens instead of 7 if you just add -OCH3 to CH3CHO.

[Babcock_Hall]

It might help to draw out the product in A.

[mjc123]

making 8 Hydrogens instead of 7 if you just add -OCH3 to CH3CHO.

You don't just add OCH₃. You add an OCH₃ group to the carbon, but the net reaction is addition of HOCH₃ to the molecule, with the H going on the carbonyl oxygen.
Similarly with A and B.

[rolnor]

It should be said that the product in case D is not stable, its a hemiacetal?

[hollytara]

And case A is the aldehyde hydrate, which (like this hemiacetal) cannot be isolated.