[Kaspijs]

According to some sources according to IUPAC a double bond has a higher priority than a single bond. However, some others state that a double bond can be interpreted as two single bonds leading to two of the elements further in chain, unattached to any other. Thus, depending on which principle is used the R/S configuration varies, for example, 2-chloro-3,3-dihydroxypropanal (attached).

So which is the correct one and why is?

Unfortunately I do not reckon the sources, yet Wikipedia proposes the first method and second comes from some sort of Russian org chem book.

[hollytara]

A double bond is counted "backward and forward" by being turned into virtual single bonds.

So an aldehyde group has a C connected to two O's and an H, then an O connected to two C's

The germinal diol or aldehyde hydroate you show has a C connected to two O's and an H, but each O is connected to only one C and one H. 

Since the second bond to C is to a higher atomic number atom than to an H, the aldehyde group has higher priority.

If you look up the book by Jakob Fredlos, it is all explained very simply.

[chenbeier]

Die Name is wrong, Check Red an black coloured Bonds.


It has two Hydroxymethyl Groups HO-CH₂- instead of Hydroxy- group.

2- chloro- 3- hydroxymethyl - 4- hydroxy- butanal