Assign relative priorities to the groups or atoms in each

Solution:

Here we have to assign the relative priority of the given sets of molecules.

The relative priorities of the given molecules can be given while considering the following rules,

1. 1st we have to choose the stereocenter or chiral center.

2. Then we will have to assign priority to the atom directly bonded to the chiral center and priority is assigned numerically with 1 being the highest priority, 2 is next etc.

3. Priority increases with increase with atomic number. An atom with the highest atomic number will have the highest priority.

The priorities of atoms commonly used  are shown below,

I > Br > Cl> S > O > N > C > H

Step 1

(a)

Here the stereocenter is the C-atom which is bonded to the above four molecules i.e CH2OH, CH3, H and CH2CH2OH.  

We can say the chiral C-atom is directly bonded to the three C-atoms and one H-atom

Now we will have to assign the priority,

In CH2OH, the bonds directly attached to C-atoms are (O, H, H)

In CH3, the bonds directly attached to C-atoms are three H-atoms (H, H, H)

In CH2CH2OH, the bonds directly attached to C-atoms are (C, H, H)

Thus the priority of CH2OH can be assigned as 1 due to the highest atomic number of O-atom.  CH2CH2OH can be assigned as 2 due to C-atom has the lower atomic number than O-atom. CH3 has priority 3 because H has the lowest atomic number.

The 4th priority will be on -H atom.

The order of decreasing priority is,

-CH2OH > CH2CH2OH > -CH3 > -H